Introduction

An endocrine gland, pituitary (hypophysis), lies within the pituitary (hypophyseal) fossa located at the base of the brain in the sphenoid bone (in the middle cranial fossa). From the inferior surface of the hypothalamus, it is seen as a protrusion resting on a small, bony cavity, known as sella turcica.

It is of the size of a pea and weighs nearly 0.5gm. Its diameter is 1-1.5cm (0.5 inch). A stalk, the infundibulum, and a funnel attach it to the hypothalamus.

Since the pituitary gland secretes hormones which control the other endocrine glands, it was named the master endocrine gland.

Structure

The hypothalamic hormones are produced in the cell bodies or axons, and move to the cells of anterior pituitary gland via the hypophyseal portal system. The posterior pituitary is under the direct neural regulation of hypothalamus.

Anatomically and functionally, pituitary gland is divided into two separate lobes:

1) Anterior Pituitary (Adenohypophysis): It accounts for 75% of the total weight of the gland. In adults, it comprises of the following two parts:

i) Pars Distalia: It constitutes the larger portion and produces growth hormone, prolactin, thyroid stimulating hormone, adrenocorticotropic hormone.

ii) Pars Tubercles: It forms a sheath around the infundibulum. It secretes only melanocyte stimulating hormone

iii) Pars Intermedia: It is the 3rd region of pituitary gland, and is present between pars distalia and pars tubercles. It degenerates during human fontal development, thus failing to appear as a separate lobe in adults. Some of its cells still persist by migrating into the adjacent parts of anterior pituitary.

2) Posterior Pituitary (Neurohypophysis): It forms the posterior part of pituitary gland and consists of the par’s nervosa and infundibulum. The posterior pituitary is made up of axons and axon terminals of more than 10,000 neurons. The cell bodies of these neurons are found in the supraoptic and paraventricular nuclei of hypothalamus. These axon terminals are linked to pituicytes (a specialized form of neuroglia).

This part stores and releases two hormones, namely oxytocin and vasopressin, which are actually synthesized by the hypothalamus and are transported via axons to the neurohypophysis.

Hormones

The pituitary gland secretes different hormones:

1) Anterior Pituitary: The hormones secreted by this part include:

i) Growth Hormone (GH),

ii) Thyroid Stimulating Hormone (TSH),

iii) Adrenocorticotropic Hormone (ACTH),

iv) Prolactin (PRL),

v) Gonadotropins

a) Follicle-Stimulating Hormone (FSH), and

b) Luteinizing Hormone (LH)

vi) Melanocyte Stimulating Hormone (MSH).

2) Posterior Pituitary: The hormones secreted by this part include:

i) Antidiuretic Hormone (ADH, or vasopressin), and

ii) Oxytocin.

Growth Hormone (GH)

Growth hormone is a peptide hormone stimulating growth, cell reproduction, and regeneration in humans and other animals. It is a type of mitogen (a chemical substance which encourages a cell to undergo mitosis) found only in specific cells. GH is a single chain polypeptide containing 191 amino acids.

Functions

1) GH is essential for growth as it gets involved in the proliferation of cells and

2) It increases the size and number of cells by initiating mitosis. It differentiates certain types of cells (like those of bones and muscles).

3) It essentially takes part in the metabolism of proteins, carbohydrates, and fats.,

Thyroid Stimulating Hormone (TSH)

Thyroid stimulating hormone (or thyrotropin) triggers the production of thyroxine (T4) triiodothyronine (T3) from the thyroid gland. The latter stimulates the metabolism of nearly all the body tissue.

functions

1) Production of Thyroid Hormone: TSH triggers the production of thyroxine (To and triiodothyronine (T3) from the thyroid gland. The T3 hormone regulates the metabolic activities of the body, and T4 increases the heart rate, basal metabolic rate, and brain development.

2) Brown Adipose Tissue (BAT): TSH triggers the production of thyroid hormone, which regulates body temperature. It also stimulates BAT (a type of mammalian tissue comprising around 5% of the new-burn's weight), which generates heat in the bodies of new-born and animals.

3) Detection of Imbalances in Thyroid Gland: TSH detects imbalances and abnormal functioning of thyroid gland (i.e., it produces insufficient amounts of thyroid hormone). In such a case, pituitary gland stimulates the release of more TSH, which again stimulates the production of more thyroid hormone.

Adrenocorticotropic Hormone (ATCH)

Adrenocorticotropic hormone (or corticotropin) is produced and secreted by anterior pituitary gland in response to biological stress. It is a polypeptide tropic hormone which forms an important constituent of the hypothalamic-pituitary-adrenal axis. ACTH also stimulates the production and release of corticosteroids.

Functions

1) ACTH triggers the adrenal cortex cells (especially, in the zona fasciculata of adrenal glands to produce) to secrete glucocorticoid steroid hormones. It does so by binding to the ACTH receptors present on the surface of adrenocortical cells of the adrenal cortex releasing hormones.

