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Citric Acid Cycle: Krebs Cycle, Tricarboxylic acid cycle or TCA cycle

Citric acid cycle consists of number of reactions which produce NADH and FADH₂ and then they are used by the oxidative phosphorylation pathway to make ATP which then passes through the electron transport system. The citric acid cycle happens in the matrix of the mitochondria of the cell. The oxydation of pyruvic acid takes place through a series of reaction. These reactions produced through a cycle known as tricarboxylic acid cycle. It is also known as TCA cycle. The first product in this cycle is cytric acid hence it is called the citric acid cycle or CAC.

British biochemist Sir Hans Adolf Krebs discovered this cycle in 1937. As a result he awarded the Nobel Prize in 1953. Accordingly this cycle also familiar as Kreb`s cycle. 

Important Features of Citric Acid Cycle

  • It helps in the cellular respiration.
  • It happens inside the mitochondria and it is restricted to only aerobic respiration.
  • It occurs in cyclic pathway which is regulated by a specific enzyme.
  • This cycle does not consume any ATP molecules.
  • Two ATP/GTP, eight NADH2, two FADH2 and six CO2 molecules are produced in the citric acid cycle.

Key Citric Acid Cycle Enzymes

  • Citrate synthase
  • Aconitase
  • Isocitrate dehydogenase
  • Oxalosuccinate decarboxylase
  • α-Ketoglutarate dehydrogenase
  • Succinyl Co-A synthatase
  • Fumarase
  • Malate dehydrogenase

Key Intermediates

Following key intermediates are produced from the citric acid cycle:

  • Cytric acid (citrate)
  • Isocitrate
  • Cis-aconitic acid
  • Cis-aconitic acid
  • Oxalosuccinic acid
  • α-ketoglutaric acid
  • Succinyl-CoA
  • Succinic acid (succinate)
  • Fumaric acid (Fumarate)
  • Malic acid (Malate)
  • Oxaloacetic acid (Oxaloacetate)

Steps of Cytric Acid Cycle

Kreb`s cycle or citric acid cycle is an oxidation process which occurs stepwise. In this case, it includes four dehydogenase steps and two decarboxylation steps. It produces reduced co-enzymes and CO2

Pyruvic acid is formed through the process of glycolysis in cell cytoplasm. After formation of pyruvic acid, it enters into the mitochondria. In the presence of six factors such as Mg++, NAD, TPP (Thiamine pyrophosphate), lipoic acid, FAD and coenzyme A, the pyruvic dehydogenase along with enzyme complex converts pyruvate to acetyle CoA.

Overall steps of citric acid cycle are described below:

Step-1: The first step is the condensation step.  In this step, acetyle CoA mix with oxaloacetate and H2O in the presence of condensing enzymes citratrate synthetage and produce one molecule of citric acid. After reaction CoA is released out. In this case, acetyl CoA is two carbon molecule, oxaloacetate is 4 carbon molecule while cytric acid or citrate is 6 carbon molecule.

Step-2: It is the isomirization step. In this step, cytric acid is converted into its isomer isocitrate by completing the following two step reactions with the help of aconitase enzyme.  

(i) Dehydration: In this case, one molecule of H2O is released out and citric acid is converted into cis-aconitic acid.

(ii) Rehydration: In this case, cis-aconitic acid joins with one molecule of H2O and produce isocitric acid.

Step-3: The third step is the dehydrogenation step. In this step, isocitrate /isocitric acid is dehydrogenated into oxalosuccinic acid by losing 2H- with the help of isocitrate dehydrogenase enzyme and Mn++.  The enzyme isocitrate dehydrogenase catalyzes this step and this enzyme is responsible to regulate the speed of the citric acid cycle. During this step, NAD (Nicodinamide adenine dinucleotide) is reduced and forms NADH2.

image of Citric Acid Cycle

Citric Acid Cycle

Step-4: The fourth step is the decarboxylation step. In this step, oxalosuccinic acid is decarboxylated into α-ketoglutaric acid by losing CO2 with the help of enzyme, oxalosuccinate decarboxylase.

Step-5: It is oxidative decarboxylation step where α-ketoglutaric acid undergoes dehydrogenation and decarboxylation at the same time with the help of enzyme, ketoglutarate dehydrogenase. The enzyme, ketoglutarate dehydrogenase catalyzes and it is responsible for regulating the speed of the citric acid cycle. In this step, NAD+, Mg++, and CoA are required. Finally, succinyl CoA, NADH2 and CO2 are produced. 

Step-6: It is the substrate level GTP or ATP synthesis step. In this step succinyl CoA is synthesized into succinic acid with the help of enzyme, succinyl-CoA synthatase. It is energy liberated step and during this step, one molecule of molecule of GTP is produced and CoA is released. 

Step-7: This step is also known as dehydrogenation step. In this step, succinic acid is dehydrogenated into four-carbon molecule fumaric acid in the presence of succinate dehydrogenase enzyme. In this case, hydrogen is given out by succinic acid and is picked up by FAD(Flavin adenine dinucleotide) to form FADH2.  

Step-8: In this step, fumaric acid is converted into a 4 carbon molecule malic acid. In this case, fumaric acid   reacts with one molecule of H2O in the presence of enzyme fumarase. 

Step-9: In this step, malic acid is dehydrogenated into oxaloacetic acid in the presence of malate dehydogenase enzyme. In this reaction, NAD+ is reduced to form NADH2. 

Oxaloacetic acid again joins with acetyle CoA and again begins a new citric acid cycle. The oxidative catabolism of pyruvate can be shown in the following equation: 

Citric Acid Cycle Products

The citric acid cycle involves 2 pyruvic acids from which the following products may be summarized:

  1. Total number of reduced co-enzyme molecules= 8 NADH2 and 2 FADH2
  2. Total number of ATP produced directly= 2ATP
  3. Total number of CO2 molecules released= 6CO2
  4. Total number H2O utilized= 6H2O

Significance of Citric Acid Cycle

  • Trough the citric acid cycle, carbon skeletom is produced that helps in growth; it also maintains the cells. 
  • During the citric acid cycle, various intermediate compounds are formed. These help in the synthesis of  nucleotides, amino acids, fats, chlorophyll,  and cytochromes, etc.
  • In this cycle,  succinyl CoA is produced which is essential for the formation of pigment like chlorophyll.
  • In cytric acid cycle, α-ketoglutaric acid, pyruvic acids and oxaloacetic acid are produced respectively which take part in the production of amino acids.
  • In this cycle, many ATPs are produced which take part in the different metabolic acitivites of the cells.

Glycolysis: Features, Steps and Significance

Glycolysis is the metabolic pathway where one molecule of glucose (C6H12O6) converts into pyruvic acid with the help of enzyme. Glycolysis occurs in the cytoplasm of the cell during both anaerobic and aerobic respiration. It is also known as EMP pathway i.e., Embden-Meyerhof-Parnas pathway named after German Biochemists Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas who first discovered the process of glycolysis in 1918.  Glycolysis is also called Entner–Doudoroff pathway.

Features of Glyclysis

  1. Glycolysis happens in the cytoplasm;
  2. Glycolysis does not need or consume oxygen;
  3. A 6-carbon glucose is converted into 3-carbon pyruvate or pyruvic acid;
  4. In this case, 2 molecules of ATP are consumed to initiate the process.
  5. Four molecules of ATP and 2 molecules of NADH are produced after the end of the process.

Step of Glycolysis

Reaction of glycolysis include the following three main steps:

  1. Phosphorylation of glucose or Preparatory Phase
  2. Cleavage of Fructose-1, 6-diphosphate
  3. Formation of 3-carbon pyruvate or pyruvic acid

Step-1: Phosphorylation of Glucose or Preparatory Phase

1. At the first step, glucose undergoes phosphorylation by ATP(Adenosine triphosphate) in presence of Mg++ to form glucose-6-phosphate in the presence of hexokinase enzyme.

