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Seasonally regulated breeding is associated with significant changes in testis mass, structure and function. This includes the variation in size, structure and function of the Leydig cells. Recently, interstitial cells have been characterised as a numerically constant population in roe deer. However, no consistent data are available regarding changes in the number of Leydig cells, their differentiation or turnover in seasonally breeding mammals. This study has quantified the numbers of both proliferating and apoptotic cells in roe deer testis bimonthly during a complete annual cycle. Proliferation was detected by immunolocalisation of PCNA and Ki-67 in tissue sections, whereas apoptosis was localised by the TUNEL technique and an antibody to caspase-3. The labelled cells were counted by using a computer-aided image-analysing system. The number of proliferating spermatogenic cells per tubule cross section showed seasonal changes with a maximum in April (14.9+/-0.6) and a subsequent decline up to December (1.6+/-0.3). Percentages of positive cells per square millimetre of interstitial area were below 1% throughout the year. The average number of apoptotic cells per tubule cross section was low and varied only between 0.5 and 1.4 (caspase-3) or 0.1 and 2.1 (TUNEL). In the interstitial compartment, only a few apoptotic cells (<or=0.7%) were found sporadically scattered within the intertubular region during all studied seasonal periods. The results suggest that a constant total number of interstitial cells arise from a conserved cell population of changing functional state rather than from a steady-state population with a definite turnover of cells during seasonal changes in testicular activity.