Here is an essay on the ‘Hormones Secreted by Pituitary Glands’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on the ‘Hormones Secreted by Pituitary Gland’ for school and college students.
Hormones Secreted by Pituitary Gland
- Essay on Thyrotropic Hormone
- Essay on Adrenocorticotropic Hormone
- Essay on Gonadotropic Hormones
- Essay on Lactogenic Hormone or Prolactin
Essay # 1. Thyrotropic Hormone:
(Thyrotropin or thyroid stimulation hormone —TSH)
This is a glycoprotein, with molecular weight of about 25,000 and is produced by the basophils.
In hypo-active gland (Dwarfism) the thyroid is ill-developed while in hyperactivity (Acromegaly or Gigantism), the thyroid becomes hyperactive and even may lead to Grave’s disease.
Mode of Action:
It stimulates the thyroid activity in all respects. It increases the quantity of intracellular colloid in the thyroid epitheloid cells and induces the liberation of thyroxine from thyroglobulin. It also increases the height of the follicle cells of thyroid, their ratio of iodine intake and hormone synthesis.
TSH also exerts influence on the swelling of the orbital contents of the eye, including the muscles, by water uptake and fat accumulation. Thus, exophthalmos resembling Grave’s disease is the consequence of hyperactivity of TSH.
Control of Secretion:
Secretion of thyrotrophin is controlled by hypothalamus and the level of circulating thyroxine. Cold promotes secretion of thyroxine through TSH release. However, this stimulus is ineffective in hypophysectomized animals or if the pituitary stalk is sectioned. Therefore, it may be assumed that this pathway includes pituitary stimulation by a chemical released into the portal vessels in hypothalamus, this substance being TRH.
Electrical stimulation of anterior hypothalamus increases TSH secretion and TRH appears in portal blood. The thyrotrophin releasing hormone stimulates the thyrotroph cells in anterior pituitary after attaching to the cell membrane by activating adenyl cyclase and promotes synthesis and secretion of TSH.
TSH secretion is also under the feedback control of thyroxine. Thyroxine depresses TSH secretion. Thyroidectomy removes this negative feedback control and results in drastic increase in TSH level of blood. In the adenohypophysis many mucoprotein containing cells proliferate. Which are the source of TSH.
Essay # 2. Adrenocorticotropic Hormone (ACTH):
Adrenocorticotrophic hormone is a polypeptide consisting of 39 aminoacid residues. The molecular weight is about 4500. The first 20 aminoacids from free NH2 terminal end possess biological activity. ACTH can withstand temperature of 100°C.
Mode of Action:
Adrenocorticotrophin is necessary for normal growth, development and maintenance of the adrenal cortex, which degenerates rapidly after removal of pituitary gland. The rate of production and secretion of adrenal steroid hormones in the zona fasciculata of the adrenal cortex is controlled by ACTH. ACTH attaches to specific receptors on the surface of the cells and causes a rapid increase in glucocorticoid synthesis.
ACTH promotes permeability of the membrane of adrenal cortical cells to certain specific ions, and adenyl cyclase is activated leading to the production of cAMP. Protein kinase activated by cAMP phosphorylates two proteins. The first one acts as a lipase and mobilized cholesterol from its bound form in the fat droplets while the second protein promotes the conversion of cholesterol to pregnenolone.
Control of Secretion:
The hypothalamus primarily controls the synthesis and release of ACTH by adenohypophysis through the corficotrophin releasing factor (CRF). CRF is synthesized and stored, and is released whenever required to activate anterior pituitary. Hypothalamus is also the area where feedback receptors, sensitive to corticoids and ACTH are located.
Essay # 3. Gonadotropic Hormones (Gonadotropins):
There are two gonadotropins controlling gonads:
(i) Follicle stimulating hormone (FSH or Prolan A),
(ii) Luteinizing hormone (LH or Prolan B or ICSH — interstitial cell stimulating hormone).
Function of FSH:
In females, it increases the number and size of Graffian follicles and also induces the secretion of oestrin. In males, it is believed to promote spermatogenesis. Since this hormone influences both male and female gametes, it is also called the gameto-kinetic factor.
Functions of LH:
In females, it causes appearance, growth and persistence of corpus luteum and also induces the secretion of progesterone. In males, it stimulates the interstitial cells of the testes to secrete testosterone.
That the adenophysis controls the gonads is evident by the following:
(i) Hypophysectomy in children (Dwarfism) retards sexual development and in adults leads to impotency.
(ii) If adenophysis be grafted in young animals; precocious sexual development takes place.
Both the hormones are glycoproteins and are believed to be produced by the pale basophil cells of the pars distalis. The molecular weight ranges from 30,000 to 100,000. Both contain hexose, hexosamine and sialic acid. The ICSH in sheep has a molecular weight of 40,000 and contains 4.5% mannose and 5.8% hexosamine.
Control of Secretion of Gonadotropins:
(a) Nervous control:
The control of the secretion of the gonadotropins by nervous system is supported by the following:
(i) In the rabbit, ovulation normally takes place after 12-18 hours of mating, in the presence of intact pituitary.
(ii) Ovulation can be induced by electrical stimulation of the hypothalmic region.
It is indicated above that the act of mating in female rabbit generates afferent impulses which are carried to the central nervous system and reflexly stimulate adenohypophysis. However, the exact nerve path is not known but hypothalamus seems to be involved.
(b) Hormonal Control:
The concentration of sex hormones in the blood also determines the secretion of gonadotropins. A high concentration of them inhibits, whereas low concentration stimulates the secretion of gonadotropins from adenohypophysis. Injections of estrogens and androgens for long period cause degeneration of anterior pituitary.
Essay # 4. Lactogenic Hormone or Prolactin (Galactin or Lactotrophic Hormone):
This is a protein and is secreted by the red acidophils of the pars distalis. It has a molecular weight of about 25,000.
It is responsible for the maintenance of corpus luteum and continued production of progesterone in female mammals. It also supplements the actions of the gonadal hormones in effecting mammary gland development, thereafter being essential for normal lactation. Hypophysectomy in lactating animals stops lactation which restarts after injecting the adenophysis extract.
Mode of Action:
(a) It chiefly stimulates the secretion of milk by developed mammary glands.
(b) It stimulates the proliferation of glandular elements of the mammary glands during pregnancy and thus helps to complete the development of the breasts.
(c) It is responsible for the maternal instincts of the mammals and nesting instinct in fowl. Prolactin stimulates the enlargement and functioning of the crop glands in the pigeons (formation of “crop milk”).
(d) It helps to maintain the secretory activity of corpus luteum and secretion of progesterone by the luteal tissue.
Control of Secretion:
The sex hormones and the placental gonadotropins inhibit the secretion of prolactin as is evident by the following:
(i) Although breasts are fully developed, yet lactation does not occur during pregnancy. It starts only after parturition and expulsion of placenta.
(ii) Prolactin secretion is inhibited by the injections of the estrogen and progesterone.
(iii) The injections of the placental gonadotropins inhibit the secretion of prolactin.
Thus, the sex hormones and the placental gonadotropins in the blood control and the secretion of prolactin hormone is absent after expulsion of placenta; hence, lactation starts.
Mammary teats sucking by the baby generates afferent impulses which reflexly stimulate the secretion of prolactin through hypothalamus.