What does the Adenohypophysis do?

The upper part of the adenohypophysis, which wraps around the infundibulum, is known as the pars tuberalis. The most significant and largest part of the adenohypophysis is known as the pars distalis. The blood supply originates from the superior hypophysial arteries, and arrives at the anterior pituitary via the hypothalamo-hypophysial portal system.

The adenohypophysis contains a number of different cell types that produce hormones. The cell types are named after the hormones they produce and a list of the cell types and the hormones are given below.

In order to understand the role of the different adenohypophysial hormones, it is necessary to study them each individually:

Adrenocorticotrophic Hormone (ACTH)

This hormone is synthesised and stored in the corticotrophe cells in the adenohypophysis. The main function of the hormone is to stimulate the adrenal glands to release cortisol in response to stress. Cortisol is released in response to stress, which can be emotional (e.g. anxiety) or physiological (e.g. fluid deprivation or injury). Stressors cause a release of Corticotrophin Releasing Hormone (CRH). This travels in the portal system of the hypothalamus to the anterior pituitary (the adenohypophysis). There, it stimulates the cleavage of Pro-opiomelanocortin (POMC) into several molecules including melanocyte stimulating hormone (MSH) and Adrenocorticotrophic Hormone (ACTH). ACTH travels in the bloodstream to the adrenal cortex stimulating the production and release of cortisol. Cortisol then travels to the tissues where it exerts its effects. Cortisol inhibits the release of CRH and ACTH from the hypothalamus and pituitary gland respectively, preventing further cortisol release. Cortisol is inactivated in the liver to inactive cortisone.

A daily pattern (circadian rhythm) is also seen, with cortisol being at it's lowest concentration at midnight, rising to a peak between 6am and 8am, falling throughout the rest of the day.

Thyroid Stimulating Hormone (TSH)/Thyrotrophin

This hormone is synthesised and stored in the thyrotrophe cells in the adenohypophysis. Thyroid stimulating hormone is released in low-amplitude pulses following a diurnal rhythm (the highest levels reached during the night). The main role of TSH is to stimulate the thyroid gland to release two of its own hormones into the bloodstream. The actions of the two thyroid hormones released (T3 (triiodothyronine) and T4 (thyroxine)) are discussed in the Thyroid section. The control of TSH release is by the hypothalamic hormone Thyrotrophin releasing hormone (TRH). The other major control factor is the negative feedback mechanism exerted by the thyroid hormones themselves at the level of the pituitary and the hypothalamus.

Growth Hormone(GH)/Somatotrophin

Growth hormone exerts its actions on many tissues. The hormone itself is synthesised and stored in the adenohypophysis in the somatotrophe cells. The main role of this hormone, as its name would suggest, is the promotion of linear growth in a variety of tissues. The hormone promotes growth is two major ways:

• The stimulation of protein synthesis
• Increasing amino acid transport through cells

Growth hormone stimulates many tissues, mainly the liver, to produce substances known as somatomedins. These substances are capable of stimulating cell division and cell proliferation. The liver produces two main somatomedins which both have great homology to insulin. The two somatomedins are therefore known as insulin-like growth factors I and II (IGF-I and IGF-II). The main actions of the somatotrophin and somatomedins are listed below.

• Increase in growth of soft and skeletal tissues
• Somatotrophin stimulates the uptake of non-esterified fatty acids (NEFA) by muscle
• Somatotrophin stimulates hepatic glycogenolysis to raise blood glucose levels
• Intestinal absorption of calcium is increased and the urinary excretions fall
• Somatotrophin seems to enhance T-cell proliferation.

The roles of somatotrophin and insulin seem to complement each other in regard to cell division and growth. Their roles, however, seem to oppose each other in respect to blood glucose levels. Insulin will reduce blood sugar levels whereas somatotrophin will act to increase blood sugar levels. The picture is even more complicated as the somatomedins will tend to reduce glucose levels due to their insulin-like actions.

The release of somatotrophin follows a diurnal variation with the greatest impulses occuring during deep sleep.

Follicle stimulating hormone (FSH) and Luteinizing hormone (LH)

The two gonadotrophins produced by the gonadotrophe cells in the adenohypophysis are follicle stimulating hormone (FSH) and luteinizing hormone (LH). The actions of these hormones vary in both men and women.

In women, LH acts on the ovaries to stimulate steroid hormone production. In males, LH acts by stimulating Leydig cells in the testes to secrete testosterone. The control of LH release is primarily by the gonadotrophin-releasing hormone (GnRH) produced by the hypothalamus. The pulsatile release of LH is dependent on the pulsatile release of GnRH. LH is also regulated by a number of other hormones such as dopamine, prolactin and most importantly by negative feedback from the sex steroids.

FSH stimulates the follicular development in the ovary in women, whereas in males it stimulates the sertoli cells to initiate spermatogenesis. FSH is regulated in similar way to LH except from the production of a specific inhibitory protein, produced by the FSH target cells, called inhibin. Inhibin allows for the specific inhibition of FSH release and plays an important role in the menstrual cycle.

Prolactin (PRL)

Prolactin is synthesised and released from lactotrophe cells in the adenohypophysis. Prolactin has two main roles:

• The growth and development of the breasts
• The maintenance of lactation in women.

Prolactin requires other hormones to complete these actions. Prolactin also has an important role in the male in the regulation of gonadal function by stimulating LH receptor synthesis in the Leydig cells.

Prolactin release is primarily under the control of the hypothalamus. The two hypothalamic hormones involved in prolactin regulation are thyrotrophin releasing hormone (TRH) and dopamine. The predominant hormone in the regulation is the inhibitory dopamine. The hypothalamus receives afferent sensory nerve cell input primarily from the nipples of lactating women. This feedback loop increases lactation during suckling by inhibiting dopamine release and stimulating TRH release.

Prolactin is released in a diurnal rhythm with the highest levels occurring at night.