This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Chapter 29. DESCRIPTIVE ENDOCRINO LOGY This chapter provides a more conventional and systematic description of the hormones of major interest in animal physiology. A lot of information is contained here, so the contents of this chapter might be best used as a “reference source” when specific details about some particular hormone are needed when that hormone is mentioned in subsequent chapters. An attempt is made to summarize the nature, source, and principal actions of each of the hormones. The listing is certainly not complete and, given the ongoing discoveries in this field, it will not remain entirely accurate. The wide array of growth factors - molecules with demonstrable actions in various cell culture systems - includes some now well- accepted hormones, and these will be noted. Others still await adequate characterization before their role in vivo becomes established, and these will usually be omitted. One rather complex grouping involves hormones known to be produced by various parts of the gastrointestinal tract and associated viscera – the so-called Diffuse Endocrine System (DES). These will be described later in this chapter. A. The Hypothalamic Hypophysiotropic Factors These molecules are commonly called the hypothalamic releasing and inhibiting factors (or hormones) and most are peptides that are synthesized and secreted by hypothalamic cells into the hypothalamo- hypophyseal portal system. This is a local and highly specialized blood vascular bed that provides direct transport from part of the hypothalamus to the nearby anterior pituitary without first passing back through the entire venous collection system (Figure 29-1).The molecules are presented to the pituitary in high concentrations because of this portal system and it is unlikely that they have any direct systemic effects, after passing through the general circulation. These hypothalamic factors provide the major form of regulation of the synthesis and/or release of hormones from the anterior pituitary. Using molecular techniques, several of the hypothalamic hormones, or very closely related molecules, have been found elsewhere in the body but little is really known of their physiological roles, presumably in or near these diverse locations. The first of the hypothalamic factors to be isolated and characterized was thyrotropin releasing hormone (TRH), noted earlier as being the smallest of the peptide hormones. Though its likely presence was postulated 50 years ago, corticotropin- releasing factor (CRF), a controller of the secretion of adrenocorticotropic hormone (ACTH) and hence a key part of the control system for the adrenal cortex (see Section C3, below), was characterized more recently 2007 version – page 233 Figure 29-1. The hypothalamo-hypophyseal portal blood system. Physical connection between the pituitary gland and the hypothalamus is by means of the stalk. Axons of hypothalamic neurons, the soma of which form “nuclei” at discrete locations in the hypothalamus, pass through the...
View Full Document
- Fall '08
- Anterior pituitary