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.
Medullary tumors originate from the
parafollicular cells (C cells) of the thyroid gland.
(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.
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.