Pituitary gland

Function

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

• sleeping patterns
  

Anterior

The anterior pituitary synthesizes and secretes hormones. All releasing hormones (-RH) referred to, can also be referred to as releasing factors (-RF). 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 (Pic. 1).

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. Its primary function is anabolic; it promotes protein synthesis and tissue building through direct and indirect mechanisms (Pic. 2). GH levels are controlled by the release of GHRH and GHIH (also known as somatostatin) from the hypothalamus.
  

Thyrotrophins:

• Thyroid-stimulating hormone (TSH), is released under the influence of hypothalamic thyrotropin-releasing hormone (TRH) and is inhibited by somatostatin. It stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3), which stimulates the metabolism of almost every tissue in the body.
  

Corticotropins:

• Adrenocorticotropic hormone (ACTH) and Beta-endorphin are released under the influence of hypothalamic corticotropin-releasing hormone (CRH). ACTH stimulates secretion of glucocorticoid steroid hormones from adrenal cortex cells, especially in the zona fasciculata of the adrenal glands.
  

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. It stimulates growth of the mammary glands and lactation in females.
  

Gonadotropins:

• Luteinizing hormone (also referred to as 'Lutropin' or 'LH'). In both males and females, LH is essential for reproduction (Pic. 3).
  

• Effect in females - LH supports theca cells in the ovaries that provide androgens and hormonal precursors for estradiol production. At the time of menstruation, FSH initiates follicular growth, specifically affecting granulosa cells. With the rise in estrogens, LH receptors are also expressed on the maturing follicle, which causes it to produce more estradiol.
  

• Effect in males - LH acts upon the Leydig cells of the testis and is regulated by GnRH. The Leydig cells produce testosterone (T) under the control of LH, which regulates the expression of the enzyme 17-β hydroxysteroid dehydrogenase that is used to convert androstenedione, the hormone produced by the gonads, to testosterone, an androgen that exerts both endocrine activity and intratesticular activity on spermatogenesis.
  

• Follicle-stimulating hormone (FSH), both released under influence of Gonadotropin-Releasing Hormone (GnRH). It stimulates the growth of ovarian follicles in female mammals, and induces spermatogenesis in male mammals.

Intermediate

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

• Melanocyte–stimulating hormone (MSH). This is also produced in the anterior lobe. When produced in the intermediate lobe, MSHs are sometimes called "intermedins". MSH stimulate the production and release of melanin (a process referred to as melanogenesis) by melanocytes in skin and hair. MSH actions in the brain have effects on appetite and sexual arousal.
  

Posterior

The posterior pituitary is actually an extension of the neurons of the paraventricular and supraoptic nuclei of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as the hypothalamic–hypophyseal tract within the infundibulum, and end in axon terminals that comprise the posterior pituitary (Pic. 4). The posterior pituitary stores and secretes (but does 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. It regulates the amount of water excreted by the kidneys.
  
• 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.
  

Development

The anterior pituitary is derived from the ectoderm, more specifically from that of Rathke’s pouch, part of the developing hard palate in the embryo. The pouch eventually loses its connection with the pharynx, giving rise to the anterior pituitary. The anterior wall of Rathke's pouch proliferates, filling most of the pouch to form the pars distalis and the pars tuberalis. The posterior wall of the anterior pituitary forms the pars intermedia. Its formation from the soft tissues of the upper palate contrasts with the posterior pituitary, which originates from neuroectoderm.

Anatomical structure

The pituitary gland is an endocrine gland about the size of a pea and weighing 0.5 grams in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The hypophysis rests upon the hypophysial fossa of the sphenoid bone in the center of the middle cranial fossa and is surrounded by a small bony cavity (sella turcica) covered by a dural fold (diaphragma sellae) (Pic. 5 and Pic. 6).

It consists of two lobes that arise from distinct parts of embryonic tissue:

• the anterior pituitary (adenohypophysis) is glandular tissue
  

• the posterior pituitary (neurohypophysis) is neural tissue
  

Anterior

The anterior pituitary is composed of three regions:

• Pars distalis
  

Microanatomy of the pars distalis showing chromophobes, basophils and acidophils The pars distalis, (distal part), comprises the majority of the anterior pituitary and is where the bulk of pituitary hormone production occurs. 

The pars distalis contains two types of cells including:

• chromophobe cells

• chromophil cells - can be further divided into acidophils (alpha cells) and basophils (beta cells) (Pic. 7)
  

These cells all together produce hormones of the anterior pituitary, and release them into the blood stream.

• Pars tuberalis
  

The pars tuberalis, (tubular part), forms a part of the sheath extending up from the pars distalis which joins with the pituitary stalk (also known as the infundibular stalk or infundibulum), arising from the posterior lobe. The function of the pars tuberalis is poorly understood. However it has been seen to be important in receiving the endocrine signal in the form of TSHB (a β subunit of TSH) informing the pars tuberalis of the photoperiod (length of day).

The expression of this subunit is regulated by the secretion of melatonin in response to light information transmitted to the pineal gland.

• Pars intermedia
  

The pars intermedia, (intermediate part), sits between the pars distalis and the posterior pituitary, forming the boundary between the anterior and posterior pituitaries. It is very small and indistinct in humans.

Posterior

The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones produced by the hypothalamus. The paraventricular nuclei produce the hormone oxytocin, whereas the supraoptic nuclei produce ADH. These hormones travel along the axons into storage sites in the axon terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones are released from the axon terminals into the bloodstream.