Pituitary Gland and its functions

Pituitary gland or hypophysis is a small endocrine gland with a diameter of 1 cm and weight of 0.5 to 1 g.

It is situated in a depression called ‘sella turcica’, present in the sphenoid bone at the base of skull. It is connected with the hypothalamus by the pituitary stalk or hypophyseal stalk.

Pituitary Gland and its functions


DIVISIONS OF PITUITARY GLAND

Pituitary gland is divided into two divisions:

1. Anterior pituitary or adenohypophysis

2. Posterior pituitary or neurohypophysis.

Both the divisions are situated close to each other. Still both are entirely different in their development, structure and function. Between the two divisions, there is a small and relatively avascular structure called pars intermedia. Actually, it forms a part of anterior pituitary.

DEVELOPMENT OF PITUITARY GLAND

Both the divisions of pituitary glands develop from different sources.

Anterior pituitary is ectodermal in origin and arises from the pharyngeal epithelium as an upward growth

known as Rathke pouch. Posterior pituitary is neuroectodermal in origin and arises from hypothalamus as a downward diverticulum. Rathke pouch and the downward diverticulum from hypothalamus grow towards each other and meet in the midway between the roof of the buccal cavity and base of brain. There, the two structures lie close together..

REGULATION OF SECRETION

Hypothalamo-hypophyseal Relationship

The relationship between hypothalamus and pituitary gland is called hypothalamo-hypophyseal relationship. Hormones secreted by hypothalamus are transported to anterior pituitary and posterior pituitary. But the mode of transport of these hormones is different. Hormones from hypothalamus are transported to anterior pituitary through hypothalamo-hypophysial portal blood vessels. But, the hormones from hypothalamus to posterior pituitary are transported by nerve fibers of hypothalamo-hypophyseal tract.

ANTERIOR PITUITARY OR ADENOHYPOPHYSIS

Anterior pituitary is also known as the master gland because it regulates many other endocrine glands

through its hormones.

PARTS

Anterior pituitary consists of three parts:

1. Pars distalis

2. Pars tuberalis

3. Pars intermedia.

Anterior pituitary has two types of cells, which have different staining properties:

1. Chromophobe cells

2. Chromophil cells.

Chromophobe Cells

Chromophobe cells do not possess granules and stain poorly. These cells form 50% of total cells in anterior pituitary. Chromophobe cells are not secretory in nature, but are the precursors of chromophil cells.

Chromophil Cells

Chromophil cells contain large number of granules and are darkly stained.

Types of chromophil cells

Chromophil cells are classified by two methods.

1. Classification on the basis of staining property:

Chromophil cells are divided into two types:

i. Acidophilic cells or alpha cells, which form 35%

ii. Basophilic cells or beta cells, which form 15%.

2. Classification on the basis of secretory nature:

Chromophil cells are classified into five types:

i. Somatotropes, which secrete growth hormone

ii. Corticotropes, which secrete adrenocorticotropic hormone

iii. Thyrotropes, which secrete thyroid-stimulating

hormone (TSH)

iv. Gonadotropes, which secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

v. Lactotropes, which secrete prolactin.

Somatotropes and lactotropes are acidophilic cells, whereas others are basophilic cells. Somatotropes form about 30% to 40% of the chromophil cells. So, pituitary tumors that secrete large quantities of human growth hormone are called acidophilic tumors.

REGULATION OF ANTERIOR PITUITARY SECRETION

Hypothalamus controls anterior pituitary by secreting the releasing and inhibitory hormones (factors), which are called neurohormones. These hormones from hypothalamus are transported anterior pituitary through hypothalamo-hypophyseal portal vessels. Some special nerve cells present in various parts

hypothalamus send their nerve fibers (axons) to median eminence and tuber cinereum. These nerve cells

synthesize the hormones and release them into median are transported by blood via hypothalamo hypophyseal portal vessels to anterior pituitary.

Releasing and Inhibitory Hormones

Secreted by Hypothalamus

1. Growth hormone-releasing hormone (GHRH): Stimulates the release of growth hormone

2. Growth hormone-releasing polypeptide (GHRP): Stimulates the release of GHRH and growth hormone

3. Growth hormone-inhibitory hormone (GHIH) or somatostatin: Inhibits the growth hormone release

4. Thyrotropic-releasing hormone (TRH): Stimulates the release of thyroid stimulating hormone

5. Corticotropin-releasing hormone (CRH): Stimulates the release of adrenocorticotropin

6. Gonadotropin-releasing hormone (GnRH): Stimulates the release of gonadotropins, FSH and LH

7. Prolactin-inhibitory hormone (PIH): Inhibits prolactin secretion. It is believed that PIH is dopamine.

HORMONES SECRETED BY ANTERIOR PITUITARY

Six hormones are secreted by the anterior pituitary:

1. Growth hormone (GH) or somatotropic hormone (STH)

2. Thyroid-stimulating hormone (TSH) or thyrotropic hormone

3. Adrenocorticotropic hormone (ACTH)

4. Follicle-stimulating hormone (FSH) 5. Luteinizing hormone (LH) in females or interstitialcell- stimulating hormone (ICSH) in males

6. Prolactin.

Recently, the hormone β-lipotropin is found to be secreted by anterior pituitary.

 

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