Parathyroid gland  -- secretes the most important hormone regulating blood calcium levels.

Examine a section of parathyroid gland. Note the very thin capsule and fine septa 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.

Principal cells of this gland can be considered antagonists to what other cells?

Endocrine pancreas is next.