Parathyroid adenomas are usually composed of highly cellular sheets of chief cells (left half of the image). Other cell types that are frequently seen include oxyphil cells (right half), water-clear cells, and transitional cells. The chief cells have amphophilic or lightly eosinophilic cytoplasm and contain prominent fat droplets. The oxyphil cells contain abundant granular eosinophilic cytoplasm and small pyknotic nuclei. The cytoplasmic appearance is secondary to abundant mitochondria contained by these cells.