The cycle of the seminiferous epithelium of the stallion was divided into eight stages, using as criteria the presence of meiotic divisions, shape of the spermatid nuclei and location of spermatids with elongated nuclei in the tubule. The mean frequencies of stages 1 to 8 were 16·9, 14·9, 3·2, 15·8, 7·4, 13·5, 12·6 and 15·7%, respectively. The duration of one cycle of the seminiferous epithelium was 12·2 days (S.E.±0·1) as determined by injecting a single dose of 700 μCi of [3H]thymidine into each spermatic artery of six stallions and removing testes at different intervals after the isotope injection. The life-span of primary spermatocytes was 19·0 days, secondary spermatocytes 0·7 days, spermatids with round nuclei 8·7 days, and spermatids with elongated nuclei 10·1 days. Radioactive spermatozoa were observed in the caput epididymidis 35 days after [3H]thymidine injection. The volumetric percentages of testicular components were: spermatogonial nuclei, 0·6; primary spermatocyte nuclei, 4·2; secondary spermatocyte nuclei, 0·1; round spermatid nuclei, 2·1; elongated spermatid nuclei, 1·0; Sertoli cell nuclei, 1·6; tubular cytoplasm, 45·7; lumina, 3·4; basement membranes, 2·6; and intertubular spaces, 38·7%. The seminiferous tubules made up 61·3% of the testicular volume. The diameters of the seminiferous tubules varied significantly among stallions, but not among stages. The average length of the seminiferous tubules per testis was 2419 m (range 1667 to 3726 m).
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