Pituitary gland originates from the ectoderm of the embryo.
Location. The pituitary gland is a small, red-grey, of pea-shaped gland attached to the hypothalamus of the brain by a stalk or infundibulum in front of the pons.
Structure of pituitary gland
The pituitary gland consists of three lobes : anterior lobe or pars anterior, also called adenohypophysis, intermediate lobe or pars intermedia, and posterior lobe or pars nervosa, also termed neurohypophysis. It has a dual origin but is entirely ectodermal. The anterior and intermediate lobes develop as an out-growth from the roof of the stomodaeum, whereas the posterior lobe arises as a downgrowth from the floor of the diencephalon. The anterior and posterior lobes are connected with the hypothalamus by hypothalamo-hypophysial portal system and by axons of hypothalamic neurons respectively.
All the three lobes of the pituitary secrete separate hormones.
A. Anterior Lobe of Pituitary.
Anterior lobe of pituitary produces six hormones .
(i) Follicle-stimulating Hormone (FSH).
It stimulates sperm formation in the male and growth of ovarian follicles in the female . In older persons, of it helps to maintain sexual activity.
(ii) Luteinsing Hormone (LH).
In the male, it induces the interstitial cells of the testes to produce male sex hormones named androgens such as testosterone. This hormone makes the male genital system to become fullgrown and functional. In the female, the luteinising hormone causes ovulation, secretion of female sex hormone, estrogen from the maturing ovarian follicle, and progesterone by the corpus luteum formed in the empty ovarian follicle.
Follicle-stimulating hormone and luteinising hormone are together referred to as gonadotrophic hormones (GTHS) or gonadotrophins.
(ii) Thyroid-Stimulating Hormone (TSH) or Thyrotrophic Hormone or Thyrotrophin.
It stimulates growth of thyroid and production of thyroid hor- mones.
(iv) Adrenocorticotrophic Hormone (ACTH) or Adrenocorticotrophin.
It stimulates the adrenal cortex to grow and secrete its glucocorticoid and mineralocorticoid hormones.
(v) Somatotrophic or Growth Hormone (STH or GH) or Somatotrophin.
Somatotrophin (Gr.soma = body ; trophe= nourishment) stimulates growth and development of all tissues by accelerating protein synthesis and cell division, and by retaining calcium in the body. GH enables the cells to take up more amino acids and mobilize fat, and make the liver to release glucose for energy supply.
Disorders. Improper secretion of GH hormone produces three important disorders.
(a) Dwarfism. It is caused by deficiency of after growth hormone from early age. Growth of long breast-feeding, bones and of the body stops prematurely, making the patient dwarf. The pituitary dwarfs produced by the deficiency of growth hormone differ from those resulting from the deficiency of thyroid hormone in produced normal intelligence.
(b) Gigantism. It is caused by excess of growth hormone from childhood. The patient grows into a pituitary giand with abnormal height and very long bones.
(c) Acromegaly. It is caused by excess of growth hormone after adolescence. The bones of the lower jaw and limbs (arms, hands, legs) become abnormally large but the body does not attain a giant stature.
(vi) Prolactin Hormone (PH) or Luteotrophic Hormone (LTH).
It stimulates the growth of milk glands during pregnancy and the secretion of milk after delivery, In males and in women who are not breast-feeding, a prolactin-inhibiting hormone produced in the hypothalamus suppresses prolactin synthesis in the anterior pituitary. The inhibition is removed in nursing mothers by nerve impulses produced when the infant sucks on the nipples.
B. Intermediate Lobe of Pituitary.
It secretes growth a hormone named melanocyte-stimulating hormone (MSH)*. It stimulates the synthesis of black pigment melanin in the skin, and also causes dispersal of melanin granules in the pigment cells, thereby darkening the colour in certain animals (fishes ; amphibians). In man it has no such role.
C. Posterior Lobe of Pituitary.
It stores and releases two hormones : oxytocin and vasopressin. These hormones are actually produced by the neurosecretory cells in the hypothalamus and stored in the terminals of their axons that pass into the posterior lobe through a stalk. They are released via posterior lobe when required.
(i) Oxytocin (OT).
It is released into the blood when the hypothalamic neurons are stimulated by widening of uterus at the time of delivery or by sucking of the breast by the infant. It induces contractions of smooth muscles –
(a) of the uterus during the birth of the young one, and
(b) myoepithelial cells of the mammary glands to cause release of milk during sucking by the infant. Because of its role, oxytocin is called “birth hormone” and “milk ejecting hormone”.
It is also called antidiuretic hormone (ADH). It decreases the loss of water in the urine by increasing the reabsorption of water in the distal convoluted tubules, collecting tubules and collecting ducts in the kidneys.
Vasopressin also stimulates the contraction of smooth muscles of the arterioles, thereby enhancing the arterial blood pressure, hence its name vasopressin.
Disorder. Deficiency of ADH reduces reabsorption of water and increases urine output, causing excessive thirst. This disorder is called diabetes insipidus. No glucose is lost in the urine of such a patient.
Interaction of Endocrine Glands.
The pituitary was earlier called “Master Endocrine Gland” because of the number of hormones it produces and the control it exercises over other endocrine glands. However, it itself is under the control of the hormones secreted by the hypothalamus of the brain. Thus, there is a chain of orders : the hypothalamus directs the pituitary output, which controls the secretion of hormones by other endocrine glands.