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Review of Endocrinology

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Cross section of a beef heifer's udder.

This section was written by Juli Rowlett, a graduate student in ANSC 438 in 2005. Thanks Juli.

Endocrinology is a subdiscipline of the broader field, physiology, and is concerned with the study of chemical messengers, hormones, substances secreted by cells of the endocrine glands (ductless glands) and tissues that regulate the activity of other cells in the body. Endocrine glands secrete their hormones into the immediate extracellular space; from here, the hormones enter the circulatory system. These ductless glands differ from the exocrine glands (e.g., salivary glands) whose products are released into ducts that lead to the digestive tract and then to the exterior of the body. Endocrinology is, therefore the study of the ductless glands or tissues and their hormonal products.

The body tries to achieve homeostasis through hormones. Maintenance of a constant internal environment is necessary for the normal functioning of the various cellular components of tissues and organs. The body maintains precise control over the concentrations of substances by having a precise set point, like a thermostat. This is where positive and negative feedback systems come into play. A negative feedback system is when there is increased metabolite availability. An example of a positive feedback system is when rising concentrations of a hormone cause another gland to release a second hormone, which is then stimulatory to an increased output of the first hormone.

Peptide hormones are composed of amino acids and include such hormones as FSH, LH, PRL, and GH. Like other proteins, they are synthesized on ribosomes where their specific amino acid sequence is determined (translated) by a specific messenger RNA sequence (codon). Steroid hormones are produced by steroidogenic tissue or adrenal or gonadal origin. The steroidogenic tissue of the gonads produces a number of sex (gonadal) steroids: androgens (masculizing), estrogens (feminizing), and progestins (related to pregnancy and gestation). The testes produce testosterone; in the ovary estradiol and progesterone are the main steroids synthesized and secreted. During pregnancy the placenta is an additional source of estrogens and progestins.  

The pituitary gland has often been referred to as the master endocrine gland of vertebrates. However, it is subservient to hormonal stimuli derived from the brain and other endocrine glands. The pituitary gland in the human male is smaller than the tip of the little finger and weighs less than one gram, its size in the human female becomes larger during pregnancy. It is recessed within the sella turica of the sphenoid bone, beneath the hypothalamus near the optic chiasm. A number of peptide hormones are produced by the pituitary. These hormones regulate such target organs as the gonads, the adrenals, and the thyroid gland. The mammary glands, uterus, kidneys, and other tissues are also controlled by hypophysial hormones.   Hormones of the pituitary include, FSH, LH, PRL, GH and oxytocin and are released into the bloodstream where they circulate to interact with their target tissues. GH plays an essential role in general body growth, whereas PRL stimulates the growth of specialized tissues such as the mammary gland during pregnancy and lactation. Oxytocin is released from the pituitary in response to suckling. Pituitary hormone secretions are clearly adaptive in nature, as hormones play essential roles under conditions of special needs. For example, the reproductive cycles of many animals are linked to seasonal changes in day length.

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