Histology of the Normal Thyroid Follicle

The thyroid gland itself consists of numerous functional units or follicles of varying size, separated by connective tissue (Fig. 15.7b). Each follicle is spherical and lined by epithelial cells surrounding a central colloid space. The base of each cell is adjacent to a capillary, and the apex consists of microvilli pointing into the colloid. 
The parafollicular or C-cells of ultimobranchial body origin secrete calcitonin. These cells are present between the follicles and usually do not touch the follicle lumen.