The thyroid gland is
composed of numerous spherical follicles.
Principal cells in the wall of each follicle
produce thyroid hormones and secrete
them on demand into capillaries.
(a) Ventral view of larynx and trachea
of a dog showing paired thyroid and
parathyroid glands. (b) Enlarged
histological section of the thyroid
illustrates follicles and colloid that fills the
lumina. (c) Cutaway view of a single
thyroid follicle showing the arrangement
of principal cells and parafollicular cells
(C cells) composing the follicular wall.
Note the nerve supply and capillaries
embracing the basal regions of these cells.