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The roe deer shows a distinct seasonal breeding pattern accompanied with significant changes in testicular structure and function during the annual cycle. It serves as a uniquely well-characterized ruminant model system to investigate the regulation of testicular activity. However, data regarding the seasonal variations taking place in the epididymis of the roe buck are not available. Therefore, this study provides a detailed morphological description of the roe buck's epididymis (cell types and segments) and a qualitative as well as quantitative characterization of the seasonal changes in the different parts of the duct. For every second month of the complete seasonal cycle, five roe bucks were castrated (n=30). Seasonal changes in the cellular composition of the epididymis were studied by computer aided image analysis of histological preparations. With regard to morphological criteria we defined 6 segments (S) within the epididymis (ductuli efferentes and S1-5) during the active period. S1-3 are located in the caput, 4 represents the corpus and 5 the cauda epididymidis. The epithelium consists of principal cells, basal cells, macrophages, lymphocytes and apical cells, except for the ductuli efferentes (cuboidal epithelium composed of ciliated and unciliated cells) and S5 (no apical cells). The quantification of the three functional compartments within the organ (lumina, epithelium and interstitial tissue) revealed distinct and region-specific seasonal changes in the cellular composition of caput, corpus and cauda epididymidis. As expected, the duct with its surrounding tissue expands towards rutting season. In the caput this enlargement of the duct is primarily caused by the growth of the epithelial compartment, whereas in the cauda it is predominantly attributed to the dilatation of the lumen, which is filled with testicular and epididymal fluid and spermatozoa towards the rut. This leads to distinct changes in the tissue composition of samples taken from the three main regions of the epididymis at different times of the year. This morphometric study provides the prerequisite for investigations of regulation mechanisms in epididymis function.