The Thyroid Gland

Identification: The thyroid gland is the only slide you will see in which large circular structures (follicles, 1) are filled with a uniformly pink staining substance, the colloid (2) (in other tissues with such circular structures they appear empty inside).

• Each circular structure is called a thyroid follicle (1).
• The pink substance within each follicle is the colloid (2), composed of the protein thyroglobulin.
• Each follicle is surrounded by a layer of simple cuboidal epithelium (3).
• In the spaces between the follicles are Parafollicular or C-cells (4)

• Follicles: Thyroid hormones (T₃ & T₄).
• Parafollicular cells: Calcitonin