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 system, known as endocrine gland cells, act as chemical messengers and perform the role of regulating other cell activities within the body (Hadley, M.E & Levine J.E, 2007 pg. 1-2).

In evolutionary terms, the first evidence of systems working in conjunction with each other to allow for the communication and functional integration between specialized cells, was visible in Metazoa. Metazoa are believed to be the earliest evolved multicellular organisms that make up most of the animal kingdom. The metazoa were the first organisms to posses various layers of specialized tissues or cell groups, evolving after the divergence of plants but before that of the fungi, placing this start of metazoan evolution and consequently endocrine evolution between 2500 and 2100Ma approximately (Hadley, M.E & Levine J.E, 2007 pg. 1-2). Both the nervous and the endocrine system are believed to have evolved alongside each other at the same time, functioning in similar ways by cell to cell signaling from a sender cell that produces and releases the chemical messenger, to a target cell that receives and translates the messenger, generating a response. Despite this similarity however, the two systems perform this cell to cell signaling in contrasting ways; in the nervous system, signaling is sent either from neurone to neurone or from a neurone to a target organ, achieved by the generation of an action potential (a nerve impulse) that propagates along the neuron axons and synapses with adjacent neuron ends. In contrast to this the endocrine system functions differently as the sender cells produce and secrete hormones into the bloodstream which then bind with receptor sites on target cells to generate a response; these responses can be either developmental, physiological or behavioral. An example of such a response would be the secretion of Epinephrine hormone released from the adrenal endocrine glands which binds with receptors present on the liver cells, generating a ‘fight or flight’ response within the body, often characterized by increased blood pressure, a rise in blood glucose levels and increased ACTH hormone secretion (Hadley, M.E & Levine J.E, 2007 pg. 2). More than fifty different hormones exist and are secreted by the vertebrate endocrine system (Hadley, M.E & Levine J.E, 2007 pg. 17). Hormones themselves are chemicals which are categorized into three main groups, lipid (generally steroid), peptide (i.e-protein) and aminederived hormones. The hormones are secreted from specialized endocrine glands which unlike exocrine glands which have specific ducts transporting their secretions to the site of action, are ductless and therefore secrete the hormones directly into the extracellular bloodstream. In vertebrates there are nine major endocrine glands that collectively found the endocrine system; these glands include the pituitary gland, the pineal gland, the hypothalamus, the thyroid gland, the parathyroid glands, the thymus, the adrenal glands, the ovaries (in females) or testes (in males), and the pancreas (Hadley, M.E & Levine J.E, 2007 pg. 17-19).

Endocrine glands arise during the development of all three embryonic tissue layers, the endoderm, the mesoderm and the ectoderm. The type of endocrine...
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