Quantitative comparison of luteal histology in the rat and rabbit: changes from mid- to late gestation

Progesterone secretion by the corpora lutea (CL) of rats and rabbits declines from a peak, at about Day 16 of gestation, to near term (Day 22 rats and 28 rabbits). However there are major differences between the two species in CL growth and blood flow over this period. In the present work quantitative histological measurements were made of CL at these stages to examine the accompanying structural changes. Eight rats and five rabbits were examined at each stage: standard morphometric techniques were used. There was gross discrepancy between the two species in the histology of their CL at peak secretory activity. Although the proportions of the major tissue components were similar, the rabbit luteal cell (52 pl) was five times larger than that in the rat (9 pl). There was substantially less vascular and interstitial space in the rabbit, all characteristics which might affect transport processes between luteal cell cytoplasm and blood. Over the period examined, there was no change in CL volume in the rat but a 37% reduction in the rabbit due to loss of luteal cells. The vascular space in the rat, however, was reduced, whereas that in the rabbit declined only in proportion to the overall decrease in CL volume. These results show that structurally there is a substantial difference in patterns of early regression between the two species which reflect different mechanisms involved.