2. By the process of isomerization Glucose 6-phosphate is isomerized into fructose 6-phosphate with the help of phosphogluco isomerase enzyme. 

3. Fructose  6-phosphate undergoes phosphorylation with the help of ATP and enzyme phosphofructokinase  to form Fructose 1, 6-bisphosphate and ADP (Adenosine diphosphate).

Step-2: Cleavage of Fructose-1, 6-bisphosphate

4. Fructose 1,6-bisphosphate is broken down to two triose (3 carbon molecule) phosphate such as dihydroxyacetone phosphate and 3 phosphoglyceraldehyde with the help of the enzyme aldolase. The dihydroxyacetone phosphate is converted to 3 phosphoglyceraldehyde with the help of enzyme triose phosphate isomerase. In this case, reaction is reversible. Here two molecules of 3-phosphoglyceraldehyde are formed from the cleavage of one fructose 1, 6-bisphosphate.

Step-3: Formation of 3-carbon pyruvate or pyruvic acid

5. With the help of NAD (nicotinamide adenine dinucleotide), H3PO(phosphoric acid) and the enzyme phosphoglyceraldehyde dehydrogenase, 3 phosphoglyceraldehyde is oxidized to 1, 3-bishosphoglyceric acid and NADH2.

iamge of Steps of Glycolysis

Steps of  Glycolysis

6. In this step 1, 3 bishosphoglyceric acid transfers phosphoric acid to ADP with the formation of 3 phosphoglyceric acid and ATP with the help of enzyme phosphoglyceric acid kinase.

7. In the next step, 3 phosphoglyceric acid is converted to 2 phosphoglyceric acid with the help of enzyme phosphoglyceromutase.

8. 2 phosphoglyceric acid is then converted to form 2 phosphoenol pyruvic acid with the help of enzyme enolase which gives out one molecule of water.

9. It is the last step of glycolysis where 2 phosphoenol pyruvic acid is converted to form pyruvic acid by the removal of phosphorus thus one molecule of ATP is synthesized from ADP. The enzyme catalyzing this step is pyruvic acid kinase.

So in the overall process,  two molecules of pyruvic acid is formed from each molecule of glucose. In animals including human being, glycogen is present in the muscle and liver cells, are phosphorylated by the glycogen phosphorylase enzyme in presence of inorganic phosphate into glucose 1 phosphate. Similarly starch of plant cells is converted to glucose 1-phosphate by the starch phosphorylase. Glucose 1-phospahte is then converted to glucose 6-phosphate with the help of enzyme phosphoglucomutase. Glucose 6-phosphate is then oxidized through the glycolytic path.

Reaction:

Thus, when one molecule of glucose (6C) undergoes the reactions in glycolysis, the overall process may be represented as follows:

In this case, 2 molecules of ATP are used up in the phase of glycolysis.

Therefore the net gains of glycolysis are:

You may also read: Fermentation Vs Respiration

Significance of Glycolysis

  • This process leads to the formation of 2 molecules of pyruvic acid which is essential for running the Krebs cycle.
  • During the course of formation of 2 molecules of pyruvic acid from a hexose 4 ATP molecules are produced as a result of substrate phosphorylation per molecule of hexose utilized. Now 2 ATP molecules are used up in the formation of hexose phosphate, so the phosphate energy of 2 other ATP molecules are used up in other general cellular purpose.
  • Dehydoxy acetone phosphate is formed as a byproduct in glycolysis which helps in the formation of glycerol which ultimately leads to fat metabolism. 
  • The different intermediary products are produced in glycolysis which can be used by the plants in different metabolic functions of the body.

Role of Plant Hormones in Agriculture and Horticulture

There are various synthetic organic compounds which are applied to plants to give some positive responses. These compounds are artificial plant hormones which help in a deficient concentration (< 0.001 M) in maintaining of growth and development of the plants. These are now widely used to get a better result in the field of agriculture and horticulture. Some of the uses of natural and synthetic hormones in the above areas are given below:

Effects on Vegetative Plant Structures

Role in the Rooting of Cuttings: There are some plants like China rose (Hibiscus), rose (Rosa), etc., generally reproduce vegetatively. They are usually propagated by cutting pieces of stem. When the cut stem piece is placed in the moist stand, then adventitious roots come out from the cut end.

The rooting of cutting now a day can be hastened by pre-treating the cuttings with powders or solution containing synthetic hormones like Indole acetic acid (IAA), Indole butyric acid (IBA), Naphthalene acetic acid (NAA), etc. By the help of such cuttings, a large number of identical plants may be raised from a single individual plant for the preservation of desired genetic pattern, generation after generation.

Role in Controlling Cambial Activity: The secondary growth in thickness of stems and roots in woody plants take place due to the activity of cambium. The primary cambium (fascicular cambium) and secondary cambium (interfascicular cambium) together form the cambium ring. The activity of this is higher in spring month; then it gradually declines and become the lowest in summer. As a consequence, the formation of annual rings occurs.

This rhythmic activity of cambium is closely linked with the hormone Indole acetic acid (IAA) which stimulates meristematic activity in the cambial tissue.

Role in Callus Formation and Healing of Wounds: Cambium performs another important function by forming callus or wound tissues for the healing of wounds. There is a possibility of infection by some pathogenic organisms in any types of injuries, especially during pruning of plants.

For protection against wounds in the exposed area, some substances come out of the wounded cells causing uninjured cells to become meristematic resulting in the formation of callus or wound tissue, whereby healing of wounds takes place. For the formation of callus, hormones like auxin (Indole acetic acid) are found to be very effective.

Role in weed Control: Unwanted plants are called weeds which can be effectively controlled by the application of synthetic hormones. Weed killing hormones are known as herbicides. One of the ideal weed killers is the herbicide 2, 4-dichlorophernoxy acetic acid (2,4-D).

Other herbicides sometimes used are 2-methyl,4-chlorophenoxy acetic acid(MCPA) and 2,4,5-trichlorophemnoxy acetic acid(2,4,5-T). The most effective way of this herbicide is used in an aqueous spray on the foliage. In this case, 1% concentration for 5 gallons per 1000 sq feet is used. 

Role in the Prevention of Sprouting of Potato Tubers: The modified, underground stem is the potato tuber which breaks dormancy at an early period causing quick loss of weight and decrease in the starch content. This creates a great problem for the agriculturists.

For overcome this difficulty now a day, synthetic hormones are applied. In this case, potato tubers are dipping in indole acetic acid solution, sprouting, i.e., bud formation of potatoes can be inhibited. Another synthetic hormone, such as methyl a-naphthalene acetate, is also effective in preventing but formation.

Effect on the Reproductive Structures of Plants

Control of Floral Initiation: Control of floral initiation is a process which is controlled by the hormone.   In many cases, naphthalene acetic acid (NAA) is used to enhance the vegetative bud of pineapple, causing the induction of flower bud.  2, 4-D also can induce flowering. Gibberellins have also been shown to initiate flower formation in some plants like Hyoscyamus and Samolus (Lang, 1956).

Control of Fruit Development (Parthenocarpic Fruit Formation): The formation of parthenocarpic fruits such as seedless fruits can be induced by the application of IAA and IBA (Gustafson, 1936) in plants like squash, tomato, pepper, Petunia, etc. Naphthalene acetic acid has also been found suitable for the development of seedless fruits in strawberry.

Role in the Control of Abscission Layer: In most species of plants a time comes when the shedding of leaves and flowers and dropping of fruits take place from the stem. The process of their removal from the plant is called abscission which generally takes place by the formation of abscission layer,

The abscission layer in the form of a thin plate of cells is formed at the base of the leaf petiole or fruit stall. The cells of this layer become softened and so weaker than they readily break from the plants by wind or by any other mechanical means.