2) It affects the secretion of steroid hormones by rapid short-term mechanisms (taking place within minutes) as well as by slower long-term mechanisms.

3) It also facilitates the uptake of lipoproteins into the cortical cells, thus enhancing the bioavailability of cholesterol in the adrenal cortex cells.

Prolactin (PRL)

Prolactin is a single-chain protein hormone secreted by lactotrophs present in the anterior pituitary gland. Various other body cells, such as immune cells, the brain and decidua of the pregnant uterus also produce and secrete this hormone. PRL is synthesized as a prohormone. After the single peptide is cleaved, the mature hormone's length is between 194 and 199 amino acids, depending on species. Three intramolecular disulfide bonds stabilize the prolactin structure.

Functions

1) PRL along with other hormones stimulates the development of mammary gland ducts.

2) It also stimulates milk production in women after delivery.

3) It is believed by the researchers that PRL does not play any role in males, while some of them consider it regulating the production of androgen.

Gonadotropins (FSH and LH)

Follicle-stimulating Hormone (FSH) is produced and secreted by gonadotrophs anterior pituitary gland called the master gland in humans and other animals’ processes in the body. FSH and Luteinizing Hormone (LH) act as synergistic in reproduction. Increased secretion of FSH by the anterior pituitary leads to ovulation.

Functions

1) In Females: FSH stimulates the growth of ovarian follicle, constituting the developing egg, cells surrounding the egg (produce hormones required for supporting pregnancy), and the fluid surrounding the egg. The anterior lobe growing follicle stimulates the cells within to produce and release estrogen's into the bloodstream in an increased amount. Estrogen's in turn thickens the endometrium (inner lining uterus) before ovulation, Increased estrogen's levels in blood also stimulate the hypothalamus and pituitary gland to slow down FSH production and release.

LH also stimulates the follicle cells to produce an increased amount of estrogen's However, ovulation is the major function of LH. The sharp rise in the blood level of LH that initiates ovulation is termed LH surge. After ovulation, the group of active follicle cells (i.e., the hormone production) become the corpus luteum producing estrogen's and progesterone (in large amounts). Progesterone in turn induces maturation of endometrium for supporting implantation of the fertilized e.g., or embryo. Failure of implantation declines the estrogen's and progesterone blood levels, sloughs off the endometrium, thus starting the menstruation.

2) In Males: Menotropins stimulate sperm production in some cases of male infertility.

Melanocyte Stimulating Hormone (MSH)

Melanocyte stimulating hormone (or intermedia or melanotropin) is a peptide produced and secreted by the intermediate lobe of the pituitary gland in most vertebrates. After secretion it circulates in the blood and binds to the receptors present on the target cell surfaces. This hormone regulates the production of melanin in specialized cells, thus influencing changes in skin pigmentation. The hormone also regulates melanin concentration and distribution in the blood vessels cells containing pigment (i.e., the melanocytes in humans and chromatophores in lower vertebrates).

Functions

1) MSH stimulates the release and distribution of melanin.

2) In the brain, it may play a signaling role.

3) Melanocytes present on the skin and eye produce melanin pigments.

4) Groups of melanocytes on the skin are called moles.

5) One of the MSH (a-MSH, particularly) plays a critical role in the gut's ability to function properly.

Antidiuretic Hormone (ADH)

Antidiuretic hormone (or arginine vasopressin) is a 9 amino acid peptide secreted by the posterior pituitary gland. Within the hypothalamic neurons, ADH and neurophysin (a carrier protein) are packed in secretory vesicles, and are released on horm01F secretion.

Around 60% of the body mass is water and this content remains stable even if the amount of water consumed each day varies from person to person. This control of body water content and solute concentration is the ability of several hormones (ADH being the k hormone) acting on the kidneys and vascular system.

Functions

1) ADH decreases the urine amount excreted and increases the water amount absorbed by the kidneys.

2) It also regulates the fluid balance in the body.

Oxytocin

It involves sexual reproduction during and after childbirth. After the cervix and uterus expands during labor, oxytocin is released in large amounts to facilitate birth and maternal bonding. The posterior lobe hormone is also secreted after stimulation of the nipples and breastfeeding. Oxytocin undergoes positive feedback mechanisms to facilitate childbirth and diabetes insipidus milk ejection.

Functions

1) In the early stages of pregnancy, the uterus is unresponsive to oxytocin. As the delivery time approaches, the muscles become sensitive to increased secretion of oxytocin. It stimulates contractions of smooth muscle tissues in the uterus wall during childbirth.

2) After childbirth, it stimulates milk ejection from the mammary glands. When an infant sucks the nipples, the nerve endocrine system cells in the hypothalamus are stimulated to release oxytocin into the bloodstream. After its release, the special cells contract and release milk into the collecting chambers which further release the milk outside. This reflex is known as the milk let-down reflex.

Functions

Various functions of different hormones secreted by the pituitary gland are shown in the below figure.