Size distribution of ferret luteal cells during pregnancy.

Steroidogenic cells in the corpus luteum of the ferret (Mustela putorius) during early (Days 6 and 13) to midpregnancy (Day 24) were characterized using electron microscopy, immunocytochemical localization of neurophysin, and smears of dispersed cells obtained by dissociating luteal cells with collagenase. The latter were stained for 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity, and the diameters of the cells were determined with an ocular micrometer. Very small cells (less than 12 microns) stained negative for 3 beta-HSD, occurred in clumps of 5-50 cells, and were presumed to be primarily endothelial cells. 3 beta-HSD-positive cells covered a wide spectrum of sizes ranging from 14 to 56 microns and did not exist as two discrete populations. The ratio of small (less than 25 microns) to large (greater than 25 microns) cells was 1.86:1.0 on Day 6, with the 17- to 20-microns cell size class predominating. On the day of implantation (Day 13), about 75% of the cells ranged from 26 to 50 microns, with the 29-microns size predominating. By Day 24, the ratio of small-to-large cells had declined to 0.15. Nearly 90% of the cells were in the 26- to 56-microns range, the predominant size being 35 microns. All size classes of luteal cells stained negative for neurophysin on all 3 days of pregnancy studied. Luteal cells obtained on Days 6, 13, and 24 of pregnancy failed to reveal any evidence of mitosis after in vivo or in vitro colchicine treatment. We interpret these results as indicating that the 3 beta-HSD-positive luteal cells of ferrets progressively increase in size as small luteal cells complete their differentiation from granulosa cells and ultimately form larger luteal cells with somewhat different ultrastructural characteristics.