Abscission can be controlled by means of the hormone. It has been observed that when auxin production diminishes then the abscission layer forms. It has been shown that by application of auxin, abscission layer formation is delayed. Pre-harvest drop of fruits (apple, orange, peaches, plums, etc.) causes a severe reduction in the yield of fruits.

Synthetic hormones like 2,4-D, NAA or naphthalene acetamide is used to prevent premature drop of fruits before they are ready for picking from the plants. It is an economically important aspect of horticulture.

Role in Thinning of Blossom and Control of Fruit Production: Plant hormones are used effectively in thinning of blossom and control of fruit production. In this case, hormones reduce fruit set by removing some of the flowers.

Role in the Fruit Growth and Maturation: Plant hormones like Indole butyric acid (IBA) can be effectively applied in the control of fruit size and their maturation, for example, tomato.  

Cranial Nerves: Nature, Origin and Distribution, Functions

The nerves of the brain which directly emerge from or enter the skull or the cranium including the brainstem are called cranium nerves. They transmit or relay various information between the different parts of the body and the brain. The main functions include special senses of vision, smell, taste, and hearing. They come out from CNS (central nervous system) above the level of the first vertebrae of the vertebral column.  

There are twelve pairs of cranial nerves in mammals including human, each of which is denoted by Roman Number. Each cranial nerve is situated on both sides. Among 12 pairs, some are purely sensory, some are purely motor and the others are mixed. The first two pairs emerge from the cerebrum; the remaining ten pairs emerge from the medulla obolongata.

image of Cranial nerves

Image Showing Cranial Nerves: Image credit-Wikimedia commons

The 12 pairs cranial nerves are shown in the following table:

Serial No

Name of Cranial Nerves

I

The olfactory nerve

II

The Optic Nerve

III

The Oculomotor Nerve

IV

The  Trochlear Nerve

V

The Trigeminal Nerve

VI

The Abducens Nerve

VII

The Facial Nerve

VIII

The Auditory Nerve

IX

The Glossopharyngeal Nerve

X

The Vagus Nerve

XI

The Accessory Nerve

XII

The Hypoglossal Nerve

Origin of the Cranial Nerves

The first two cranial nerves such as olfactory nerve I and the optic nerve II originate from the cerebrum while the cranial nerves III – XII arise from the brain stem.

12 pairs of cranial nerves, their nature, distribution and functions are described below:

I. Olfactory

Nature: Pure Sensory nerve

Origin and Distribution: It is originated from the olfactory lobe of cerebrum and extended to the mucus membrane of the nazal cavity. It is capable of regeneration.

Functions: It transmits the sense of smell from the nasal cavity.

II. Optic

Nature: Sensory nerve

Origin and Distribution: It is originated from the optic lobe of the midbrain and extended to the retinal wall of the eye.

Functions: It carries visual information from the retina of the eye to the brain.

III. Oculomotor

Nature: Motor nerve

Origin and Distribution: It is originated from the ventro-lateral side of the midbrain and innervates the eye muscles.

Functions: It controls the movements of eye muscles. It also helps the constriction of the pupil, and regulates an open eyelid

IV. Trochlear

Nature: Motor nerve

Origin and Distribution: It is originated from the dorso-lateral side of the midbrain and innervates the eye muscles.

Functions: It controls the rotational movements of eye muscles.

V. Trigeminal

Origin and Distribution: It is originated from lateral side of the medulla obolongata and divides into the following three branches and extended to the different organs.

(i) Opthalmic: It innervates the eye lid and the mucus membrane of the nazal cavity. It is sensory in nature.

Functions: It transmits senses from the eye-lid and the mucus membrane to the brain.

(II) Maxillary: It is extended to eye-lid, upper and lower jaws. It is sensory in nature.

Functions: It transmits senses from the eye-lid, upper and lower jaws to the brain.

(III) Mandibular: It is extended to the the muscles of the ventral buccal cavity. It is a mixed type nerve.

Functions: It controls the movement of lower jaw and transmits heat, pressure and touch senses to the brain.

VI. Abducens

Origin and Distribution: It is originated from lateral side of the medulla obolongata and innervates the lateral rectal eye muscle.

Nature: It is mixed type nerve.

Functions: It is responsible for lateral movement of the eye.

VII. Facial

Origin and Distribution: It is originated from the lateral side of the medulla obolongata and having two branches for different organs.

(i) Palatine: It is extended to the roof of the buccal cavity.

Nature: It is sensory in nature.

Functions: It is responsible for taste of food.

(ii) Hyomandibular: It is extended to the buccal cavity and lower jaw.

Nature: It is mixed in nature.

Functions: It is responsible for the taste of food and controls nick membrane.

VIII. Auditory/ Vestibulocochlear

Nature: Sensory nerve

Origin and Distribution: It is originated from lateral side of the medulla obolongata and extended to the inner ear.

Functions: It is responsible for hearing and maintains balance of the body.

IX. Glossopharyngeal

Nature: It is mixed nerve

Origin and Distribution: It is originated from the lateral side of the medulla obolongata and extended to the tongue and pharynx.

Functions: It is responsible for taste and movment of the pharynx.

X. Vagus

Origin and Distribution:It is originated from the lateral side of the medulla obolongata and it divides into the following four branches:

(i) Laryngeal: It is extended to the larynx.

Nature: It is mixed in nature.

Functions: It regulates the activities of the larynx.

(ii) Cardiac: It is extended to the heart.

Nature: It is mixed in nature.

Functions: It regulates the activities of the heart.

(iii) Gastric: It is extended to the wall of the stomach.

Nature: It is mixed in nature.

Functions: It regulates the activities of the stomach.

(iv) Pulmonary: It is extended to the lungs.

Nature: It is mixed in nature.

Functions: It regulates the activities of the lungs.

XI. Spinal Accessory

Nature: Motor nerve

Origin and Distribution: It is originated from the floor of the medulla obolongata and extended to the pharynx, larynx and neck.

Functions: It maintains the muscle movement of related organ such as shoulder and neck.

XII. Hypoglossal

Origin and Distribution: It is originated from the lateral side of the medulla obolongata and innervates the tongue and neck.

Nature: Motor nerve

Functions: It regulates the movement of the tongue.

Spermatogenesis Vs Oogenesis

Gametes are produced by the animals through meiosis cell division from diploid mother cells (gonads). There are two types of gonads such as testis in males and ovaries in females. The process of gamete production (ovum and sperm) is known as gemetogenesis (Gr. Gamos=marriage; genesis=origin).

Testes produce male gametes or sperms through the process of spermatogenesis while ovaries produce ovum or female gamete through the process of oogenesis.

In this case, sperms and ova are produced from the germinal epithelial cells of the testes and ovaries of sexually matured male and females, respectively. 

Spermatogenesis

Males can produce sperm when they reach puberty at the age between 10-16 years old. Approximately 200 million sperms produce in a day. These sperms happen in the seminiferous tubules of the testes of male.  In this case, the seminiferous tubules are separated by the blood-testis barrier from the systematic circulation. 

The spermatogenesis is a process to produce sperms which occurs in the male gonads or testes. The human testes consist of many seminiferous tubules which are lined by the cells of germinal epithelium.

This germinal epithelium plays an important role to produce sperms through the process of spermatogenesis. The germinal cell also contains some somatic cells, known as sertoli cells which have a role in nourishing the developing spermatozoa or sperms.

image of Spermatogenesis

Image Showing Spermatogenesis Process: Image credit-wikimedia commons

The spermatogenesis is a continuous process and it can be described in four different headings:

  • 1. Multiplication Phase
  • 2. Growth Phase
  • 3. Maturation Phase and
  • 4. Spermiogenesis

Multiplication Phase

The male germinal cells are also referred to as the primordial cells or primary germinal cells and they can produce the sperms. These primordial cells reproduce by repetitive mitosis cell division and produce the diploid (2n) cells, known as spermotagonia.

