Thyroid gland -- secretes a hormone regulating the basal metabolic rate and a hormone regulating blood calcium levels.

Examine a section of thyroid gland attached to trachea on slide 2. Identify thyroid follicles, composed of follicle cells in a simple cuboidal epithelium, which are organized into lobes by supporting tissue septa continuous with the thin capsule (Fig. 20-19). Examine the electron micrograph of a small thyroid follicle in Fig. 20-20.

Explain how thyroid follicle cells produce thyroglobulin, store it, convert it to iodine-containing thyroid hormones for secretion.

Clinical note: Increased height of thyroid follicular cells and infoldings of the follicle wall are indications of hyperthyroidism or Grave’s disease, a disorder characterized by restlessness, sleeplessness, tremor, and a noticeable exophthalmos. Treatment is by subtotal thyroidectomy or use of radioactive iodine to destroy or inactivate thyroid follicles.

Identify the calcitonin-secreting, pale parafollicular cells (or clear cells; Figs. 20-19 and 20-20), which are closely associated with the thyroid follicles but are not usually in contact with their lumens. With the light microscope, parafollicular cells can be very difficult to distinguish from follicle cells.

How do the origins of these two cell types differ embryologically?

Why do you think parafollicular cells stain much less heavily than follicular cells?

Parathyroids are next.