Anterior Pituitary
Anterior Pituitary
The anterior pituitary consists of an anterior lobe (pars distalis) as shown in Figure 36-5, and an intermediate lobe (pars intermedia), which is absent in some animals (including humans). The anterior pituitary produces seven hormones, and all but one are released by the anterior lobe.
Four hormones of the anterior pituitary are tropic hormones (from the Greek tropƒ, to turn toward) that regulate other endocrine glands (Table 36-1). Thyroid-stimulating hormone (TSH) or thyrotropin stimulates production of thyroid hormones by the thyroid gland. Two tropic hormones are commonly called gonadotropins because they act on the gonads (ovaries of females, testes of males). These are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH promotes egg production and secretion of estrogen in females, and supports sperm production in males. LH induces ovulation, corpus luteum production, and secretion of the female sex steroids, progesterone and estrogen. In males, LH promotes production of male sex steroids (primarily testosterone). It once was called interstitial cell stimulating hormone (ICSH) in males, before it was discovered to be identical to LH in females. The fourth tropic hormone, adrenocorticotropic hormone (ACTH), increases production and secretion of steroid hormones from the adrenal cortex.
Prolactin and the structurally related growth hormone (GH) are proteins. Prolactin is essential for preparing mammary glands for lactation; after birth it is required for production of milk. Prolactin also has been implicated in parental behavior in a wide variety of vertebrates. Beyond its more traditional role in reproductive processes, prolactin regulates water and electrolyte balance in many species. More recently, prolactin has been shown to be a chemical mediator of the immune system and is important in formation of new blood vessels (angiogenesis). Unlike tropic hormones, prolactin acts directly on its target tissues rather than through other hormones.
GH (also called somatotropin) performs a vital role in governing body growth through its stimulatory effect on cellular mitosis, on synthesis of messenger RNA and protein, and on metabolism, especially in new tissue of young vertebrates. Growth hormone acts directly on growth and metabolism, as well as indirectly through a polypeptide hormone, insulin-like growth factor (IGF) or somatomedin, produced by the liver.
The only anterior pituitary hormone produced by the intermediate lobe is melanocyte-stimulating hormone (MSH). In cartilaginous and bony fishes, amphibians, and reptiles, MSH is a direct-acting hormone that promotes dispersion of the pigment melanin within melanocytes, causing darkening of the skin. In birds and mammals, MSH is produced by cells in the anterior pituitary rather than the intermediate lobe, but its physiological function remains unclear. MSH appears unrelated to pigmentation in endotherms, although it will cause darkening of the skin in humans if injected into the circulation. Until recently, many endocrinologists thought MSH was a vestigial hormone in mammals, but interest has been rekindled by studies showing that it enhances memory and growth of the fetus. In addition, MSH has been isolated from specific regions of the hypothalamus, where it has been linked to regulation of ingestive behaviors and metabolism in adult mammals. Future studies will determine if a similar role also exists for MSH during development. MSH and ACTH are derived from a precursor molecule that is transcribed and translated from a single gene.
The anterior pituitary consists of an anterior lobe (pars distalis) as shown in Figure 36-5, and an intermediate lobe (pars intermedia), which is absent in some animals (including humans). The anterior pituitary produces seven hormones, and all but one are released by the anterior lobe.
Four hormones of the anterior pituitary are tropic hormones (from the Greek tropƒ, to turn toward) that regulate other endocrine glands (Table 36-1). Thyroid-stimulating hormone (TSH) or thyrotropin stimulates production of thyroid hormones by the thyroid gland. Two tropic hormones are commonly called gonadotropins because they act on the gonads (ovaries of females, testes of males). These are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH promotes egg production and secretion of estrogen in females, and supports sperm production in males. LH induces ovulation, corpus luteum production, and secretion of the female sex steroids, progesterone and estrogen. In males, LH promotes production of male sex steroids (primarily testosterone). It once was called interstitial cell stimulating hormone (ICSH) in males, before it was discovered to be identical to LH in females. The fourth tropic hormone, adrenocorticotropic hormone (ACTH), increases production and secretion of steroid hormones from the adrenal cortex.
Prolactin and the structurally related growth hormone (GH) are proteins. Prolactin is essential for preparing mammary glands for lactation; after birth it is required for production of milk. Prolactin also has been implicated in parental behavior in a wide variety of vertebrates. Beyond its more traditional role in reproductive processes, prolactin regulates water and electrolyte balance in many species. More recently, prolactin has been shown to be a chemical mediator of the immune system and is important in formation of new blood vessels (angiogenesis). Unlike tropic hormones, prolactin acts directly on its target tissues rather than through other hormones.
GH (also called somatotropin) performs a vital role in governing body growth through its stimulatory effect on cellular mitosis, on synthesis of messenger RNA and protein, and on metabolism, especially in new tissue of young vertebrates. Growth hormone acts directly on growth and metabolism, as well as indirectly through a polypeptide hormone, insulin-like growth factor (IGF) or somatomedin, produced by the liver.
The only anterior pituitary hormone produced by the intermediate lobe is melanocyte-stimulating hormone (MSH). In cartilaginous and bony fishes, amphibians, and reptiles, MSH is a direct-acting hormone that promotes dispersion of the pigment melanin within melanocytes, causing darkening of the skin. In birds and mammals, MSH is produced by cells in the anterior pituitary rather than the intermediate lobe, but its physiological function remains unclear. MSH appears unrelated to pigmentation in endotherms, although it will cause darkening of the skin in humans if injected into the circulation. Until recently, many endocrinologists thought MSH was a vestigial hormone in mammals, but interest has been rekindled by studies showing that it enhances memory and growth of the fetus. In addition, MSH has been isolated from specific regions of the hypothalamus, where it has been linked to regulation of ingestive behaviors and metabolism in adult mammals. Future studies will determine if a similar role also exists for MSH during development. MSH and ACTH are derived from a precursor molecule that is transcribed and translated from a single gene.
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