Parathyroid gland (Fig. 20-22) -- secretes the most important hormone regulating blood calcium levels.

Examine a section of parathyroid gland (slide 69). Note the very thin capsule and fine septa (Fig. 20-23) and identify the clumps of chief (principal) cells which secrete PTH and of oxyphil cells which are packed with large mitochondria but whose function remains unknown. (Some examples of slide 69 also have thyroid and thymus tissue.)

Explain how the parathyroid glands are related functionally and embryologically to the parafollicular cells of the thyroid gland.

Adrenal gland -- consists of two endocrine tissues (cortex and medulla) that are functionally distinct (Fig. 20-14).

Examine adrenal glands on slides 20, 75 and 143. These are flattened glands with concentric layers of secretory tissue (Fig. 20-14). In the distinct capsule associated with adipose tissue, notice the small arteries entering the gland and the vascular plexus just inside the capsule (Fig. 20-15).

On the same two slides (20 and 143), examine the cortex and identify the three layers or zones in which the steroid-secreting cells have slightly different arrangements, with groups of cells separated by fine, well-vascularized support tissue septa. The outermost zona glomerulosa (Fig. 20-15) has cells secreting mineralocorticoids arranged in irregular rounded clumps ("glomeruli"). The middle and widest layer, the zona fasciculata (Fig. 20-15), has cells making glucocorticoids arranged in strands ("fascicles"). The innermost layer, the zona reticularis (Fig. 20-15), shows irregular, branching ("reticular") cords of cells secreting small quantities of sex hormones (androgens).

Study the electron micrographs of steroid-secreting cells in Fig. 20-13.

Why do steroid-secreting cells stain poorly in routine light-microscope staining procedures?

Addison's disease and beyond.