Cell Biology - Evolution of the Endocrine System

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Vertebrate Biology Essay
Conversion Programme in the Natural Sciences Dr. Colin Stolkin & Professor P.B Gahan

The Vertebrate Endocrine System in specific reference to their role in Human Menstruation, Ovarian and Uterine cycles

Rio Summers April 2009

Word Count: 2693

The vertebrate endocrine system can be best explained as an internal body system collecting multiple organs and glands that produce and regulate hormone secretion into the bloodstream in order to control the many functions of the body. The endocrine system functions alongside the nervous and exocrine system to control and maintain growth, metabolism and sexual development. Hormone substances which are synthesized and secreted from cells involved within the endocrine
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2003, pg. 23). The vertebrate hormones are secreted in response to three types of stimulus. The first stimulus can be the presence or change in concentration of a specific substance in the blood; for example an decrease in blood glucose levels triggers the secretion of glucagon hormone from the ?-cells of the islets of Langerhans in the pancreas. Another stimulus which initiates the release of a hormone, is the presence or change in concentration of another hormone in the blood such as the presence of thyroid stimulating hormone which clearly stimulates the thyroid gland to secrete the amine hormone, thyroxin. The third and final type of stimuli to cause a change in hormone action is by nervous stimulation which involves neurons of the autonomic nervous system and can be clearly illustrated in the secretion of adrenaline from the adrenal gland in

response to fear. (Adds et al. 2003, pg. 23-24). However once the hormone has been secreted and is circulating in the blood stream, it will only affect its target organ due to the presence of specific receptor sites or receptor molecules on the cell surface membrane of the target organ or cell. Peptide hormones form hormone-receptor complexes with the proteinbased receptors on the plasma membrane of the

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