Seasonal or pathological findings? Morphofunctional characteristics of the equine endometrium during the autumn and spring transition

Abstract

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.

Microscopic examination of endometrial biopsies of retired sports mares: An explanation for the clinically observed subfertility?

“…Only during the physiological breeding season (15th March-15th September, in the northern hemisphere) are the behavioral, ovarian, and endometrial activities synchronized to allow a successful mating (Aurich, 2005;Sharp, 2011). Endometrial inactivity is commonly observed during winter anestrus and in the beginning of the transitional period in spring, whereas an irregular glandular differentiation may occur during the transitional cycles (Killisch et al, 2014). If these findings are diagnosed during the breeding season, they represent a pathological alteration associated with a permanent or temporary reduced fertility .…”

Endometrial biopsies of old mares – What to expect?!

“…Functional disturbances may lead to permanent or temporary infertility depending on the individual cause (Schoon et al 1999b, Ellenberger et al 2005. Their prognostic significance during winter anoestrus (Aupperle et al 2003) and transition periods (Killisch et al 2017) is questionable whereas they are an important finding with impact on the fertility of the mare in the breeding season. There is no association between the occurrence of functional disturbances and the age of the mare (Schoon et al 1999b, 2000, Schilling 2017).…”

The Healthy and Diseased Equine Endometrium: A Review of Morphological Features and Molecular Analyses

“…The activity of the hypothalamic-pituitary gland-ovarian axis gradually increases during STP and decreases during ATP [35]. Accordingly, biopsies collected during early, mid-, and late STP show a gradual increase in glandular activity [9], whereas those obtained during early, mid-, and late ATP display a successive decrease in glandular activity [9]. An irregular differentiation during the transitional periods is observed in 22-30% of the biopsies and is regarded as a physiological finding likely evoked by irregular ovarian cycles [9].…”

“…Endometrial diseases of mares are an important cause of subfertility. These include endometrosis, endometritis, glandular differentiation disorders, and angiosis (synonym: angiosclerosis) [1][2][3][4][5][6][7][8][9]. Individual mares often display the concurrent presence of several endometrial diseases [1,3].…”

“…Except for suppurative endometritis, endometrial diseases can only be revealed by the histopathological examination of an endometrial biopsy [1,3]. Importantly, the endometrial biopsy of a mare should be interpreted under consideration of signalment, clinical history, and season of the year [3,7,9]. Fertility-influencing factors include the age of the mare [7], the length of barrenness [1,7], and a previous long-term use of the mare in athletic performances [13].…”