Progesterone receptors and proliferating cell nuclear antigen expression in equine luteal tissue

Steroid hormones act via specific receptors, and these play an important physiological role in the ovary. The objective of this study was to evaluate the cellular distribution of progesterone receptors and their staining intensity in different equine luteal structures during the breeding season, as well as their relationship to luteal cell composition, cell proliferation pattern and plasma progesterone (P4) concentration. There was an increase in proliferating cell nuclear antigen (PCNA) expression in large luteal cells from the corpus hemorrhagicum (CH) to mid-luteal phase, followed by a decrease toward the late luteal stage. In the CH, the number of large luteal cells was lower than in other structures. Only large luteal cells showed positive staining for P(4) receptors. An increase in staining intensity for P(4) receptors was observed between CH and mid-phase corpus luteum, and CH and late-phase corpus luteum. Synthesis of P(4) started at a very early stage of the luteal structure and was accompanied by an increase in P(4) receptors and PCNA expression, and proliferation of large luteal cells, until mid-luteal phase. These data suggest that large luteal cells might play an important role in the regulation or synthesis of P(4) in equine luteal structures.