Immunolocalization of progesterone receptors in the canine ovary and theirrelation to sex steroid hormone concentrations

The aim of the present study was to describe the normal cellular distributi on of progesterone receptors in the canine ovary at different stages of the oestrous cycle. Samples of both ovaries were obtained from 75 healthy adul t bitches of various breeds and ages, including five pregnant bitches and t hree bitches that had just delivered. The presence of progesterone receptor s was visualized by immunohistochemistry on paraffin wax sections using a m onoclonal antibody. Nuclear staining for progesterone receptors was observe d in the surface epithelium, cortical tubules, rete ovarii, follicle cells, thecal cells, luteal cells, granulosa cell cords and ovarian stroma. The s taining intensity for progesterone receptors in the follicle cells increase d with the stage of follicle development, indicating an intrafollicular rol e of progesterone in the mechanism of ovulation and luteinization. The stro nger staining intensities for progesterone receptors in thecal cells compar ed with follicle cells may be explained by the fact that thecal cells media te some effects of steroid hormones on the follicle cells in secondary and tertiary follicles. Little correlation was found between the expression of progesterone receptors in follicle cells and oestradiol, progesterone or te stosterone concentrations. This finding indicates a different regulating me chanism for progesterone receptors in canine ovarian follicles compared wit h other tissues of the genital tract. During pregnancy all groups of ovaria n cells had lower staining intensity scores than during the oestrous cycle, although the sex steroid hormone concentrations in pregnant bitches were s imilar to those in non-pregnant bitches during the luteal phase of the oest rous cycle. The lower expression of progesterone receptors during pregnancy may be due to higher tissue concentrations of progesterone that are not re flected in the serum because of haemodilution and increased metabolism and clearance during pregnancy.