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Endocrine System

Objective :

  • To learn the procedure for collection of different parts of endocrine system
  • To learn the structure of the endocrine system of rat

Theory :

            All vertebrates (animals with backbones) have a well-developed and highly organized endocrine system. The system consists of the following glands: the pituitary, the pineal, the thyroid, the thymus, the pancreas, a pair of adrenals (each adrenal actually acts as two glands-the adrenal cortex produces unique hormones and functions independently of the adrenal medulla), a pair of parathyroids, and a pair of ovaries or testes. Early research in physiological psychology focused on the nervous system, but it soon became evident that the endocrine system also influenced behavior and that the effects of the two systems were interrelated contributors to behavior. The endocrine system essentially consists of ductless glands that produce chemical substances called hormones. The hormones elicit physiological reactions, either locally or at some distant target site. When acting at a distance, the hormones travel to the site by way of the circulatory system. The endocrine glands perform a wide variety of functions and are grouped into a common system because of the similarity in their mode of secretion, which entails secretion into the blood stream rather than through a system of ducts. The pituitary gland develops from a dorsal evagination of the oropharyngeal ectoderm (Rathke's pouch or craniphayngeal duct) for the adenohypophysis, and a ventral downgrowth of diencephalicneuroectoderm for the neurohypophysis. The pituitary gland, also known as the hypophysis, may be divided into two major compartments: the anterior adenohypophysis composed of the pars distalis, pars tuberalis, and pars intermedia; and the posterior neurohypophysis, which includes the pars nervosa or infundibular process, infundibulum infundibular stem, and tuber cinerreum.

            The pituitary lies within the sellaturcica of the sphenoid bone, and receives blood via the posterior and anterior hypophyseal arteries, which originate from the internal carotid arteries. Both oxytocin and vasopressin (antidiuretic hormone (ADH) are synthesized in supraoptic and paraventricular nuclei as large precursor molecules, which contain both active hormones and their associated neurophysins. The thyroid gland originates as a thickened plate of epithelium in the floor of the pharynx. It is intimately related to the aortic sac in its development, and this association leads to the frequent occurrence of accessory thyroid parenchyma in the mediastinum. The thyroid gland is the largest of the organs that function exclusively as endocrine glands. The basic structure of the thyroid is unique for endocrine glands, consisting of follicles of varying size (20-250 μm) that contain colloid produced by the follicular cells. C-cells or parafollicular cells of the thyroid gland, named after their major secretory product (calcitonin), are located either within the thyroid follicles between the basal regions of the follicular cells and the basement membrane of the follicle, or in an interfollicular location. The pancreas consists of two functionally and anatomically separate parts. The main body of the gland that secretes digestive enzymes into the duodenum by way of one or more pancreatic ducts and the endocrine portion of the gland which secretes hormones into the blood stream. The latter portion consists of many small Islets of Langerhans, scattered throughout the gland.

            In the rat, the gonads produce sex hormones. In the male, the testes contain glandular interstitial tissue in addition to the sperm producing seminiferous tubules. The interstitial cells function in the synthesis of the male sex hormone, testosterone. In the female, the ovarian follicles produce estrogens. These follicles, which also contain the eggs, rupture when an egg becomes ripe. The old follicle then develops rapidly into a new structure, the corpus luteum, which also secretes hormones. Phylogenetically, parathyroids first appear in amphibians, coincident with the transition from an aquatic to a terrestrial life. Since the calcium ion plays a key role in many fundamental biological processes, the precise control of calcium ions in extracellular fluids is vital to the health of humans and animals. To maintain a constant concentration of calcium, endocrine control mechanisms primarily consists of the interaction of parathyroid hormone (PTH), calcitonin, and vitamin D. Parathyroid glands in most animal species consist of two pairs of glands situated in the anterior cervical region. Rats are the exception, because they have a single pair of parathyroid glands that are located close to the thyroid. Embryologically they arise from the third and fourth pharyngeal pouches, in close association with the primodria of the thymus. The entodermal bud that forms the thyroid gland arises on the midline at the level of the first pharyngeal pouch. This gives rise to the thyroglossal duct that migrates caudally. The adrenal medulla constitutes approximately 10% of the volume of the adrenal gland. The bulk of the medulla is composed of chromaffin cells, which are the sites of synthesis and storage of catecholamines.

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