Pituitary gland

The pituitary gland is a major control centre for several hormone-producing or endocrine glands and the chief link between the brain and the endocrine system. It is therefore also called the ‘master gland’. It secretes several hormones which control the function of other endocrine glands such as the thyroid and adrenal glands. Its own secretory activity is controlled by a particular part of the brain known as the hypothalamus.

Smaller in size than a cherry, the pituitary gland is located in a bony cavity, or fossa, at the base of the skull, immediately above the nose. It is connected by a short stalk to the hypothalamus, which lies immediately above it. The gland is divided into an anterior or front lobe (adenohypophysis) and a posterior or rear lobe (neurohypophysis).

Anterior pituitary

The anterior lobe secretes most of the pituitary hormones, including growth hormone, adrenocorticotropic hormone (ACTH), thyroid stimulating hor- mone (TSH), luteinizing hormone (LH), otherwise known as interstitial cell secreting hormone (ICSH), follicle stimulating hormone (FSH), prolactin, and melanocyte stimulating hormone (MSH). The output of hormones by the anterior lobe is controlled by releasing factors, chemical substances that are produced in the hypothalamus. This part of the brain receives information about the condition in the body and can react by triggering the release of hormones by the pituitary gland. During stress for example, the hypothalamus secretes the releasing factor for ACTH; this hormone from the pituitary in turn stimulates the production of Cortisol in the adrenal glands. In this way the bodily responses that try to withstand stress are initiated.

As its name implies, growth hormone controls processes that are concerned with growth. It mobilizes fat, heightens the levels of glucose (sugar) in the blood and it enhances protein synthesis in all kinds of body cells. In children growth hormone determines what their height will be when they have grown up.

This effect is the result of the influence on the growing speed of the bones. A deficiency of growth hormone causes a short stature (or pituitary dwarfism), a condition which can be corrected by injecting preparations of the hormone.

In a fully grown individual growth hormone is not superfluous, because it still is necessary for the repair processes in tissues. In adults, too much growth hormone causes enlargement of the end parts of the body, such as hands, feet, ears, nose or the jaw leading to the condition called acromegaly. In lower animals, such as some fish and amphibians, melanocyte stimulating hormone (MSH) disperses pigment granules of melanocytes (special cells in the skin concerned with changing the colour of the animal). In the human being melanocyte stimulating hormone has no such rapid action, but it slowly increases pigmentation by steadily promoting melanin synthesis.

Adrenocorticotropic hormone (ACTH) controls the secretion of the hormone Cortisol from the adrenal glands. Similarly, thyroid stimulating hormone (TSH) stimulates the secretion of thyroid hormones from the thyroid gland. Undcrsecretion or oversecretion of ACTH and TSH causes a disturbance of adrenal and thyroid function.

Luteinizing hormone (LH) in combination with follicle stimulating hormone (FS) in women promotes the development and release of eggs – ova – from the ovaries, and the production of female sex hormones. This occurs in an alternating pattern, each cycle lasting about one month: the menstrual cycle. If the ovum is not fertilized, after two weeks menstruation follows and the cycle starts again. If this delicate balance of alternating production of LH and FSH by the pituitary is disturbed, infertility may be one of the consequences.

In men, LH (known also as ICSH) is necessary for normal development of sperm and production of the male hormone testosterone in the testes. Deficiency of LH or ISCH in adolescents prevents the development of secondary sexual characteristics. Prolactin is particularly important because it is necessary for breast development during pregnancy and for successful milk production during breast feeding. Prolactin deficiency causes failure to produce milk after childbirth. In non-pregnant women, excessive prolactin secretion causes a milky discharge from the breasts (galactorrhea), usually associated with cessation of menstruation and infertility.

Posterior pituitary

The posterior lobe of the pituitary gland consists of nerve fibres that originate in the hypothalamus. These nerve fibres convey and store two hormones: antidiuretic hormone (ADH), also known as vasopressin, and oxytocin.

Antidiuretic hormone controls the loss of water through the kidneys and thus plays an important role in maintaining water balance in the body. The rate of secretion of ADH is under nervous control from the hypothalamus and its secretion is dependent on the relative amount of water and dissolved chemicals in the fluids surrounding the body’s cells. Too much water reduces ADH secretion, and increases water excretion; too little water increases ADH secretion and promotes water retention. In disorders of the posterior pituitary or the hypothalamus too little ADH causes a failure to retain water filtering through the kidneys. The sufferer passes large volumes of dilute urine, often as much as 5 to 10 litres a day, and feels thirsty. The condition is known as diabetes insipidus.

Oxytocin is mainly involved with childbirth when it stimulates contractions of the uterus during labour and facilitates delivery of the baby. A synthetic preparation of oxytocin is often used to induce labour when uterine contraction is weak and labour delayed.

Neuropeptides

Of recent interest is the observation that pituitary hormones can influence the way in which the brain functions. Research on these neuropeptides, as they are called, has shown that they may have an influence on the ability to memorize. ACTH and vasopressin make learning easier, whereas oxytocin hampers memory. These facts are however mainly derived from animal studies, and it is not yet known whether such neuropeptides in future will provide us with something like ‘learning pills’.