Structural aspects of luteal function and regression in the ovary of the domestic dog

Corpora lutea (CL) in various stages of regression were collected from 52 cross-bred bitches. Plasma progesterone concentrations were measured by RIA and various morphological and histochemical parameters of the regressing CL were studied. These included overall glandular and cellular dimensions; the processes of lipid accumulation and pigmentation; and the activities of the enzymes succinic dehydrogenase, non-specific esterase, and acid phosphatase. The ultrastructural appearance of the luteal cells in the various stages of regression were also described. Four of these stages, i.e. R1, R2, R3 and R4, approximately 2 months apart and beginning at 60 days after ovulation, were recognized. The first 3 were typical of the phases of regressive metoestrus, early anoestrus and late anoestrus, respectively, using conventional terminology. By R4 a new cycle had begun. The mature secretory stage with zero regression (R0) was used as a baseline whereby degrees of regressive change in the other stages could be judged. On this basis, R1 was considered a stage representing functional luteolysis and early lipid accumulation and R2 represented a lag phase in the regressing CL with ongoing lipid accumulation but the luteal cells remaining relatively intact. These regressive changes had intensified by R3, with luteal cells becoming autolytic and the resulting cell debris being removed by invading macrophages. By R4, the macrophages had fused forming giant cells and the corpus albicans had the appearance of a non-immunologically dependent granuloma.