Thyroid Gland
Thyroid Gland is a dumbbell shaped, bilobed gland located in the thoracic. region at the root of the throat. The two lobes are almost symmetrical and lie one on either side of the tracheal tube. Each lobe measures about 5x 2x 2 cm. The two lobes are Joined together by a narrow strip of tissue. called isthmus or middle lobe, which crosses the 2- 4th tracheal rings. In some cases, the right lobe is bigger than the left lobe. A pyramidal lobe varying in size extends from the isthmus upward in the neck. Accession thyroid bodies are located beneath the main thyroid gland.
Morphology
The weight of the thyroid gland of the adult varies between 20 to 25 gm and is influenced by diet, age, sex and reproductive state of the individual Thyroid gland is highly vascular. Blood is supplied by the paired superior and inferior thyroid arteries and directly from the aorta. Venous blood is collected by internal jugular and innominate veins. Blood flows at the rate of 4 6 ml per gland per minute. This high rate of blood flow ensures adequate supply of inorganic iodine to the gland. Thyroid gland is innervated by sympathetic fibers derived from the superior, middle and inferior cervical ganglia and parasympathetic fibers derived from the superior and inferior laryngeal branches of the vague nerve. These nerves control the blood supply to the gland.
Development
Thyroid gland is endodermal in origin arising from the primitive foregut During embryonic development, the primitive thyroid gland arises as out pushing from the middle of the neck Infront of the thyroid cartilages Thyroid gland is derived from the fourth pharyngeal pouch, which gives reset the lateral lobes of the gland. During intra-uterine development, thyroid gland can be recognized in the seventh week. The follicular structure of the gland is evident by 12 -14th week of the fetus. Gradually, with the development of the gland, it auguries the capacity of concentrating iodine from the circulating blood and other functions of synthesis and secretion follows.
Histology: -
Microscopic examination of the sections of the thyroid gland reveals the presence of numerous acini or follicles about 200 u in diameter. The number of the follicles in a normal gland is about 100, 000.The size of the follicles also varies. Large follicles are found near the periphery of the gland while smaller follicles are arranged at the center. Each follicle is made up of a Cuboidal follicular epithelium. In the resting condition, the epithelium is low, but the height is dependent upon Under the extent of stimulation of the gland. stimulation by thyroid stimulating hormone the low cuboidal epithelium becomes converted into tall columnar epithelium. The thyroid follicles are usually spherical or oval in shape and measure about 20- 150 diameter. A single layer of cubical follicular epithelial cells lines each follicle. The follicular cells are surrounded by a basement membrane consisting of fine connective tissue fibers on which rest the bases of the follicular cells. Follicles are surrounded by a highly vascular stroma containing Channels and nerve endings. The follicular lumen is filled with a colloidal material, the thyroglobulin. Thyroglobulin constitutes about 75% of the colloid material, which is the main storage form of the thyroid hormones.
Each follicular cell is loaded with of granular cytoplasm containing large mitochondria and distinct Golgi apparatus. The luminal end of each follicular cell facing the colloidal material is thrown into a number of microvilli. The nucleus is situated at the bases of the follicular cell and its cytoplasm is filled with well-developed rough endoplasmic reticulum Golgi complex. In addition to these, large number of cytoplasmic vesicles filled with the colloid material are also found in the cytoplasm. Based the staining properties, the cytoplasmic vesicles can be distinguished by three types, eosinophile or acidophilic, basophilic and mixed tow type of follicular epithelial cells have been distinguished on the basis of electron microscopic and histochemical studies. The first are the principal cells present in large number and contain small number of mitochondria, and different proteases and oxidases required for the synthesis of thyroid hormones. The second type of cells occurs in small number but contain large number of mitochondria and these are scattered in between the principal cells. These cells are known as parafollicular cells or C-cells those secrete thyrocalcitonin.
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