In this article we will discuss about:- 1. Origin of Pituitary Glands 2. Location and Structure of Pituitary Glands 3. Disorders.
Origin of Pituitary Glands:
It develops from ectoderm of the embryo.
Location and Structure of Pituitary Glands:
The pituitary gland is located just below the hypothalamus. The pituitary gland is situated in a depression the sella turcica of sphenoid bone of the skull. The pituitary gland is the smallest endocrine gland.
It is about 1.3 cm in diameter and weighs about half a gram. The gland is attached to the brain by a stalk the infundibulum which is continuous with the hypothalamus above. The pituitary gland is formed of two main lobes of different origin.
These lobes are the much larger anterior lobe or adenohypophysis or pars distalis and the smaller posterior lobe or neurohypophysis or pars nervosa. Adenohypophysis originates as Rathke’s pouch from dorsal wall of stomodaeum in the embryo, but later its connection with the stomodaeum disappears.
The neurohypophysis originates as an outgrowth from the floor of the diencephalon. Thus the pituitary gland is dual in origin (from stomodaeum – foregut and diencephalon). A third lobe, called the intermediate lobe or pars inter-media is a part of adenohypophysis.
Adenohypophysis comprises about 75% part of the pituitary gland. The hypophysial portal veins carry blood containing neurohormones (releasing factors) from the hypothalamus to the adenohypophysis. Neurohypophysis comprises about 25% part of the pituitary gland. The axons of neurosecretory cells (secretory neurons) extend into the neurohypophysis where these axons terminate as axon terminals.
These terminals are embedded in a neuroglial tissue formed of large and branched cells called pituicytes. Obviously no hormones are synthesized in neurohypophysis but two hormones, synthesized in the hypothalamic neurosecretory cells remain stored in very small vesicles in the axons and terminals. These vesicles are called Herring bodies.
(a) Hormones of the anterior lobe:
The anterior lobe of the pituitary gland secretes the following hormones, most of them are trophic hormones.
(i) Growth hormone (STH or GH) or Somatotrophin (Soma— body, trophe— nourishment):
This hormone stimulates growth. Growth hormone promotes protein anabolism, the absorption of calcium from the bowel and the conversion of glycogen to glucose.
Target Cells:
Various body cells which undergo growth.
(ii) Thyroid stimulating hormone (TSH) or Thyrotropin:
This hormone controls the growth and activity of the thyroid gland. It influences the uptake of iodine, the synthesis of the hormones, thyroxine and tri-iodothyronine by the thyroid gland and the release of stored hormones into the blood stream.
Target Cells:
Cells of thyroid.
(iii) Adrenocorticotropic hormone (ACTH):
This hormone stimulates the cortex of the adrenal gland to produce its hormones.
Target Cells:
Cells of adrenal cortex.
(iv) Prolactin hormone (PRL) or Mammotropin hormone (MTH) or Luteotropic hormone (LTH):
Prolactin is also called the “hormone of maternity” because its main physiological effect is to activate growth of breasts during pregnancy and secretion of mammary glands after child birth. The name luteotrophic hormone (LTH) refers to because it also stimulates the corpus luteum of the ovary to secrete progesterone hormone.
Target Cells:
Cells of mammary glands.
(v) Gonadotropic hormones:
These are as follows:
Follicle-stimulating hormone (FSH):
It stimulates growth of ovarian follicles and their secretion of oestrogens in the female, and spermatogenesis (formation of sperms) in the male.
(b) Luteinizing hormone (LH):
In female it stimulates the corpus luteum of the ovary to secrete progesterone. In male it activates the Leydig’s (interstitial) cells of the test is to secrete androgenes hence it may be called interstitial cell stimulating hormone (ICSH) in male.
Target Cells. Cells of gonads (testes and ovaries)
(b) Hormone of the Intermediate lobe:
The intermediate lobe of the pituitary gland secretes melanotrophin or melanocyte stimulating hormone (MSH). This hormone causes dispersal of pigment granules in the pigment cells, thereby darkening the colour in certain animals like fishes and amphibians. It is believed that it is associated with the growth and development of melanocytes in man which give colour to the skin.
Target Cells. Melanocytes in skin.
(c) Hormones of the Posterior lobe:
The secretion of the posterior lobe is known as pituitrin and it contains two hormones:
(i) Oxytocin or pitocin.
(ii) Antidiuretic hormone (ADH) or vasopressin. Once again it is reminded that the posterior lobe of the pituitary gland does not secrete any hormone. Its hormones are secreted in the hypothalamus.
(i) Oxytocin (ОТ):
Oxytocin promotes contraction of the uterine muscle and contraction of the myoepithelial cells of the lactating breast, squeezing milk into the large ducts behind the nipple. In late pregnancy the uterus becomes very sensitive to oxytocin.
The amount secreted is increased just before and during labour and by sucking of the baby. Because of its role, oxytocin is called “birth hormone” and “milk ejecting hormone”. Milkmen inject synthetic oxytocin, called pitocin, into their cows and the buffaloes to get more milk.
Target Cells:
Cells of mammary glands.
(ii) Antidiuretic hormone (ADH) or Vasopressin or Pitressin:
This hormone has two main functions:
(a) Antidiuretic effect:
It increases the reabsorption of water in the distal convoluted tubule and collecting ducts of the nephrons of the kidneys. As a result, the reabsorption of water from the glomerular filtrate is increased,
(b) Pressor effect:
Involuntary muscles in the walls of the intestine, gall bladder, urinary bladder and blood vessels are stimulated to contract by ADH. Contraction of the walls of the blood vessels raises the blood pressure and this may be its most important pressor effect.
Target Cells:
Cells of kidneys.
The pituitary gland is also called “Master Endocrine Gland” of body or the “Chief Executive of Endocrine System” or “The Leader of Endocrine Orchestra” as it secretes the number of hormones (e.g., TSH, ACTH etc.) which regulate the working of other endocrine glands.
But it is not proper to call it as master endocrine gland because it is itself under the control of the releasing hormones secreted by the hypothalamus of the brain. Thus the hypothalamus is, in fact, the supreme commander of endocrine regulation.
Pituitary Disorders:
(a) Pituitary Dwarfism:
It is caused by the deficiency of growth hormones (GH) from childhood. It is characterised by small but well proportioned body and sexual immaturity. The dwarfs produced by the deficiency of growth hormone are different from those which are formed from the deficiency of thyroid hormone in having normal intelligence.
(b) Gigantism:
It is caused by excess of growth hormone from early age. It is characterised
by large and well proportioned body. If size of pituitary gland increases, it affects (suppresses) optic chiasma and ultimately affects vision.
(c) Acromegaly (Aero- extremity, megaly- large):
It is caused by excess of growth hormone after adult size is reached. It is characterised by disproportionate increase in size of bones of face, hands and feet.
(d) Diabetes Insipidus:
It is caused by the deficiency of ADH. It is characterised by excessive dilute urine.
(e) Simmonds’ Disease:
Cause — atrophy or degeneration of anterior lobe of pituitary gland. Symptoms— the skin of face becomes dry and wrinkled, premature ageing.
(f) High Blood Level of ADH:
It is caused by excessive secretion of ADH. It is characterised by excessively dilute blood and low plasma sodium.