There are two types of spermatogonia such as A1 spermatogonia and B type spermatogonium. In this case, B type spermatogonium finally forms mature sperm.

Growth Phase

In this phase, B type spermotogonial cells gather huge quantity of chromatin substance and nutrition. They multiply several times by mitosis and form identical diploid (2n) cells. These cells are now known as primary spermatocytes.

Maturation Phase

In this phase, each primary spermatocytes then undergoes meiosis-I and produces two haploid (n) cells, known as secondary spermatocytes. Each secondary spermatocyte goes through the Meiosis-II and produces four haploid (n) cells, known as spermatids.

As a result, four haploid spermatids are produced from a diploid (2n) spermatogonium through a meiosis or maturation division. Spermatids have unflagelatted and round shaped-body which form motile haploid (n) sperms or spermatozoa through maturation. 

Spermiogenesis

Spermiogenesis is the maturation process of the spermatids into sperm cells through the complex process. During this phase, the following modification occurs in the spermatids:

  • The non-motile, round, more cytoplasmic spermatids transformed into motile, elongated, less cytoplasmic sperms.
  • The nucleus of spermatid shrinks and discards water; RNA and nucleolus form the head of the sperm.
  • To form acrosome of the sperm, the golgi apparatus comes together near the anterior end of the sperm nucleus.
  • The mitochondria become spiral to form the middle part of the sperm.
  • The two centriols of the spermatids form the axial filament and tail of the sperms.
  • The entire process of spermatogensis takes about 70 days to form sperm from spermatogonium.
  • The sperms are stored and become functionally mature in the epididymis.
  • 200-300 million sperms are produced from one ejaculation, of which 40% are motile and 60% non-motile.

Difference Between Spermatogenesis and Spermiogenesis

Spermatogenesis

Spermiogenesis

Motile spermatozoa are formed from spermatogonium through the process of spermatogenesis.

Mature spermatozoa are produced from spermatids through the process of spermiogenesis. 

A spermatogonium produces four functional spermatozoa through the spermatogenesis.

A spermatid can produce one spermatozoon through the spermiogenesis.

To complete the spermatogenesis process, it includes the following phases: multiplication phase, growth phase, maturation phase, and a differentiation phase.

It includes only differentiation process.

It is the complete process to produce mature sperm cells from the primordial germ cells.

It is the part of spermatogenesis where mature spermatozoa are produced.

In this process, haploid (n) gametes are produced from diploid (2n) germ cells.

It happens in the haploid (n) cells and no changes occur in the amount of genetic material through this process.

In this case, growth and divisions occur.

Growth and division do not occur.

Oogenesis

Oogenesis is the process for producing ovum. It occurs in the cells of the germinal epithelium of the ovary. These cells are known as primary germinal cells or primordium cells. The oogenesis process is completed in the following four successive stages:

Multiplication Phase

The primordial cells or primary germinal epithelium multiply by the repeated mitosis cell division and produce many diploid (2n) oogonia(singlular: oogonium).

Growth Phase

In this phase, oogonia receive nutrition and become massive large in size. During this period, synthesis of different cellular organelles happens. Along with the growth in size, the nuclei of the oogonia undergo first prophase of the meiosis cell division.

In this stage, egg cells are known as primary oocyte. Oocyte development takes place within follicles.

image of Oogenesis

Image Showing Ogenesis: Image credit-​Wikimedia

Maturation Phase

The maturation phase is accomplished by the maruration or meiotic division. After the meiosis-I, the primary oocyte divides unequally to produces a large-sized haploid (n) secondary oocyte and a small-sized haploid(n) first polar body.

The secondary oocyte passes through the metaphase stage of meiosis-II to forms a large mature ootid and a small polar body.

The first polar body also divides into two secondary polar bodies through the meiosis-II. As a result, a diploid primary oocyte produces one large haploid (n) ootid and three small haploid (n) polar bodies through maturation phase.

Metamorphosis Phase

In this phase, the ootid is modified into a mature ovum. But in this case, no huge changes occur as like in spermatid. A few changes held in the internal component for fertilization.

The polar bodies ooze out from the egg and degenerate while the haploid (n) non-motile, large and spherical ovum ready for fertilization.

In the ovary of a female foetus, 7 million primary oocytes forms, but at the time of birth they reduce to 2-4 millions. 40,000 primary oocytes left out at puberty.  Approximately 400 ova are produced throughout the female life.

Difference between Oogenesis and Spermatogenesis

Oogenesis

Spermatogenesis

It is the process for production of sperms from spermatogonia. 

It is the process for production of ova from oogonia. 

This process happens in the ovaries of the females.

It happens in the testes of the males.

It starts during fetal development.

It starts at puberty.

Maximum stages occur in the ovary and last few in oviduct.

All stages occur in testes.

It is an irregular process and occurs after the puberty until menopause.

It is a nonstop process and occurs after the puberty till death.

At menopause, fertility stops.

Fertility does not stop but it reduces in aged.

Each meiosis cell division produces one gamete.

Each meiosis cell division produces four gametes.

In this process, non-motile gametes are produced.

Motile gametes are produced.

In this case, unequal cytokinesis occurs. 

In this process, equal cytokinesis occurs.

One gamete or ovum is  produced.

Four male gametes or sperms are produced.

Four polar bodies are formed.

No polar bodies are produced.

Gamete produces in monthly cycle.

Gamete produces in any time.

In this process, growth phase is long.

Growth phase is too short.

Ovum holds lot of food reserve.

Sperm hold less food reserve

Roles of Hormone in Human Body Growth

There are many hormones in the human body. Among them growth hormone and the thyroid hormones play an important role in the growth of the body. Growth hormones are secreted from the hypophysis or pituitary gland, also known as master gland while the thyroid hormones are produced from the thyroid gland. 

Growth Hormones and its Role in Human Body Growth

Growth hormone is a protein molecule that contains 191 amino acids in a single chain and has a molecular weight of 22.005. It is also called somatotropic hormone or somatotropin. This hormone causes growth of almost all tissues of the body that are capable of growing; hence it is called growth hormone. It helps to promote the increasing height and weight of the body.

It performs to increase the growth of the body in the following ways:

Growth of Skeletal System

Growth hormone stimulates the cartilage and bone growth. It happens due to multiple effects of growth hormone on bone:

  • Growth hormone helps to increase deposit of protein by chondrocytic and osteogenic cells that cause bone growth;
  • It increases the  rate of reproduction of chondrocytic and osteogenic cells and
  • It also helps to convert chondrocytes into osteogenic cells, thus causing deposit of new bone.

Metabolism

Growth hormone performs some specific metabolic effects, including:

  • It increases the rate of protein synthesis in most cells of the body;
  • It helps to activate the fatty acids from adipose tissue;
  • It boost up the free fatty acids in the blood;
  • It helps to decrease the rate of glucose utilization all over the body;
  • It helps to utilize of fatty acids for producing energy;
  • Growth hormone influences carbohydrate metabolism by decreasing glucose uptake in tissues and increasing the production of glucose by the liver.
  • Growth hormone also increases the secretion of insulin to maintain glucose level in the body.

Increasing Ionic Levels

Growth hormone influences to increase all the electrolytes in the body and helps to increase the body growth. Growth hormone influences absorption of Ca++ from the gut and reabsorption of different electrolytes in the kidney.

Increasing the Size of the Organ

Growth hormone stimulates the growth of muscles and other soft organs except the brain by promoting protein synthesis.

