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 gland 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 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.
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.
The anterior pituitary arises from an invagination of the oral ectoderm and forms Rathke’s pouch. This contrasts with the posterior pituitary, 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.
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.
Although rudimentary in humans (and often considered part of the anterior pituitary), the intermediate lobe located between the anterior and posterior pituitary is important to many animals. 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.
The intermediate lobe is, in general, not well developed in tetrapods, and is entirely absent in birds.
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, in general, 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.
The arrangement in lampreys, which are among the most primitive of all fish, may indicate how the pituitary originally evolved in ancestral vertebrates. Here, the posterior pituitary is a simple flat sheet of tissue at the base of the brain, and there is no pituitary stalk. Rathke’s pouch remains open to the outside, close to the nasal openings. Closely associated with the pouch are three distinct clusters of glandular tissue, corresponding to the intermediate lobe, and the rostral and proximal portions of the anterior pituitary. These various parts are separated by meningial membranes, suggesting that the pituitary of other vertebrates may have formed from the fusion of a pair of separate, but associated, glands.
Most armadillos also possess a neural secretory gland very similar in form to the posterior pituitary, but located in the tail and associated with the spinal cord. This may have a function in osmoregulation.
There is a structure analogous to the pituitary in the octopus brain.
The anterior pituitary synthesizes and secretes the following important endocrine hormones:
- Growth hormone (also referred to as ‘Human Growth Hormone’, ‘HGH’ or ‘GH’ or somatotropin), released under influence of hypothalamic Growth Hormone-Releasing Hormone (GHRH), (also known as growth-hormone-releasing factor (GHRF)); inhibited by hypothalamic somatostatin
- Thyroid-stimulating hormone (TSH), released under influence of hypothalamic Thyrotropin-releasing hormone (TRH) (or TRF); inhibited by somatostatin
- Adrenocorticotropic hormone (ACTH), released under influence of hypothalamic Corticotropin-Releasing Hormone (CRH), (or CRF)
- Beta-endorphin, released under influence of hypothalamic Corticotropin-Releasing Hormone (CRH)or (CRF)
- 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.
- 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.
The posterior pituitary synthesizes and secretes the following important endocrine hormones:
- 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, 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.
The intermediate lobe synthesizes and secretes the following important endocrine hormone:
- Melanocyte–stimulating hormones (MSHs), although this function is often (imprecisely) attributed to the anterior pituitary. These are sometimes called “intermedins,” as these are released by the pars intermedia.
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 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
Diseases involving the pituitary gland
Some of the diseases involving the pituitary gland are:
- Hyperpituitarism, the increased (hyper) secretion of one or more of the hormones normally produced by the pituitary gland.
- Hypopituitarism, the decreased (hypo) secretion of one or more of the hormones normally produced by the pituitary gland. If there is decreased secretion of most pituitary hormones, the term panhypopituitarism (pan meaning “all”) is used.
- Pituitary tumours.
- Pituitary adenomas, noncancerous tumors that occur in the pituitary gland.