Adenohypophysis regulation

List functions and describe mechanisms regulating release of hormones from the adenohypophysis... 

The adenohypophysis is derived embryonically from glandular tissue, specifically, Rathke s pouch. This tissue originates from the oropharynx, or the roof of the mouth. It then migrates toward the embryonic nervous tissue destined to form the neurohypophysis. When these two tissues come into contact, Rathke s pouch loses its connection with the roof of the mouth and the pituitary gland is formed. Unlike the neurohypophysis, which releases hormones originally synthesized in the hypothalamus, the adenohypophysis synthesizes its own hormones in specialized groups of cells. Similar to the neurohypophysis, however, the release of these hormones into the blood is regulated by the hypothalamus. 

The adenohypophysis does not have a direct anatomical connection with the hypothalamus therefore, regulation of hormone secretion by way of neuronal signals is not possible. Instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus (see Figure 10.2). Systemic arterial blood is directed first to the hypothalamus. The exchange of materials between the blood and the interstitial fluid of the hypothalamus takes place at the primary capillary plexus. The blood then flows to the adenohypophysis through the hypothalamic-hypophyseal portal veins. Portal veins are blood vessels that connect two capillary beds. The second capillary bed in this system is the secondary capillary plexus located in the adenohypophysis. 

Located in close proximity to the primary capillary plexus in the hypothalamus are specialized neurosecretory cells. In fact, the axons of these cells terminate on the capillaries. The neurosecretory cells synthesize two types of hormones releasing hormones and inhibiting hormones (see Table 10.2). Each of these hormones helps to regulate the release of a particular hormone from the adenohypophysis. For example, thyrotropin-releasing hormone produced by the neurosecretory cells of the hypothalamus stimulates secretion of thyrotropin from the thyrotrope cells of the adenohypophysis. The hypo-thalamic-releasing hormone is picked up by the primary capillary plexus travels through the hypothalamic-hypophyseal portal veins to the anterior pituitary leaves the blood by way of the secondary capillary plexus and exerts its effect on the appropriate cells of the adenohypophysis. The hypophyseal hormone, in this case, thyrotropin, is then picked up by the secondary capillary plexus, removed from the pituitary by the venous blood, and delivered to its target tissue. 

Connected to the brain by a stalk (Fig. 30-1), the pituitary gland releases at least ten peptide or protein hormones that regulate the activity of other endocrine (hormone-producing) glands in distant parts of the body. The pituitary is composed of several distinct parts the anterior lobe (adenohypophysis), a thin intermediate portion (pars intermedia), and a posterior lobe (neurohypophysis). Each has its own characteristic endocrine functions. 

The biosynthesis of the glucocorticoids and mineralocorticoids are regulated by independent mechanisms. The glucocorticoids, such as cortisol, are biosynthesized and released under the influence of peptide hormones secreted by the hypothalamus and anterior pituitary (adenohypophysis) to activate the adrenal cortex (the... 

Growth hormone (GH), somatotropin, somatotropic hormone, STH, a single-chain proteohormone (Mr 21.5 kDa) produced by somatotrophs of the adenohypophysis. GH is an anabolic hormone that regulates the metabolism of proteins, sugars, fats... 

The mechanisms of hormonal regulation provide us with many instances of insertions into new systems. The secretion of milk, due to the biochemical differentiation of one type of Mammalian cell (p. 306) is provoked and controlled by prolactin, resulting from the biochemical specialization of another type of cell, the adenohypophysis. But prolactin is secreted by the adenohypophysis of fish, amphibians and reptiles. Its intervention in the secretion of milk in mammals is thus an insertion into a new biochemical system. 

One of the important aspects of the biochemical evolution of Vertebrates has been the acquisition by the cells of the mesoderm of enzyme systems for the biosynthesis of new types of steroid (heteromorphic evolution). One of the physiological effects of this evolution is the ionic regulation brought about by the action of the corticosteroid hormones at the urinary tube. In the Amphibiae this action is established in conjunction with a pre-existing system, that of the regulation controlled by the adenohypophysis. 

From a pathological perspective, the detailed MS study of pituitary tumors can be used to determine whether proteins that regulate cell growth or hormone secretion are over- or under-expressed in neoplastic tissue, and whether they can be used as blood biomarkers of tumor mass and activity. For example, secreta-gogin, which is important for pancreatic 8-cell insulin secretion, is a newly discovered protein in the normal adenohypophysis it and its mRNA are underexpressed in null cell/gonadotroph pituitary adenomas [17]. Further studies are needed to determine whether secretagogin plays a physiological role in pituitary hormone secretion. 

Corticotropin-Releasing Factor. A hormone-like factor found recently in the midbrain affects the adenohypophysis. This factor, like the hormones of the hypophyseal posterior lobe, is a neurosecretion. Progress has been made in its enrichment it may be related to the hormones of the neurohypophysis. It is not yet clear whether this factor is a link in the chain of the self-regulation of the hypophysis-adrenal cortex system, or whether it merely releases the stress reaction. 

Schreiber, V., 1974, Adenohypophysial hormones Regulation of their secretion and mechanisms of their action, MTP Internal. Rev. Sci., Biochem. of Hormones, Series One 8 61. 

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