The diagram represents the path traversed in the neck by the developing thyroid gland embryo logically. Which point along this path is the commonest site of a Thyro-glossal cyst?
Ans. C. Point III.Embryology of the Thyroid Glanda. The thyroid gland begins to develop during the third week as an endodermal thickening in the midline of the floor of the pharynx between the tuberculum impar and the copula. Later, this thickening becomes a diverticulum that grows interiorly into the underlying mesenchyme and is called the thyroglossal duct. As development continues, the duct elongates, and its distal end becomes bilobed. Soon, the duct becomes a solid cord of cells, and as a result of epithelial proliferation, the bilobed terminal swellings expand to form the thyroid gland.b. The thyroid gland now migrates interiorly in the neck and passes either anterior to, posterior to, or through the developing body of the hyoid bone. By the seventh week, it reaches its final position in relation to the larynx and trachea. Meanwhile, the solid cord connecting the thyroid gland to the tongue fragments and disappears. The site of origin of the thyroglossal duct on the tongue remains as a pit called the foramen cecum. The thyroid gland may now be divided into a small median isthmus and two large lateral lobes.c. In the earliest stages, the thyroid gland consists of a solid mass of cells. Later, as a result of invasion by surrounding vascular mesenchymal tissue, the mass becomes broken up into plates and cords and finally into small clusters of cells. By the third month, colloid starts to accumulate in the center of each cluster so that follicles are formed. The fibrous capsule and connective tissue develop from the surrounding mesenchyme.d. The ultimobranchial bodies (from the fifth pharyngeal pouch) and neural crest cells are believed to be incorporated into the thyroid gland, where they form the parafollicular cells, which produce calcitonin.e. Agenesis of the Thyroid: - Failure of development of the thyroid gland may occur and is the commonest cause of cretinism.f. Incomplete Descent of the Thyroid: - The descent of the thyroid may be arrested at any point between the base of the tongue and the trachea. Lingual thyroid is the most common form of incomplete descent. The mass of tissue found just beneath the foramen cecum may be sufficiently large to obstruct swallowing in the infant.g. Ectopic Thyroid Tissue: - Ectopic thyroid tissue is occasionally found in the thorax in relation to the trachea or bronchi or even the esophagus. It is assumed that this thyroid tissue arises from entodermal cells displaced during the formation of the laryngotracheal tube or from entodermal cells of the developing esophagush. Persistent Thyroglossal Duct: -Conditions related to a persistence of the thyroglossal duct usually appear in childhood, in adolescence, or in young adults.i. Thyroglossal Cyst: - Cysts may occur at any point along the thyroglossal tract They occur most commonly in the region below the hyoid bone. Such a cyst occupies the midline and develops as a result of persistence of a small amount of epithelium that continues to secrete mucus. As the cyst enlarges, it is prone to infection and so it should be removed surgically. Since remnants of the duct often traverse the body of the hyoid bone, this may have to be excised also to prevent recurrence.j. Thyroglossal Sinus (Fistula): - Occasionally, a thyroglossal cyst ruptures spontaneously, producing a sinus. Usually, this is a result of an infection of a cyst. All remnants of the thyroglossal duct should be removed surgically.
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Head & Neck