The corpus luteum of the domestic cat: Histologic classification and intraluteal hormone profile

In the present study, we aimed to histologically stage and characterize the structural life span of the CL in the domestic cat. Moreover, the intraluteal levels of progesterone and estrogens were determined throughout the pregnant and nonpregnant (pseudopregnant, PP) luteal phases. On the basis of observed histomorphology of CL, the following stages were identified: CL formation (preimplantation period, PP1), development/maintenance (Days 10-36 of pregnancy, PP2), early regression (Days 38 and 39 of pregnancy, PP3), late regression (Day 48 of pregnancy, PP4), and corpus albicans. The main cellular markers included luteal cell shape, the type and degree of vacuolation, nucleus condition, and the ratio of nonsteroidogenic to luteal cells. Intraluteal levels of progesterone and estrogens differed significantly throughout stages of pseudopregnancy (P < 0.01, progesterone; P < 0.0001, estrogens). The progesterone level in PP2 was higher compared with PP3 and PP4 (P < 0.05). The estrogen level in PP1 was higher compared with PP2 (P < 0.05), PP3 and PP4 (P < 0.005), as well as in PP2 compared with PP3 and PP4 (P < 0.005). The staging of the domestic cat luteal phase established here provides a basis for further research on feline luteotropic and luteolytic factors. These data will contribute to comparative studies in felids.