Briefly mention the mechanism of action of FSH.

1. Endocrine Glands and Hormones:

(i) There are two coordinating, integrating and interdependent systems in our body namely the nervous system and endocrine system.

(ii) The endocrine system achieves coordination and integration by transmitting information through hormones also known as chemical messengers.

(iii) Endocrinology is the study of endocrine glands and hormones.

(iv) The term hormone was coined by Starling in $1905$. The endocrine system consists of hypothalamus, pituitary, pineal, thyroid, parathyroid, adrenal, pancreas, thymus and gonads (testis and ovary).

(v) Some other organs, e.g., the gastrointestinal tract, kidney, heart, etc., also produce hormones.

(vi) Prostaglandins are long-chain unsaturated fatty acids that regulate vasoconstriction, vasodilation, contraction of uterine muscles, etc.

(vii) Pheromones are molecules used for chemical communication between individual animals.

2. Pituitary Gland:

(i) Anterior pituitary is controlled by the hypothalamus, which secretes both releasing and release-inhibiting neurohormones.

(ii) Pituitary gland has been nicknamed the ‘master gland’ or ‘bandmaster of the endocrine orchestra’. The pituitary gland is divided into three major parts: pars distalis, pars intermedia, and pars nervosa.

(iii) Pars distalis produces six trophic hormones.

(iv) Pars intermedia secretes only one hormone, while pars nervosa (neurohypophysis) secretes two hormones.

3. Pineal Gland:

(i) The pineal gland (epiphysis) secretes melatonin, which plays a very important role in the regulation of $24$ hour (diurnal) rhythms of our body (e.g., rhythms of sleep and state of being awake, body temperature, etc.).

(ii) The pineal gland also secretes serotonin. Serotonin ($5$-Hydroxytryptamine) acts as a vasoconstrictor, increasing blood pressure, whereas adrenoglomerulotropin stimulates the zona glomerulosa of the adrenal cortex to secrete aldosterone.

4. Thyroid Gland:

(i) The thyroid gland hormones play an important role in the regulation of the basal metabolic rate, development, and maturation of the central neural system, erythropoiesis, metabolism of carbohydrates, proteins, and fats, and menstrual cycle.

(ii) Another thyroid hormone, i.e., thyrocalcitonin secreted by C-cells regulates calcium levels in our blood by decreasing it.

5. Parathyroid Glands:

The parathyroid glands secrete parathyroid hormone (PTH, Collip’s hormone) which increases the blood $C{a}^{2+}$ levels and plays a major role in calcium homeostasis.

6. Thymus:

(i) The thymus gland secretes thymosin which plays a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity.

(ii) Thymus is active in young ones but gradually becomes inconspicuous after sexual maturity.

7. Adrenal Glands:

(i) Adrenal glands are paired endocrine glands located above the kidney, called suprarenal glands.

(ii) Adrenal glands are composed of the outer part adrenal cortex and the inner part adrenal medulla.

(iii) The adrenal cortex secretes mineralocorticoids, glucocorticoids and sex corticoids.

(iv) The principal mineralocorticoid is aldosterone. Two basic functions of aldosterone are the conservation of sodium and water and the elimination of potassium.

(v) The main glucocorticoids are cortisol (hydrocortisone) and corticosterone. Glucocorticoids stimulate gluconeogenesis, lipolysis, proteolysis, erythropoiesis, cardiovascular system, blood pressure, and glomerular filtration rate and inhibit inflammatory reactions by suppressing the immune response.

(vi) The adrenal medulla secretes epinephrine and norepinephrine. These hormones increase alertness, pupillary dilation, sweating, heartbeat, rate of respiration.

(vii) Epinephrine (adrenaline) contributes to ‘fight or flight’ response which occurs in condition of emergency. Adrenaline is also known as the ‘emergency hormone’.

8. Pancreas:

(i) The endocrine portion of pancreas consists of clusters of cells called Islets of Langerhans, which secrete glucagon and insulin.

(ii) Glucagon stimulates glycogenolysis and gluconeogenesis resulting in hyperglycemia.

(iii) Insulin stimulates cellular glucose uptake and utilisation and glycogenesis resulting in hypoglycemia.

(iv) Insulin deficiency and/or insulin resistance results in a disease called diabetes mellitus.

9. Gonads- Testes and Ovaries:

(i) The main function of gonads is to produce gametes, but they also secrete sex hormones.

(ii) The testis secretes androgens, which stimulate the development, maturation and functions of the male accessory sex organs, appearance of the male secondary sex characters, spermatogenesis, male sexual behaviour, anabolic pathways and erythropoiesis.

(iii) The ovary secretes oestrogen and progesterone.

(iv) Oestrogen stimulates growth and development of female accessory sex organs and secondary sex characters.

(v) Progesterone plays a major role in the maintenance of pregnancy as well as in mammary gland development and lactation.

10. Hormones of Heart, Kidney and Gastrointestinal Tract:

(i) The atrial wall of the heart produces atrial natriuretic peptide (ANP), a polypeptide with $28$ amino acids which decreases the blood pressure.

(ii) Kidney produces erythropoietin which stimulates erythropoiesis.

(iii) A temporary structure with endocrine function is placenta which produces many hormones like chorionic gonadotropin, placental lactogen, estrogen, progesterone and relaxin.

(iv) The presence of hCG (human chorionic gonadotropin) in urine forms the basis of all pregnancy diagnosis tests.

11. Mechanism of Hormone Action:

(i) Hormones act through two kinds of receptors: membrane-bound receptors and nuclear membrane receptors.

(ii) Specificity of hormone action is based on cell receptors. Hormones bind with receptors to form hormone receptors complex.

(iii) Amino-acid derivatives and peptide hormones have receptors attached to the cell membrane. 

(iv) Steroid and thyroid hormones have intracellular receptors.

(v) Hormones that interact with membrane-bound receptors normally do not enter the target cell but generate second messengers, which regulate cellular metabolism.