Pituitary Gland – Control And Hormones of Anterior And Posterior Lobe

The pituitary gland, or hypophysis, is attached to the hypothalamus by a short stalk. It rests in a depression of the sphenoid bone, the sella turcica, which provides protection. The pituitary gland consists of two major parts that have different functions: an anterior lobe and a posterior lobe. Although the pituitary gland is small, it regulates many body functions. The pituitary gland is controlled by neurons and hormones that originate in the hypothalamus. The hypothalamus serves as a link between the brain and the endocrine system and is itself an endocrine gland.

Hormones of the Pituitary Gland

Control of The Anterior Lobe

Special neurons (neurosecretory cells) in the hypothalamus regulate the secretion of hormones from the anterior lobe by secreting releasing and inhibiting hormones. The hypothalamic hormones enter the hypophyseal portal veins, which carry them directly into the anterior lobe without circulating throughout the body. In the anterior lobe, the hormones exert their effects on specific groups of cells. There is a releasing hormone for each hormone produced by the anterior lobe. There are inhibiting hormones for growth hormone and prolactin. As the names imply, releasing hormones stimulate the production and release of hormones from the anterior lobe, while inhibiting hormones have the opposite effect. The secretion of releasing and inhibiting hormones by the hypothalamus is regulated by various hormonal negative-feedback mechanisms.

Control of The Posterior Lobe

The posterior lobe is controlled by the neural negative. Special neurons that originate in the hypothalamus have axons that extend into the posterior lobe of the pituitary gland. Nerve impulses passed along these neurosecretory axons cause the release of hormones from their terminal boutons within the posterior lobe, where they diffuse into the blood. Note that the posterior lobe hormones are formed by neurosecretory cells originating in the hypothalamus and not by cells of the posterior lobe of the pituitary gland. They are only released within the posterior lobe.

Anterior Lobe Hormones

The anterior lobe of the pituitary gland is sometimes called the “master gland” because it affects so many body functions. It produces and secretes six hormones: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle- stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL).

Growth Hormone

As the name implies, growth hormone (GH) stimulates the division and growth of body cells. Increased growth results because GH promotes the synthesis of proteins and other complex organic compounds. GH also increases available energy for these synthesis reactions by promoting the release of fat from adipose tissue, the use of fat in cellular respiration, and the conversion of glycogen to glucose. Although GH is more abundant during childhood and puberty, it is secreted throughout life.

Regulation of growth hormone secretion is by two hypothalamic hormones with antagonistic functions. GH- releasing hormone (GHRH) stimulates GH secretion, and GH- inhibiting hormone (GHIH) inhibits GH secretion. Whether the hypothalamus releases GHRH or GHIH depends upon changes in blood chemistry. For example, following strenuous exercise, a low level of blood sugar (hypoglycemia), and an excess of amino acids in the blood trigger the secretion of GHRH. Conversely, high levels of blood sugar (hyperglycemia) stimulate the secretion of GHIH.

Disorders If hypersecretion of GH occurs during the growing years, the individual becomes very tall- sometimes nearly 2.5 m (8 ft) in height. This condition is known as gigantism. If the hypersecretion of GH occurs in an adult after full growth in height has been attained, it produces a condition known as acromegaly (ak-ro- meg’-ah-le). Because the growth of long bones has been completed, only the bones of the face, hands, and feet continue to grow. Over time, the individual develops heavy, protruding brow ridges, a jutting mandible, and enlarged hands and feet. Both gigantism and acromegaly may result from tumors of the anterior lobe. Affected persons may have other health problems due to hypersecretion of other anterior lobe hormones.

If hyposecretion of GH occurs during childhood, body growth is limited. In extreme cases, this results in pituitary dwarfism. Affected persons have well- proportioned body parts but may be less than 1 m (3 ft) in height. They may suffer from other maladies due to a deficient supply of other anterior lobe hormones.

Thyroid-Stimulating Hormone

Thyroid-stimulating hormone (TSH) stimulates the thyroid gland to produce thyroid hormones. Blood concentrations of thyroid hormones control the negative- feedback mechanism for TSH production. Low levels of thyroid hormones activate the hypothalamus to secrete thyrotropin-releasing hormone (TRH), which stimulates release of TSH by the anterior lobe. Conversely, high concentrations of thyroid hormones inhibit the secretion of TRH, which decreases production of TSH. Because TSH controls the thyroid gland, disorders of TSH secretion lead to thyroid disorders.

