The hormones of the pituitary gland - Wilkinson & Brown - 2015 - Article

The hypophysis

Another name for the pituitary gland is the hypophysis. The secretion of hormones by the pituitary gland is regulated by the hypothalamus. Through these connections the external and internal stimuli influence the release of pituitary hormones, which is a neural-endocrine interaction. The hormones secreted by the pituitary gland stimulates secretion of hormones in other endocrine glands. The pituitary gland consists of the anterior pituitary (adenohypophysis or pars distalis) and the posterior pituitary (neurohypophysis). The pituitary stalk connects the pituitary gland to the hypothalamus.

Posterior pituitary

It contains neural tissue. Axons from the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) of the hypothalamus project to the pituitary stalk, particularly the infundibulum, and then terminate in the posterior pituitary gland. The hormones secreted by the PVN and SON, oxytocin and vasopressin, are stored in these nerve terminals in the posterior pituitary. The blood supply of the posterior pituitary comes from the inferior hypophyseal artery. With stimulation of the PVN and SON, oxytocin and vasopressin are released into the bloodstream to go to their targets in the body.

Anterior pituitary

It has two parts: (1) the anterior pituitary and (2) the intermediate pituitary. The pituitary stalk with the hypophyseal portal system of blood vessels connects the anterior pituitary to the hypothalamus. Hormones of the hypothalamus are released into this hypophyseal portal system and are then carried to the anterior pituitary. The pituitary stalk delivers blood to the median eminence of the hypothalamus through the superior hypophyseal artery. There it forms a series of tiny blood vessels, which are called ‘primary plexus’, and here the hypothalamic hormones are released. Via the hypophyseal portal veins the hormones travel to the secondary plexus, where they stimulate pituitary cells to release their hormones to enter in the general circulation.

Intermediate pituitary

It is not present in all mammals and has different sizes in the vertebrates. In human fetal growth it is well developed. However, after birth it becomes indistinct, because the melanotrophs distribute throughout the anterior lobe. The cells secreting melanotrophs are stimulated by the hypothalamic neurons and produce and secrete neuropeptides derived from the pro-opiomelanocortin gene (POMC).

Hormones

Posterior pituitary

The posterior pituitary gland secretes oxytocin and vasopressin, but they are actually hypothalamic or neurosecretory hormones, because they are produced in the neurosecretory cells of the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) of the hypothalamus. However, after this they travel to the posterior pituitary through the infundibulum, where they are stored and released into the venous circulation.

Oxytocin promotes the uterine contractions during childbirth and stimulates milk ejection from the mammary glands during lactation. Vasopressin raises the blood pressure and has an antidiuretic function, which is stimulating water reabsorption in the kidneys. It can enhance memory. The posterior pituitary also releases two large proteins called neurophysins, which carry oxytocin and vasopressin.

Anterior pituitary

The anterior pituitary secretes six hormones and the intermediate lobe two hormones. One of the secreted hormones by the anterior pituitary is growth hormone, which is also called somatotropin or somatotropic hormone. This is a stimulating hormone on almost all body cells: bone, muscle, brain, heart, etc. Because of its effects on muscle cells it increases muscle mass. It also has an effect on bone growth and height through the release of somatomedin, which is a peptide growth factor secreted by the liver. Glucose output from the liver is increases by stimulation of growth factor.

Lactotroph cells of the anterior pituitary produce prolactin, which starts milk production in the mammary glands. It is also important for growth, osmoregulation, fat and carbohydrate metabolism, reproduction, and parental behaviour. This is often done by interaction with other hormones. The corticotroph cells of the anterior pituitary produce ACTH to stimulate the production and secretion of glucocorticoid hormones in the adrenal cortex. The levels of ACTH and cortisol are high in the early morning. ACTH also plays a role in regulating the immune system.

The thyrotroph cells of the anterior pituitary produce thyroid-stimulating hormone (TSH), which stimulates the production and secretion of thyroxine (T4) and triiodothyronine (T3) from the thyroid gland. The gonadotroph cells of the anterior pituitary produce gonad-stimulating or gonadotropic hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH leads to the development of the gametes and release of gonadal hormones. in women the primary follicle in the ovary grows because of FSH, just as the ovum develops and estradiol is secreted because of FSH. LH stimulates the ovulation by rupturing the follicle and releasing the ovum. In men sperm production and release of inhibin is stimulated by FSH. LH stimulates the secretion of andorgens by the Leydig cells.

The opioid peptide, β-endorphin, comes from the POMC polypeptide, which is made in the anterior and intermediate pituitary. The POMC is also developed into the ACTH, because enzymes break the peptide into the different smaller pieces than with β-endorphin. Proprotein convertases enzymes break POMC into active hormones. ACTH in the intermediate pituitary is converted to α-MSH. Β-LPH is converted into β-endorphin and γ-lipotropin in the anterior pituitary. We don’t know the function of β-endorphin and γ-LPH in the bloodstream, but β-endorphin does have a role in analgesia, learning and memory, psychiatric diseases, feeding, thermoregulation, blood pressure regulation and reproductive behaviour. Β-endorphin is released during exercise and somehow reaches the brain from the circulation. However it can’t pass the blood-brain barrier, so the ‘runner’s high’ is probably the result of the ‘high’ in the brain itself.

Intermediate pituitary

The production of α-MSH and β-endorphin also happens in the intermediate pituitary out of the POMC pre-propeptide. α-MSH affects the pigmentation and it is released in the human fetus. People with Addison’s disease have low cortisol levels, high ACTH secretion and pronounced darkening of the skin through secretion of α-MSH and ACTH. α-MSH is also released by the hypothalamus to influence the feeding behaviour. α-MSH modulates the immune system.

Folliculostellate cells

Folliculostellate (FS) cells take care of all signalling and information transfer between the cells in the anterior pituitary gland. The hormonal secretion is influenced by paracrine regulators through the close association of the FS and endocrine cells. Examples of paracrine factors are growth factors, cytokines and nitric oxide. The FS cells release activin and follistatin to regulate FSH secretion. Through feedback signals of the hormonal cells the FS cells are also controlled. Therefore the FS cells are important in the regulation of the endocrine system.