In Milk production

Growth hormone stimulates the mammary gland and increases milk production which helps in physical growth of infants.

Production of RBC

Growth hormone stimulates the erythropoesis process to production of more RBC in the body.

As Medicine

Growth hormone is used in medicine to treat children`s growth disorders and adult growth hormone deficiency. In recent years, growth hormone replacement therapies have become popular in the battle against aging and obesity.

Growth hormone also performs the following functions:

  • Growth hormone stimulates collagen synthesis in skeletal tendons and muscles; as result, muscle strength is increased;
  • Growth hormone is essential for regulating growth of the bone and helps to increase the height of the children.
  • Growth hormone helps to promote the breakdown of the fat in the body. In this way, it helps to fat people to lose weight. 
  • Growth hormone helps to decrease the risk of cardiovascular disease. Deficiency of growth hormone can change the metabolism of lipoprotein and raise the risk of cardiovascular disease.
  • Growth hormone helps in male reproductive function and sexual maturity by improving erectile dysfunction. If the body does not produce normal level of growth hormone, loss of sexual desire and erectile dysfunction occur.
  • Growth hormone helps in emotional and mental health. In many cases, adults suffer from depression due to deficiency of growth hormone. In this case hormone therapy can improve the mood and cognition function.
  • Growth hormone stimulates the renewal of neurons, endothelial cells and oligodendrocytes. It also helps in the formation of myelin sheath and dendritic diversity.
  • Growth hormone controls the immune function and also helps to increase thymocyte.

Besides, abnormal secretion of growth hormone leads to various abnormalities such as:

Dwarfism: Pituitary dwarfs are produced due to lack of Growth hormone since infancy.

Gigantism: Gigants are produced due to increased secretion of growth hormone before the union of epiphysis of long bones.

Acromegaly: Prolonged excess growth hormone thickens the bones of the jaw, fingers and toes. As a result, heaviness of the jaw and increased size of digits is referred to as acromegaly.

Role of Thyroid Hormones in Growth

Thyroid hormones are secreted from thyroid gland and play a great role for general growth of the body. They perform to influence the body growth in the following ways:

  • Thyroid hormones stimulate the secretion of growth hormone and potentiate the action of growth hormone.
  • They cause to increase of synthesis of structural proteins; as a result, increases the growth of the body
  • They are essential for differential and maturation of different tissues. The ossification of bones does occur without proper secretion of thyroid hormones and the man fails to develop into an adult.
  • They enhance the metabolic rate of all ttypes food, hence helps in body growth.
  • They are required for development and maturation of neurons as well as myelinogenesis.
  • They are responsible for growth and maintenance of the skeletal muscles.
  • They are essential for maturation and differentiation of cartilages; they help in fusion of epiphysis and growth of bones in length and girth.
  • They are essential for maintaining the normal functions of the digestive system.

They perform some other functions which have direct or indirect effects on human growth as:

  • They stimulate erythropoisis;
  • They increase milk production;
  • They maintain normal reproductive function;
  • List Element
  • They help to influence some other hormone for human growth;

Besides, over secretion of thyroid hormone cause hyperthyroidism, as a result, the goiter occurs. Insufficient secretion of thyroid hormones cause hypothyroidism, as a result, cretinism occurs in the babies and myxodema disease occurs in adult.

Endocrine Glands: Hormones, Functions and Disorders

There are two types of glands in the body such as endocrine glands and exocrine glands. Endocrine glands are also known as ductless glands that constitute the endocrine system. The endocrine system forms network in the body and produce hormones directly into the blood stream. Endocrine glands perform various functions in the body. They influence the metabolism, reproduction, growth and so on. If endocrine gland does not work properly, it may cause various problems in your body.

In human body, the major endocrine glands include:

The endocrine glands, hormones and their major functions are described in the following table:​

Name of Hormones

Endocrine Glands

Functions

Growth Hormone Releasing Hormone (GHRH).

Hypothalamus

It helps to release of Growth Hormone (GH).

Thyrotrophin Releasing Hormone(TRH) 

Hypothalamus

It helps to release of Thyroid Stimulating Hormone (TSH) and release of prolactin (PRL).

Corticotrophin Releasing Hormone(CRH)

Hypothalamus

It helps to release of Adreno Corticotrophic Hormone (ACTH).

Gonadotrophin Releasing Hormone(GnRH)

Hypothalamus

It helps to release of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH).

Somatostanin(SS)

Hypothalamus

It inhibits to secrete of growth hormone(GH)

Dopamine (DA)

Hypothalamus

It inhibits to secrete of prolactin  (PRL).

Human Growth Hormone(HGH) or Somatotropic Hormone(STH)

Pituitary Gland

It controls growth of tissue, bone, muscles and internal organs.

It influences metabolic process.

 It also breaks down stored glycogen from the muscle and liver.

Thyroid Stimulating Hormone(TSH)

Pituitary Gland

It stimulates the secretion of thyroxin from the thyroid glands.

It controls intake of iodine by the thyroid gland.

Adreno Corticotrophic Hormone(ACTH)

Pituitary Gland

It stimulates the adrenal cortex and regulates the secretion from the adrenal glands.

It also influences the production of melanocyte.

Luteinizing Hormone(LH)

Pituitary Gland

It stimulates the testes to secrete testosterone or endrogene hormone.

It is responsible for formation of the corpus luteum, production of progesterone in females.

Follicle Stimulating Hormone(FSH)

Pituitary Gland

It influences the production of sperm by the seminiferous tubules of the testes in males.

It controls the development of grafian follicles(ova) in females.

Luteotrophic Hormone(LTH)

Pituitary Gland

It stimulates the development of breast and lactation in females.

It influences the development of secondary sexual characteristics in males and females.

Melanocyte Stimulating Hormone(MSH)

Pituitary Gland

It controls the pigments in melanocytes and thus determines skin color.

Anti Diuretic Hormone(ADH) or Vassopressin

Pituitary Gland

It increases the blood pressure.

It stimulates the kidney to reabsorb more water, preventing excessive water loss by urination.

Oxytocin

Pituitary Gland

It causes contraction of the uterus in females during childbirth and regulates lactation.

Thyroxine(T4)

Thyroid Gland

It increases metabolism of iodine, proteins and carbohydrates.

It increases cardiac output and heart rate.

It increases milk production.

Tri-iodothyronine (T3)

Thyroid Gland

It increases metabolic rate of iodine.

It increases the sensitivity of nervous system.

Thyrocalcitocin

Thyroid Gland

It increases calcium absorption capability of bones.

It influences the metabolism and transportation of phosphate.

Parathyroid hormone (it is also known as parathormone or parathyrin)

Parathyroid Gland

It breaks down the bone and causing calcium release.

It increases the ability of the body to absorb calcium from food.

It also increases the ability of kidney to held on calcium.

Thymosin

Thymus gland

It influences the formation of lymphocytes and antibody.

It helps in deposition of minerals in the bones.

Glucocorticoids or cortisol

Adrenal gland

It influences the synthesis of glycogen in muscle and liver.

It increases absorption rate of glucose and lipids from the intestine.

Mineralocorticoids or aldosterone

Adrenal gland

It increases the NaCl and water absorption capacity of kidney.

It increases the plasma content of the blood.

It also increases the excretion rate of potassium (K).

Gonadocorticoids

Adrenal gland

It regulates the sex differentiation of the embryo.

It helps to develop of sex glands, gonads and secondary sex characteristics.

Epinephrine or Adrenaline

Adrenal gland

It increases the heart rate, blood pressure and carbohydrate metabolism rate.

It decreases the urine production rate.

Norepinephrine or Noradrenaline

Adrenal gland

It maintains a constant blood pressure by stimulating certain blood vessels to constrict when the blood pressure falls bellow normal

Insulin

Pancreas

It increases the metabolic rate of glucose to decrease it levels in the blood.