Adrenocorticotropic Hormone

Adrenocorticotropic (ad-re-no-kor-ti-ko-tro-p’-ik) hormone (ACTH) controls the secretion of hormones produced by the adrenal cortex (the superficial portion of the adrenal gland). ACTH production is controlled by c orticotropin- releasing hormone (CRH) from the hypothalamus. CRH release is controlled by blood levels of ACTH and glucocorticoids from the adrenal cortex through negative- feedback mechanisms. Low levels of ACTH in the blood trigger the production and release of CRH. High blood levels of ACTH inhibit the production of CRH. Low levels of glucocorticoids from the adrenal cortex activate the hypothalamus to secrete CRH, which stimulates the release of ACTH from the anterior lobe. High levels of glucocorticoids inhibit CRH secretion, and thus inhibit the production and secretion of ACTH. Excessive stress may stimulate the production of excessive amounts of ACTH by overriding the negative- feedback control.

Gonadotropins

The follicle-stimulating hormone (FSH) and luteinizing (lu-te-in-^–z-ing) hormone (LH) affect the gonads (testes and ovaries). Their release is stimulated by gonadotropin-releasing hormone (GnRH) from the hypothalamus. The onset of puberty in both sexes is caused by the start of FSH secretion. In females, FSH acts on the ovaries to promote the development of ovarian follicles, which contain ova and produce estrogens, the primary female sex hormones. In males, FSH acts on testes to promote sperm production. In females, LH stimulates ovulation and the development of the corpus luteum, a temporary gland in the ovary that produces progesterone, another female sex hormone. In males LH is often referred to as interstitial cell stimulating hormone (ICSH) because it affects the interstitial cells of the testes, where it stimulates the secretion of testosterone.

Prolactin

Prolactin (PRL) helps to initiate and maintain milk production by the mammary glands after the birth of an infant. Prolactin stimulates milk secretion after the mammary glands have been prepared for milk production by other hormones, including female sex hormones. In males, PRL increases the activity of LH in the testes, thus increasing testosterone production. Prolactin secretion is regulated by the antagonistic actions of prolactinreleasing hormone (PRH) and prolactin-inhibiting hormone (PIH) produced by the hypothalamus.

Posterior Lobe Hormones

Posterior lobe hormones are good examples of neuroendocrine secretion. The posterior lobe stores and releases two hormones: the antidiuretic hormone and oxytocin. Both of these hormones are secreted by neurons that originate in the hypothalamus and extend into the posterior lobe. The hormones are released into the blood within the posterior lobe and are distributed throughout the body.

Antidiuretic Hormone

The antidiuretic hormone (ADH) promotes water retention by the kidneys to reduce the volume of water that is excreted in urine. ADH secretion is regulated by special neurons that detect changes in the water concentration of the blood. If water concentration decreases, secretion of ADH increases to promote water retention by the kidneys. If water concentration increases, secretion of ADH decreases, causing more water to be excreted in urine. By controlling the water concentration of blood, ADH helps to control blood volume and blood pressure.

Disorders

A severe hyposecretion of ADH results in the production of excessive quantities (20-30 liters per day) of dilute urine, a condition called diabetes insipidus. Diabetes means “overflow,” and insipidus means “tasteless.” Thus, diabetes insipidus essentially means to have overflow of tasteless urine. Conversely, mellitus means “sweet,” so diabetes mellitus is an overflow of sweet urine. In diabetes insipidus, the affected person is always thirsty and must drink water almost constantly.

This condition may be caused by injuries or tumors that affect any part of the ADH regulatory mechanism, such as the hypothalamus or posterior lobe of the pituitary gland, or nonfunctional ADH receptors in the kidneys.

Oxytocin

Oxytocin (ok-se-to’-sin) (OT) is released in large amounts during childbirth. It stimulates and strengthens contraction of the smooth muscles of the uterus, which culminates in the birth of the infant. It also has an effect on the mammary glands. Stimulation of a nipple by a suckling infant causes the release of OT, which, in turn, contracts the milk glands of the breast, forcing milk into the milk ducts, where it can be removed by the suckling infant.

Unlike other hormones, oxytocin secretion is controlled by a positive-feedback mechanism. For example, the greater the nipple stimulation by a suckling infant, the more OT released and the more milk available for the infant. When suckling ceases, OT production ceases. OT is also produced in males and non-pregnant females, where it plays a role in creating parental caretaking behaviors and feelings of pleasure associated with sexual intercourse.