Pituitary gland

In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g (0.02 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 pituitary is functionally connected to the hypothalamus by the median eminence via a small tube called the infundibular stem (Pituitary Stalk). The pituitary fossa, in which the pituitary gland sits, is 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.

Sections

The pituitary gland consists of two components: the anterior pituitary (or adenohypophysis) and the posterior pituitary (or neurohypophysis), and is functionally linked to the hypothalamus by the pituitary stalk (also named the "infundibular stem", or simply the "infundibulum"). It is from the hypothalamus that hypothalamic tropic factors are released to descend down the pituitary stalk to the pituitary gland where they stimulate the release of pituitary hormones. While the pituitary gland is known as the 'master' endocrine gland, both of the lobes are under the control of the hypothalamus; the anterior pituitary receives its signals from the parvocellular neurons and the posterior pituitary receives its signals from magnocellular neurons

Anterior pituitary (Adenohypophysis)

The anterior pituitary synthesizes and secretes the following important endocrine hormones:

Adrenocorticotropic hormone (ACTH), release under influence of hypothalamic Corticotropin Releasing Hormone (CRH).

Thyroid-stimulating hormone (TSH), release under influence of hypothalamic Thyrotropin Releasing Hormone (TRH).

Growth hormone (also referred to as 'Human Growth Hormone', 'HGH' or 'GH' or somatotropin), release under influence of hypothalamic Growth Hormone Releasing Hormone (GHRH); inhibited by hypothalamic Somatostatin.

Prolactin (PRL), also known as 'Luteotropic' hormone (LTH), release under influence of multiple hypothalamic Prolactin Releasing Factors (PRH).

The two 'Gonadotropins';

Luteinizing hormone (also referred to as 'Lutropin' or 'LH', or in males, 'Interstitial Cell Stimulating Hormone' (ICSH)), and

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

and;

melanocyte–stimulating hormones (MSH's) or "intermedins" as these are released by the pars intermedia which is "the middle part"; adjacent to the posterior pituitary lobe, pars intermedia is a specific part developed from the anterior pituitary lobe.

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.

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 pituitary (Neurohypophysis)

The posterior pituitary stores and releases:

Oxytocin, most of which is released from the paraventricular nucleus in the hypothalamus

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

Intermediate lobe

There is also an intermediate lobe in many animals, but is rudimentary in humans. For instance, in fish, it is believed to control physiological color change. In adult humans, it is just a thin layer of cells between the anterior and posterior pituitary. The intermediate lobe produces melanocyte-stimulating hormone (MSH), although this function is often (imprecisely) attributed to the anterior pituitary.

Variations among vertebrates

The pituitary gland is found in all vertebrates, but its structure varies between different groups.

The division of the pituitary described above is typical of mammals, and is also true, to varying degrees, of all tetrapods. However, only in mammals does the posterior pituitary have a compact shape. In lungfishes it is a relatively flat sheet of tissue lying above the anterior pituitary, and in amphibians, reptiles and birds, it becomes increasingly well developed. The intermediate lobe is generally not well developed in tetrapods, and is entirely absent in birds.

Apart from lungfishes, the structure of the pituitary in fish is generally different from that in tetrapods. In general, the intermediate lobe tends to be well developed, and may equal the remainder of the anterior pituitary in size. The posterior lobe typically forms a sheet of tissue at the base of the pituitary stalk, and in most cases sends irregular finger-like projection into the tissue of the anterior pituitary, which lies directly beneath it. The anterior pituitary is typically divided into two regions, a more anterior rostral portion and a posterior proximal portion, but the boundary between the two is often not clearly marked. In elasmobranchs there is an additional, ventral lobe beneath the anterior pituitary proper.

Functions

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

Growth (Excess of HGH can lead to gigantism and acromegaly.)

Blood pressure

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

Breast milk production

Sex organ functions in both men and women

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

Additional images

See also

Pituitary disease

Head and neck anatomy

Dantian

References

External links

The Pituitary Gland, from the UMM Endocrinology Health Guide

Oklahoma State, Endocrine System

Pituitary apoplexy mimicking pituitary abscess

Category:Glands Category:Endocrine system Category:Head and neck Category:Neuroendocrinology