Of the endocrine glands, the anterior pituitary, or adenohypophysis, occupies the prime place because, through the secretion of various hormones, it controls the functioning of certain other endocrine glands, namely, the adrenal cortex, the thyroid, and the gonads. In addition, hormones from the anterior pituitary influence the growth and metabolism of the organism through direct action on skeletal, muscular, and other tissues. The pituitary maintains control over the various target organs by a feedback mechanism which is sensitive to circulating levels of hormones from the target organs. Pituitary hormones are also released in response to metabolic conditions which they help to control. One factor influencing the secretion of growth hormone, for example, is the level of blood glucose.
There are eight anterior pituitary hormones whose existence has been firmly established for some time. They include the two gonadotropic hormones, interstitial-cell stimulating hormone (ICSH, or luteinizing hormone, LH) and follicle-stimulating hormone (FSH); thyrotropic hormone (thyroid-stimulating hormone, TSH); lactogenic hormone (prolactin); growth hormone (GH, or somatotropin, STH); adrenocorticotropic hormone (ACTH, or adrenocorticotropin, or corticotropin); and the two melanocyte-stimulating hormones (α-MSH and β-MSH). In addition, two peptides have been isolated which are structurally related to ACTH and the MSHs. These peptides have been designated β-lipotropic hormone (β-LPH) and γ-lipotropic hormone (γ-LPH). The hormones of the adenohypophysis are composed of amino acids in peptide linkage and are therefore classed as either polypeptides or proteins, depending on their size. In addition, three of the hormones, TSH, ICSH, and FSH, contain carbohydrate and are therefore categorized as glycoproteins. See Amino acids, Hormone, Protein
In the female, FSH initiates the development of ovarian follicles. ICSH, acting synergistically with FSH, is necessary for the final stages of follicular maturation and the production of estrogen. ICSH also stimulates the development of the corpus luteum. In the male, FSH stimulates spermatogenesis through its action on the germinal epithelium of the testis, and ICSH primarily activates the Leydig cells which produce androgen.
TSH stimulates the growth of the thyroid gland and the secretion of thyroid hormones.
Through a process which requires other hormones as well, lactogenic hormone stimulates the mammary gland to secrete milk. There is evidence that in some species of mammals lactogenic hormone also plays a role in maintaining the corpus luteum in the ovary. In contrast to all nonprimate species investigated, a distinct lactogenic hormone has never been isolated from the pituitaries of the monkey or the human. In these two species a hormone containing lactogenic activity can indeed be isolated from the pituitary, but it also has growth-promoting activity and in major respects corresponds to growth hormones isolated from nonprimate species. Thus, in nonprimates, a distinct growth hormone, having no lactogenic activity, can be isolated from the pituitary gland. In the primates, however, growth-promoting and lactogenic activities are present in the same molecule. In spite of its dual activities, the primate hormone is usually referred to simply as growth hormone, which promotes an increase in body size. It stimulates the growth of bones, muscles, and other tissues and enhances the effects of other pituitary hormones on their target organs. Although certain cellular effects of growth hormone are known, such as increasing incorporation of amino acids into muscle protein, the biochemical mechanisms whereby this hormone exerts its effects at the cellular level remain a mystery.
The hormone ACTH stimulates the growth of the adrenal cortex and the secretion of cortisol and other cortical hormones. An interesting aspect of the ACTH molecule is that it contains in part a sequence found in the melanocyte-stimulating hormones and lipotropic hormones (see below). In accordance with their chemical similarities, it is not surprising that all of these hormones exhibit both melanocyte-stimulating and lipolytic activities.
The melanocyte-stimulating hormones are also called intermedins, since they can be isolated from the intermediate lobe of the pituitary in those animals which have a distinct intermediate lobe. Melanocyte-stimulating activity refers to the ability of these hormones to cause dispersion of pigment granules in melanocytes, producing a darkening of the skin. Two types of MSHs have been isolated. α-MSH is the most potent melanocyte-stimulating hormone; β-MSH has about 50% of the activity of α-MSH, while ACTH and the LPHs have about 1%.
In 1965–1966, β-lipotropic hormone and γ-lipotropic hormone, were isolated from the pituitaries of sheep and chemically characterized. The term lipotropic refers to the lipolytic activity of these substances on adipose tissue. Subsequently, it was learned that they also possessed melanocyte-stimulating activity. Lipolytic activity refers to the ability of certain hormones to stimulate the breakdown of lipid in adipose tissue to free fatty acids and glycerol. As has been mentioned, ACTH and the MSHs are also lipolytic hormones, and are, in fact, of greater potency in this regard than the LPHs. See Lactation, Pituitary gland, Thyroid gland