It increases the glycogen synthesis rate of liver and muscle.

Glucagon

Pancreas

It increases the activities of α-cells and β-cells.

It decreases the secretion rate of digestive gland.

Somatostanin

Pancreas

It inhibits the activities of α-cells and β-cells.

Gastrin

Gastrointestinal glands

It helps to secrate the pepsinogen and HCl.

Secretin

Gastrointestinal glands

It stimulates the pancreas and bile ducts to release sodium bicarbonate to neutralize the acid.

Cholecystokinin (CCK)

Gastrointestinal glands

It stimulates the release of digestive enzymes in the pancreas.

It also stimulates the contraction of the gall bladder to empty bile into the duodenum.

Gastric inhibitory peptide(GIP)

Gastrointestinal glands

It decreases the stomach contractions churning the chime, and slows the emptying of the stomach into the duodenum.

Enterocrinin

Gastrointestinal glands

It stimulates the intestine to secrete its own intestinal juice.

Peptide YY (PYY) or it also known as peptide tyrosine tyrosine

Gastrointestinal glands

It helps to slow down the passage of food along the gut.

Enterogastrone

Gastrointestinal glands

It inhibits the motility and acid secretion of stomach.

Melatonin

Pineal gland

It plays an important role to regulate sleep patterns.

It regulates the body's daily (circadian) clock.

Testosterone

Testis

It helps in maturation and functional maintenance of male reproductive tract.

It helps in the development of secondary sex characters.

It influences sex behavior.

Estrogen

Ovary

It helps in maturation and cyclic fluctuation of female reproductive tract.

It helps in the development of secondary sex characters.

It stimulates duct system of mammary gland.

It influences sex behavior.

Progesterone

Ovary

It regulates the female tract with the help of estrogen.

It develops alveolar system of mammary gland.

It helps to prepare uterus for implantation of blastocyst.

It helps in maintenance of blastocyst.

Relaxin

Ovary

It helps to maintain cervical tone.

It facilitates parturition.

Some Endocrine System Disorders

Sometimes, endocrine glands produce too high or too low levels of hormones that have number of effects on our health. There are many conditions that occur due to cause of endocrine glands disorders. Among them some important disorders are described below briefly:

Cushing's disease

This disease occurs if endocrine gland produces high level of hormone, cortisol. It shows the following symptoms:

  • Increasing body weight;
  • Fatty deposits occur in the face, shoulders;
  • Weakening of muscles and bones;
  • Irregular of periods in females;
  • Purple stretch marks on the arms, thighs, and abdomen;
  • Skin becomes thin that bruises easily;
  • High blood pressure;
  • Red cheeks;
  • Growth of excess hair on the face, neck, chest, thighs and abdomen;
  • Healing occurs slowly if cuts, scrapes, insect bites and infection;
  • Decreasing sex drive and fertility in males.
  • Disorders of mood and behavior;
  • Severe fatigue;
  • Headache;

Hyperthyroidism

Hyperthyroidism occurs if thyroid gland secretes more hormones than necessary. Some general symptoms of hyperthyroidism include:

  • Loss of weight;
  • Rapid heartbeat;
  • Nervousness, anxiety and irritability;
  • It makes sleeping problem;
  • Increasing appetite;
  • Changing menstrual cycles;
  • Fatigue;
  • Weakness of muscles;
  • Increasing sensitivity to heat;
  • Thinning of skin;
  • Sweating;
  • Diarrhea, etc.

Hypothyroidism

It occurs due to secrete less thyroid hormone. Some general symptoms of Hypothyroidism include:

  • Increasing the body weight;
  • Fatigue;
  • Dry and rough pale skin;
  • Weakness;
  • Loss of memory;
  • Dry and coarse hair;
  • Constipation;
  • Loss of hair;
  • Slow heart rate;
  • Irregular menstrual cycle;
  • Depression;
  • Much sensitivity to cold;
  • Swelling of thyroid gland;

Addison Disease

It is also known as adrenal insufficiency. It occurs if adrenal gland produce insufficient cortisol and aldosterone. All age groups and both sexes suffer from addison`s diseases ant it can be life-threatening. Addison`s diseases show some common symptoms which include:

  • Weakness of muscle;
  • Severe fatigue;
  • Loss of weight;
  • Decreasing appetite;
  • Low blood pressure;
  • Abdominal pain;
  • Skin darkness;
  • Low blood sugar level;
  • Diarrhea or vomiting;
  • Decreasing heart rate;
  • Hair loss;
  • Lesion in the mouth;
  • Irritability;
  • Depression;
  • Energy loss;
  • Irregular menstrual cycle;
  • Sleep disturbances, etc;

Acromegaly

It happens if pituitary gland produces too much growth hormone (GH). Generally, a middle-aged person suffers from acromegaly disease. As a result, bones of hand feet and face increase in size. Some common symptoms of acromegaly include:

  • Enlargement of feet and hands;
  • Enlargement of facial features;
  • Weakness of muscles;
  • Tiredness;
  • Enlargement of vocal cords and sinuses;
  • Thickened oily skin;
  • Headache;
  • Enlargement of tongue;
  • Too much sweating;
  • Weakening of vision;
  • Irregular menstrual cycle;
  • Sexual dysfunction; etc.;

Diabetes

It happens due to high blood sugar level. It occurs if pancreas does not produce necessary amount of insulin.  Some common symptoms of diabetes include:

  • Weight loss;
  • Tiredness;
  • Increasing thirst or hunger;
  • Frequent urge to urinate;
  • Irritability, etc.

Thyroid Gland: Hormones, Functions and Disorders

The thyroid gland is a butterfly-shaped endocrine gland. It is brownish-red that rich in blood vessels. Vital nerves that are responsible for voice quality also pass through the thyroid gland.

The thyroid gland is found in all vertebrate, but they are quite variable to shape and anatomical position. In some lower vertebrates, only thyroid follicles are present.

In human, the thyroid gland consists of two lobes that lie on either side of the roof of the trachea and are usually connected by a thin isthmus extending over the anterior surface of the second, third and fourth tracheal rings.

Two layers of fibrous connective tissues cover the thyroid. The gland is about two inches (5 cm) long. The weight of the normal thyroid gland of the adult people ranges from 20-60 gm. The gland is actually situated in front of the throat just below the thyroid cartilage, known as the Adam`s Apple.

The thyroid consists of two fairly symmetrical lateral lobes. Each lobe measures about 5 x 2 x 2 cm in size. The two lobes are connected by a strip of tissue, known as the isthmus. Each lobe consists of many follicles of variable size. Each follicle is lined by a single layer of granular cubical epithelium and filled with protein material, called colloid. Thyroid follicles contain a lesser number of high cubical mitochondria rich-cells. These are called parafollicular cells.

Hormones of Thyroid Gland

The thyroid gland contains two types of cells found in the thyroid follicles and secretes the following three types of hormones:

Thyroxine: It is also known as T4, which is secreted by principal cubical epithelial cells.

Tri-iodothyronine: It is also known as T3, which is also produced by principal cubical epithelial cells.

Thyrocalcitonin: It is produced by the parafollicular cells.

Thyroxine and Tri-iodothyronine

These two hormones are iodinated derivatives of tyrosin (amino acid). They are generally referred to as thyroid hormones. Thyroid gland performs the following functions through these two hormones. It has been observed that tri-iodothyronine is more active than thyroxin.

