The small luteal cell of the sheep.

Corpora lutea of sheep were examined by electron microscopy at day 10 of the oestrous cycle and at days 15, 25, 50, 100, 125 and 140 of pregnancy. Small luteal cells were present in all corpora lutea, and were two to three times as numerous as large luteal cells. The former were irregular in shape, with tapering cytoplasmic processes. Their major cytoplasmic organelles were a predominantly smooth endoplasmic reticulum, mitochondria with tubular and lamellar cristae, and one or more Golgi complexes. The enzyme delta 5-3 beta-hydroxysteroid dehydrogenase was present in their cytoplasm. Small luteal cells were often interposed between large luteal cells and capillaries, and formed close, complex surface relationships with large luteal cells. Small and large luteal cells differed in many ways, including the restriction of numerous approximately 0.2 micron cytoplasmic granules to the large cells, and no cells of intermediate structure were observed. These features of small luteal cells suggest a steroid hormone synthetic function, and direct interaction with large luteal cells.