PITUITARY GLAND By vikas lodhi

Pituitary

In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 grams (0.018 oz) in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity (sella turcica) covered by a dural fold (diaphragma sellae). The posterior pituitary (or neurohypophysis) is a lobe of the gland that is functionally connected to the hypothalamus by the median eminence via a small tube called the pituitary stalk (also called the infundibular stalk or the infundibulum). The anterior pituitary (or adenohypophysis) is a lobe of the gland that regulates several physiological processes (including stress, growth, reproduction, and lactation). The pituitary gland sits in the hypophysial fossa, situated in the sphenoid bone in the middle cranial fossa at the base of the brain. The pituitary gland secretes nine hormones that regulate homeostasis.

Structure

The pituitary gland is a pea-sized gland that sits in a protective bony enclosure called the sella turcica. It is composed of three lobes: anterior, intermediate, and posterior. In many animals, these three lobes are distinct. However, in humans, the intermediate lobe is but a few cell layers thick and indistinct; as a result, it is often considered part of the anterior pituitary. In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition of the posterior pituitary. It belongs to the diencephalon.

Anterior

The anterior pituitary (adenohypophysis), arises from an invagination of the oral ectoderm and forms Rathke's pouch. This contrasts with the posterior pituitary (neurohypophysis), which originates from neuroectoderm.

Endocrine cells of the anterior pituitary are controlled by regulatory hormones released by parvocellular neurosecretory cells in the hypothalamus. The latter release regulatory hormones into hypothalamic capillaries leading to infundibular blood vessels, which in turn lead to a second capillary bed in the anterior pituitary. This vascular relationship constitutes the hypothalamo-hypophyseal portal system. Diffusing out of the second capillary bed, the hypothalamic releasing hormones then bind to anterior pituitary endocrine cells, upregulating or downregulating their release of hormones.

The anterior pituitary is divided into anatomical regions known as the pars tuberalis, pars intermedia, and pars distalis. It develops from a depression in the dorsal wall of the pharynx (stomodial part) known as Rathke's pouch.

Posterior

The posterior lobe develops as an extension of the hypothalamus. The magnocellular neurosecretory cells of the posterior side possess cell bodies located in the hypothalamus that project axons down the infundibulum to terminals in the posterior pituitary. This simple arrangement differs sharply from that of the adjacent anterior pituitary, which does not develop from the hypothalamus. The release of pituitary hormones by both the anterior and posterior lobes is under the control of the hypothalamus, albeit in different ways.

Function

Anterior

The anterior pituitary synthesizes and secretes the following important endocrine hormones. All releasing hormones (-RH) referred to, can also be referred to as releasing factors (-RF).

Somatotrophins:

* Human growth hormone (HGH), also referred to as 'growth hormone' (GH) , and also as somatotropin, is released under the influence of hypothalamic growth hormone-releasing hormone (GHRH), and is inhibited by hypothalamic somatostatin

Thyrotrophins:

* Thyroid-stimulating hormone (TSH), is released under the influence of hypothalamic thyrotropin-releasing hormone (TRH) and is inhibited by somatostatin.

Corticotropins:

* Adrenocorticotropic hormone (ACTH), and Beta-endorphin are released under the influence of hypothalamic corticotropin-releasing hormone (CRH).

Lactotrophins:

* Prolactin (PRL), also known as 'Luteotropic' hormone (LTH), whose release is inconsistently stimulated by hypothalamic TRH, oxytocin, vasopressin, vasoactive intestinal peptide, angiotensin II, neuropeptide Y, galanin, substance P, bombesin-like peptides (gastrin-releasing peptide, neuromedin B and C), and neurotensin, and inhibited by hypothalamic dopamine.

Gonadotropins:

* Luteinizing hormone (also referred to as 'Lutropin' or 'LH').

* Follicle-stimulating hormone (FSH), both released under influence of Gonadotropin-Releasing Hormone (GnRH)

These hormones are released from the anterior pituitary under the influence of the hypothalamus. Hypothalamic hormones are secreted to the anterior lobe by way of a special capillary system, called the hypothalamic-hypophysial portal system.

Posterior

The posterior pituitary stores and secretes (not synthesize) the following important endocrine hormones:

Magnocellular Neurons:

* Antidiuretic hormone (ADH, also known as vasopressin and arginine vasopressin AVP), the majority of which is released from the supraoptic nucleus in the hypothalamus.

* Oxytocin, most of which is released from the paraventricular nucleus in the hypothalamus. Oxytocin is one of the few hormones to create a positive feedback loop. For example, uterine contractions stimulate the release of oxytocin from the posterior pituitary, which, in turn, increases uterine contractions. This positive feedback loop continues throughout labour.

Intermediate

The intermediate lobe synthesizes and secretes the following important endocrine hormone:

* Melanocyte–stimulating hormone (MSH), although this function is often (imprecisely) attributed to the anterior pituitary. MSHs are sometimes called "intermedins," as they are released by the pars intermedia.

Functions

Hormones secreted from the pituitary gland help control the following body processes:

* Growth

* Blood pressure

* Some aspects of pregnancy and childbirth including stimulation of uterine contractions during childbirth

* Breast milk production

* Sex organ functions in both males and females

* Thyroid gland function

* The conversion of food into energy (metabolism)

* Water and osmolarity regulation in the body

* Water balance via the control of reabsorption of water by the kidneys

* Temperature regulation

* Pain relief

                

  Vikas Lodhi

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