Might as well admit it: the histology of the parathyroid glands is highly forgettable. I’m not sure why this is – perhaps because we don’t talk about it very often, perhaps because it’s so banal – but I do know that it’s one of those things that needs a lot of repetition to become permanent.
The parathyroid glands are four dinky little things (3-4 mm, about 35 mg each) usually located on the posterior surface of the thyroid gland. They exist in two pairs; the upper pair is derived from the fourth branchial cleft and descends with the thyroid gland, and the lower pair is derived from the third branchial cleft and descends with the thymus. Â Their main function is the regulation of serum calcium levels. They do this by secreting a hormone called parathormone (or PTH) when serum calcium levels go down. PTH does all kinds of things (it activates osteoclasts to chew up bone, increases renal reaborption of calcium, increases renal conversion of vitamin D to its active form, and increases calcium absorption from the gut) but the bottom line is that it raises the serum calcium.
It’s the histology that’s like a blank spot in most medical students’ (and physicians’) minds. The parathyroid is composed of two types of cells: chief cells (small, round, bland cells) and oxyphil cells (large cells with abundant eosinophilic cytoplasm). The chief cells are the secretors of PTH, and they make up the bulk of the cellularity of the parathyroid. Scattered throughout are small islands of oxyphil cells (you can see one at 12 o’clock in the image above). There is a varying proportion of fat, too, that increases with age (like every other part of the body, it seems).
That’s it! Now go memorize it! Maybe this is the final repetition of parathyroid histology that will stick in your brain forever.
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