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Electron micrograph (EM) of follicular epithelia of thyroid of goat (1-2 years) showing cuboidal and columnar cells (F) and parafollicular cell (P) and luminal colloid (L).
However several unusual variants of MTC including encapsulated tumors, anaplastic variants, oxyphilic variants, tumors with mixed follicular and parafollicular cell differentiation as well as melanin producing MTCs have been reported.
(1) C cells are located between the follicular epithelial cells and their basement membranes and, hence, are also referred to as parafollicular cells, a term first coined by Jose Fernandez Nonidez.
Five pairs of pharyngeal pouches lined by the endoderm give rise to the middle ear cavity, auditory tube, tympanic membrane (first one), palatine tonsils (second one), thymus and inferior parathyroid glands (third one), superior parathyroid glands (fourth one) and finally, parafollicular cells of the thyroid gland (fifth one).
These are follicular cells and parafollicular cells. Major portion of thyroid gland is made up of follicular cells and these cells secrete iodine-containing hormones called thyroxine (T4) and triiodothyronine (T3).
Neoplastic transformation can occur in either the follicular or parafollicular cells of the gland.
Calcitonin is a 32-amino acid polypeptide hormone produced by the parafollicular cells of the thyroid gland whose secretion is mainly regulated by serum calcium levels.
The second group consists of carcinoma arising from parafollicular cells of ultimobranchial body of neural crest, which is the medullary carcinoma.
They can be divided into four categories: (i) the medullary carcinoma (which is derived from parafollicular cells), (ii) anaplastic or undifferentiated carcinoma, (iii) follicular thyroid carcinoma (FTC), (iv) and papillary thyroid carcinoma (PTC).
The second hormone is calcitonin (CT) which originates from the C cells or parafollicular cells of the thyroid and thymus glands.
It is made up of three principal components; the lining follicular cells the basal parafollicular cells and the luminal colloid.