Seasonal changes in histological-morphometric parameters of testes in the European brown hare

The European brown hare is characterised by a breeding season from January to August and a nonbreeding period between September and December. We studi ed the changes in testicular mass and histology associated with the annual transitions between activated and arrested spermatogenesis. Histological pr eparations were carried our from three adult males per month from May 1994 until February 1995. Morphometric measurements of the seminiferous tubules and the interstitial Leydig cells were performed including computer-assiste d image analysis. Testes mass showed the typical seasonal periods of growth and involution, with average values between 10.66 and 12.03 g from Februar y to July and the lowest weight in October (2.06 +/- 0.30 g). Testes growth was reactivated in December. The ratio of testes and body weight showed th e same circannual course as testes weight. Changes in testes mass correspon ded with variations in diameter and cell content of the seminiferous tubule s. The tubular diameter and epithelial height were low in October (21.4 +/- 4.4 mum and 5.8 +/- 0.5 mum. respectively) and increased to the highest va lues in May (45.7 +/- 2.3 mum and 18.4 +/- 0.6 mum) Differentiation of sper matogenic cells in the seminiferous epithelium cycle stage VIII showed a ce ll type specific seasonal pattern. In January, even before the distinct inc rease of sperm production to the high level from February to May, the numbe r of spermatogonia per tubulus cross-section was markedly higher than durin g other periods (32.5 +/- 1.3 cells). The numbers of spermatocytes and sper matides reached their maximum in May; then the proportion of spermatocytes in the Zygotene stage was 47,5 +/- 2,1 % of the total cells. Subsequently n umbers of all spermatogenic cells declined to the nadir in October. The Ley dig cells and their nuclei were also significantly larger during the breedi ng season compared to the non-breeding season. The results demonstrate that the regulation of seasonal testicular activity is characterised not only b y pronounced changes in total mass, bur also in the size and composition of compartments containing spermatogenic and steroidogenic cells.