Functions of Thyroid Hormones

  • Thyroid hormones are known as calorigenic hormones because they increase oxygen uptake and metabolism by the tissues and thus accelerate energy production.
  • They increase the sugar level in the blood. In a low dose, it causes protein synthesis, but in a high dose, it depresses protein synthesis.
  • Thyroxine decreases serum cholesterol and phospholipids.
  • Thyroxine helps in brain development, bone health and muscle control.
  • Thyroid hormones help in the skeletal, muscular, sexual and mental growth.
  • Thyroxine increases heart rate, cardiac output and blood pressure. It also dilates peripheral vessels.
  • Thyroid hormones increase the utilization of O2 and the formation of CO2. These effects increase the rate and depth of respiration.
  • Thyroxine increases absorption of foods, secretion of digestive juices and movement of the gastrointestinal tract.
  • Thyroid hormones help in the development of RBC. Hypofunctions of the thyroid cause anaemia.
  • By the calorigenic effect of thyroxine, body temperature increases, and thus it regulates body temperature.
  • It regulates central and peripheral nervous systems.
  • It controls the menstrual cycle in the female.
  • It also helps in metamorphosis. For example, the tadpole larva develops into a toad by the activities of the thyroid gland.

Hypofunction of Thyroid Gland or Hypothyroidism

Less secretion of thyroid gland produces cretinism in infancy or childhood and myxoedema in adults.

Cretinism

It is the condition in children due to severe thyroid deficiency. It is also known as congenital iodine deficiency syndrome. It hampers both mental and physical development, if untreated. It mainly occurs due to dietary iodine deficiency. Many people suffer from this congenital iodine deficiency syndrome around the world. It is a significant public health problem in many countries of the world. Many developed countries already have eliminated through iodine supplementation of food.

The most characteristic features of this defect are:

  • Stunted growth,
  • Goitre,
  • Inadequate length in infants,
  • Deformed bone and teeth,
  • Reduced adult stature,
  • Hair loss,
  • Rough, dry, wrinkled skin,
  • Bloated idiotic-look face,
  • Protruding large tongue and umbilicus,
  • Pot-bellied abdomen,
  • Impeded ovulation,
  • Sex organs and sex glands degenerate,
  • Mental deterioration,
  • Sex characters are retarded,
  • Neurological impairment,

Myxedema

This disease generally occurs more in adult female than male due to decrease or absence of thyroid hormone. It is also known as severely advanced hypothyroidism. It happens when the body does not make sufficient thyroid hormone. If untreated or undiagnosed, it causes severe advanced hypothyroidism. Critically advanced hypothyroidism can lead to Myxedema crisis, a life-threatening situation.

Causes of Myxedema (hypothyroidism)

  • Removal of the thyroid gland due to surgical operation,
  • Medication due to cancer treatments,
  • Radiation therapy can cause decreasing hormone production,
  • Iodine deficiency or an excess of iodine,
  • Hashimoto’s disease,
  • Sudden illness or infection,
  • Pregnancy, etc

Symptoms of Myxedema

  • The face appears as swollen puffy oedematous and thus exhibits Mongoloid appearance,
  • The thick dry skin, hairs falling from the axilla, pubis and head,
  • Swelling of the tongue and larynx, etc.,
  • Degeneration of sex dullness,
  • Loss of memory,
  • Slowing of heart rate,
  • Low blood pressure, etc.,
  • Sudden illness, like a heart attack or stroke,
  • Trauma
  • Stress,
  • Exposure to cold,
  • Low blood oxygen levels,
  • Decreasing breathing,
  • Decreasing blood sodium levels,
  • Shock,
  • Increasing CO2 levels in the blood,
  • Decreasing body temperature,
  • Confusion or mental slowness,
  • Constipation,
  • Weight gain,
  • Drooping eye,
  • Coma,

Hyperfunctions of Thyroid or Hyperthyroidism

This clinical syndrome due to hypersecretion of the thyroid gland is known as exophthalmic goitre or Graves disease.

Symptoms

  • The enlargement of the thyroid gland (swelling of neck),
  • Protrusion of eyeball occurs with less twinkling of eyelids. It is due to deposition of fat in the retro-ocular region.
  • The decrease in body weight,
  • Soft moist and flushed skin,
  • Increasing heart rate, cardiac output,
  • Rising of blood sugar,
  • A sharp mental condition, etc.

Thyrocalcitonin

It is also known as calcitonin. It is a protein hormone secreted from the parafollicular cells or C-cells (mitochondria-rich cells) of the thyroid gland. It is also secreted from the cells of the glandular ultimobranchial bodies in fishes, birds, and other non-mammalian vertebrates.

Functions of Thyrocalcitonin

  • It helps to rise of calcium level in the blood.
  • It lowers the level of blood calcium and phosphate.
  • It promotes the formation of bone.

Concluding Remarks

The thyroid gland is a very important endocrine gland that is situated in your neck region. It produces three hormones such as thyroxine (T4), tri-iodothyronine (T3) and thyrocalcitonin. Among them, T3 and T4 are the most important hormones which play an important role to do normal function in your body.

There are many thyroid disorders due to abnormal secretion from the thyroid gland. These disorders occur in babies, children, teenagers and adults. It is estimated that about one in 20 people suffer from some kind of thyroid disorder. If untreated or undiagnosed, it can lead to a severe, life-threatening situation.

Adrenal Gland: Different Parts, Hormones and Functions

The adrenal gland is one kind of endocrine gland, which is also known as a suprarenal gland. It is two in number that is placed on the upper pole of each kidney at the level of 1st lumbar vertebra. The right adrenal gland of humans is pyramidal in shape, while the left adrenal gland is crescent-shaped and slightly larger than the right adrenal.

Each gland measures about 3 cm in width, 5 cm in length and about 1 cm in thickness, which is surrounded by the fibrous capsule. It surrounds the kidney. In the adult human body, the combined weight of the two glands ranges from 7-10 grams. It is yellowish in color. The adrenal gland consists of two parts, the outer cortex and inner medulla.

image of Adrenal gland and its various layers

Image Showing Adrenal Gland and Its Various Parts

Adrenal Cortex

It is the outermost layer of the adrenal gland. This gland produces some essential hormones such as aldosterone, cortisol and androgens. The cortex consists of polyhedral epithelioid cells which are arranged in three layers:

1. Zona Glomerulosa: It is the outermost zones which are located just under the fibrous capsule of the gland. This outer layer produces a mineralocorticoid hormone, such as aldosterone. Generally, it is secreted by the activity of the aldosterone synthetase enzyme. This hormone helps to regulate long term blood pressure.

2. Zona Fasiculata: It is the middle layer which is located between the zona reticularis and the zona glomerulosa. This layer is the largest layer among three layers, which accounts for about 80% of the cortex. Cells in this layer are arranged in columns and contain lots of lipid droplets, smooth endoplasmic reticulum (ER) and mitochondria. Glucocorticoids such as cortisol hormones are produced from this layer.

3. Zona Reticularis: It is the inner irregular network rows of the cell layer. It is placed directly adjacent to the madula. This layer contains small cells with a small amount of cytoplasm and lipid droplets. It also contains some lipofusion pigment. It produces hormones like androgens.

image of Adrenal gland and its various layers

Image Showing Zones of Adrenal Cortex

Hormones of Adrenal Cortex

A number of cortical hormones are secreted from the cortex. They are grouped into three types:

Glucocorticoids: These groups of hormones are mainly secreted from zona fasiculata. They include cortisol, cortisone, corticosterone.

Minerelocorticoids: This group of hormones is secreted mainly from the zona glomerulosa. They include aldosterone and deoxycorticosterone (DOC).

Sex steroids: This group of hormones is secreted mainly from zona reticularies. They include androgen, estrogen, progesterone, etc.

The Functions of Adrenal Cortex​​​​

Effect on Carbohydrate Metabolism: Glucocorticoids help for the metabolism of carbohydrates as follows:

  • It increases the formation of glycogen in the liver and muscles.
  • It helps to increase the formation of glucose from protein and fat in the liver.
  • It also helps to increase glucose absorption from the intestine.
  • It helps to decrease glucose uptake by the tissues.
  • It helps to increase the sugar level in the blood.

