Ultrastructure of the corpus luteum of the white‐tailed deer during pregnancy

The fine structure of corpora lutea from 14 white‐tailed deer was studied from early through near‐term pregnancy. The corpora contained both thecal and granulosal lutein cells. The small, elongate and cylindrical thecal lutein cells contained many lipid droplets, juxtanuclear Golgi elements, abundant agranular endoplasmic reticulum, and other cytoplasmic organelles. These cells were observed throughout pregnancy. Some of the thecal lutein cells became modified during early to midpregnancy. The modified thecal lutein cells possessed many lipid droplets, several lysosomes, packets of PAS‐positive glycogen granules, and numerous small membranous whorls of agranular endoplasmic reticulum. The granulosal lutein cells had highly folded and/or ruffled plasma membranes, abundant tortuous tubular and cisternal agranular endoplasmic reticulum, numerous rod‐shaped to round mitochondria with tubular and lamellar cristae, Golgi elements, and a few packets of granular endoplasmic reticulum. Many granulosal lutein cells became modified during mid‐ to near‐term pregnancy by the addition of numerous small membrane‐bound osmiophilic droplets and variable numbers of large nonmembrane‐bound lipid droplets. The granulosal and thecal lutein cells appeared more active in steroid biosynthesis than the modified granulosal and thecal lutein cells.