The deep anoestrous phase in winter is part of the anovulatory season in mares and is bordered by the autumn and spring transitional periods (ATP/STP). To define an annual time span for effective prognostic biopsy sampling, the aim of this study was to provide a morphofunctional characterization of the endometrium during ATP and STP. To outline both transitional periods, endometrial specimens were taken in September, October and November (n = 76) as well as February, March and April (n = 184) with the requirement of a detailed clinical documentation. Tissue samples were examined histologically with special emphasis on the functional endometrial morphology. Additionally, an immunohistochemical evaluation was performed on selected specimens regarding the expression of oestrogen receptor α, progesterone receptor and Ki67-antigen. An absent to low endometrial activity was ascertained in more than 60% of all specimens from late October onwards, whereas a comparably lacking or low activity in STP was observed until early April. Approximately 30% (ATP) to 22% (STP) of all samples exhibited a predominantly "irregular" endometrial differentiation. During the transitional periods, the clinically evaluated ovarian status (transrectal palpation, transrectal ultrasonography and/or serum progesterone and oestrogen analyses) and the endometrial functional morphology were in accordance with approximately 70% of all cases. The expression of steroid hormone receptors and Ki67-antigen was generally low. Given that endometrial maldifferentiations were frequently found during ATP and STP, its occurence might display a characteristic and physiological feature of the transitional periods. Regarding the functional endometrial morphology, a diagnostic biopsy sampling should therefore be performed between late April and before September.

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