Effect on Minerals and Water Metabolism: Mineralocorticoids increase reabsorption of NaCl, bicarbonates, and water. It depresses of potassium and phosphate reabsorption by the renal tubules.

Effect on Protein Metabolism: Glucocorticoids break down tissue protein into amino acids; thus, body proteins are lost, and nitrogen excretion increases.

Effect on Fat Metabolism: Glucocorticoids increase the fat absorption from the intestine. They also increase lipid and cholesterol levels in the blood and decrease the synthesis of lipids from carbohydrates.

Effect on different System: Glucocorticoids decrease the number of eosinophils and lymphocytes in the blood. They regulate the composition of blood, blood volume, blood pressure, etc. It also controls bone formation, muscular activities, the function of the nervous system and the digestion system.

Adrenal Medulla

The vital part of the adrenal gland is known as the medulla, which is surrounded by the adrenal cortex. It consists of densely packed polyhedral cells that are surrounded by blood tissue. These cells are known as chromaffin cells. The adrenal medulla produces catecholamine adrenaline and noradrenaline hormones. 

This organ can produce about 80% adrenaline or epinephrine and 20% noradrenaline or norepinephrine. These are protein hormones. Epinephrine (adrenaline) and nor-epinephrine together called catecholamines that are produced from the adrenal medulla and nerve endings such as sympathetic and parasympathetic nerves.

Functions of Epinephrine

  • Epinephrine helps to increase conductivity, contractility, heart rate and cardiac output, etc.
  • It helps to constrict all blood vessels, but it dilates coronary vessels and vessels of skeletal muscles
  • It helps to increase the blood pressure and heart rate through the constriction of blood vessels.
  • Epinephrine can cause dilation of bronchioles and increase the rate and depth of respiration.
  • It helps to increase the excitability, contractibility and tone of skeletal muscle. It also causes a delay in fatigue.
  • It helps to inhibit the tone of the involuntary muscles, which are found in the stomach, intestine and bronchiole.
  • It also helps to inhibit intestinal movement and cause the construction of spleen, dilatation of the pupil, etc.
  • It constricts the erector pilorum (muscle of skin), which causes the erection of hairs.
  • It constricts cutaneous vessels, thus decreases the flow of blood, which prevents loss of body temperature.
  • It increases the renal circulation; as a result, decrease the urine volume.
  • It helps to increase the blood sugar by stimulating the breakdown of liver glycogen, the formation of glucose from lactic acid, etc.
  • It also stimulates the secretion of saliva, lacrymal secretion, sweat secretion, etc.

Functions of Nor-epinephrine

Nor-epinephrine is also known as noradrenaline. Chemically, it is catecholamine and phenethylamine. It acts as a chemical messenger and transmits signals across nerve endings. It releases higher levels during the situation of stress or danger. Generally, it secretes the lowest amount during sleep and increases during wakefulness.

  • It encourages vigilance and enhances restlessness and anxiety.
  • It can cause an increase in blood pressure and heart rate.
  • It causes the release of glucose and increases blood flow to skeletal muscles.
  • It also decreases the level of blood flow to the gastrointestinal system.
  • It helps to increase tear production and make the eye moist.
  • It increases the number of blood pumps in the heart.
  • It increases blood pressure through the constriction of blood vessels.
  • It helps to release of rennin in the kidney and maintain the level of sodium in the bloodstream.
  • It helps to generate body temperature.
  • It helps to increase glucose production in the liver, which helps to increase energy sources.
  • It helps to increase glucose uptake in the skeletal muscle.
  • It helps to increase glucagon in the pancreas, which increases glucose production by the liver.
  • It helps to increase the fat conversion to the substrate in the adipose tissue. These substrates use as an energy source by various tissues and muscles.
  • It deduces the digestive activity in the stomach and intestine by the inhibitory effect of this hormone.
  • It increases the level of circulating free fatty acids.
  • Clinically, norepinephrine is used to maintain blood pressure in certain types of shocks.

Disorders and Diseases of the Adrenal Glands

There are multiple disorders and diseases caused by the adrenal glands when it does not work precisely. The following most common disorders and diseases of the adrenal glands are:

Addison’s Disease

It is caused due to hyposecretion or insufficient secretion of cortisol hormone.

Symptoms

  • Pigmentation in the skin.
  • Loss of muscular weakness.
  • Loss of appetite.
  • Vomiting.
  • Diarrhea.
  • Fall of blood pressure.
  • Hypoglycemia.
  • Nausea
  • Reduction of fluid and blood volume.
  • Low body temperature
  • Renal insufficiency.
  • Fatigue;
  • Abdominal pain;
  • Weight loss;
  • Dizziness upon standing;
  • Depression of sexual activity, etc.

Over production or hyper-functions of adrenal cortex causes the following diseases:

Cushing`s Syndrome

It is caused by over secretion of cortisol hormone. There are the following many symptoms of cushing`s syndrome:

  • Moon face or rounded face due to deposition of fat on nose;
  • Fish-like mouth with buffalo neck;
  • Excessive hair growth in male;
  • Masculinisation with growth of beard and moustache in female;
  • Wasting of muscle;
  • Osteoporosis of bones;
  • Hypertension;
  • Hyperglycemia;
  • Increasing of Na+ and decreasing of K+ in plasma;
  • Mental derangement, etc.

Adrenogenital Syndrome

It is caused due to the excessive secretion of adrenal androgen. It shows the following symptoms:

Symptoms of Adrenogenital Syndrome in Female: 

  • Deepening voice;
  • Growth of beard,
  • Increasing growth of muscle, etc;

Symptoms of Adrenogenital Syndrome in Male

  • Feminization;
  • Development of breast;
  • Atrophy of testis, etc.

Comparison Among Auxin, Gibberellin and Cytokinin

The following table shows the comparison among Auxin, Gibberellin and Cytokinin:

Auxin

Gibberellin

Cytokinin

It is an organic acid: Indole radicle-Auxin a(auxenotriolic acid): C18H32O5, auxin b(auxenolonic acid): C18H30O4 and hetero auxin(indole acetic acid): C10H9O2N.

It is also organic acid-5-ringed diterpenoids. Its chemical formula is C19H22O6 (GA3).

It is also an organic alkali: 6-furfuryl amino purine similar to adenine. Its chemical formula is C10H9N5O.

It acts as both nitrogen free and nitrogenous compound.

Here nitrogen free compounds are auxin a  and auxin b while hetero auxin is nitrogenous compound.

It is nitrogen free compounds.

It is nitrogenous compound.

It is produced in apical meristem of growing region of plants.

It is originated from germinating plants and cotyledons.

It is originated from endosperm and meristem.

Auxin is transported basipetally, i.e., polar transport.

It is transported in all directions.

It is transported in all direction or may be active at the site of synthesis.

Auxin plays an active role in cell division.

It does not perform in cell division.

It helps to encourage cell division.

It initiates adventitious root production at the cut ends of branches.

It controls adventitious root production at the cut ends of branches.

It initiates adventitious root production at the cut ends of branches.

It stops premature leaf fall.

Gibberellin does take part to stop premature leaf fall.

It does take part to stop premature leaf fall.

It controls tropic movements such as phototropism and geotropism.

It does not control tropic movement.

It does not take part in tropic movement.

It also plays an important role in apical dominance.

It does not take part in apical dominance.

Apical dominance can be concentrated.

It helps to produce seedless fruits.

It helps to removes the dormancy of seed.

It stimulates the germination of seeds.

Auxin

Auxin

Gibberellin

Gibberellin

Zeatin

Corn Cytokinin(Zeatin)

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