الفهرس 
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Abstract
The thyroid gLand is one of the largest endocrine organs, it weighs about 15-20 grams and extends from 5 th tracheal ring inferioly to the side of the thyroid cartilage superiorly. The thyroid gland is a highly vascular bibbed gland with an isthmus which lies over the second and third cartilagenous rings of the trachea (Romanes, 1971, Johannes, 1974 and Duplessis, 1975). It is coverd by two capsules; the outer one is continuous with and is part of the pretracheal fascia, wherease the inner capsule is regarded as the true one. It is formed of a delicate fibroelastic connective tissue and sends septa into the gland to provide internal support and carry blood supply, bymphatics and nerves into its substance. These septa divide the gland into incomplete lobules, composed of spherical structural units called thyroid follicles (Junq tie ira and Carneiro, 1980).
The thyroid follicles represent both the structural and functional units of the thyroid gland and are essentially storage compartments. Their walls are made up of simple cuboidal follicular epithelial cells. The foblicular cavity is usually filled with PASpositive acid ophilic secretory glycoprotein known as thyroglobulin. Normally, the follicles vary from being irregularly rounded to tubular in shape measuring from 50-500 mm in diameter (Arey, 1974 and Willams, 1981). There are about 30 million follicles in the human thyroid gland packed together in a delicate reticular network that contains an extensive capillary bed. Each follicle is surrounded by a basement membrane allowing communications between the adjacent follicles (Williamson and Pearse, 1923 and Fujita, 1975).
There are two types of epithelial cells in the thyroid follicles, the principale follicular cells and the C- cells or parafollicular cells. The follicular cells are generally cuboidal in the normal gland having a height of 14-15 mm. They have their apical ends facing towards the follicular cavity and their bases resting on a thin basement membrane.
The cell boundaries are distinct and fairly obvious under the light microscope. The principal cells become low cuboidal or even squamous in the relatively inactive gland. They enlarge and become tall columnar cells during periods of increased activity (Tom onari, 1959; Doniach, 1967).
The cytoplasm of the follicular cells is highly basophilic, this basophilia increases with the activity of the cells. The cytoplasm also contains lipid droplets and PAS-positive granules (Waterman, 1956, Weiss and Greep, 1977 and Leeson and Leeson, 1981). It was found that there is an additional type of cells occasionally seen, the colloid cell of L.angendroff; which are slender cells with darkly stained cytoplasm that often appears to be filled the human thyroid gland packed together in a delicate reticular network that contains an extensive capillary bed. Each follicle is surrounded by a basement membrane allowing communications between the adjacent follicles (Wiltiamson and Pearse, 1923 and Fujita, 1975).
There are two types of epithelial cells in the thyroid follicles, the principale follicular cells and the C- cells or parafollicular cells. The follicular cells are generally cuboidal in the normal gland having a height of 14-15 mm. They have their apical ends facing towards the follicular cavity and their bases resting on a thin basement membrane.
The cell boundaries are distinct and fairly obvious under the light microscope. The principal cells become low cuboidal or even squamous in the relatively inactive gland. They enlarge and become tall columnar cells during periods of increased activity (Tom onari, 1959; Doniach, 1967).
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