Search results
Results from the WOW.Com Content Network
The optimality hypothesis states too much variability in the MHC can result in a failure of T-cells to distinguish themselves non-selves, and thereby increase the risk of autoimmune disease. This would confer greater fitness to individuals without a large degree MHC diversity. [6] [13] Autoimmune diseases are associated with MHC loci. In humans ...
The Organizational-Activational Hypothesis states that steroid hormones permanently organize the nervous system during early development, which is reflected in adult male or female typical behaviors. [1] In adulthood, the same steroid hormones activate, modulate, and inhibit these behaviors.
The ability of a target cell to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormone can bind. Hormone receptors are dynamic structures. Changes in the number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones.
In order to be active, steroid hormones must free themselves from their blood-solubilizing proteins and either bind to extracellular receptors, or passively cross the cell membrane and bind to nuclear receptors. This idea is known as the free hormone hypothesis. This idea is shown in Figure 1 to the right.
Hormone producing cells are found in the endocrine glands, such as the thyroid gland, ovaries, and testes. [10] Hormonal signaling involves the following steps: [11] Biosynthesis of a particular hormone in a particular tissue. Storage and secretion of the hormone. Transport of the hormone to the target cell(s).
Hyperplasia is considered to be a physiological (normal) response to a specific stimulus, and the cells of a hyperplastic growth remain subject to normal regulatory control mechanisms. [5] However, hyperplasia can also occur as a pathological response, if an excess of hormone or growth factor is responsible for the stimuli.
These hormones, unknown at the time of Cannon and Washburn's work, have since been identified as crucial players in the complex system of appetite regulation. Ghrelin, often referred to as the "hunger hormone," is now known to be secreted by the stomach and stimulates appetite, while leptin, produced by fat cells, signals satiety to the brain.
The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated early in embryonic development in mammals. [ 3 ] [ 4 ] The X chromosomes that become inactivated are chosen randomly, except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the X chromosome from the sperm is ...