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The Pennsylvania State University,4, University Park 16802
3 Reprint requests to R. P. Amann at the Animal Reproduction Laboratory, Colorado State Univ., Ft. Collins 80523.
Abstract
Fifty-two Holstein bulls were castrated at 12, 16, 20, 24, 28 or 32 weeks to (1) determine whether testicular development is influenced by season of birth (April or October) and (2) examine quantitatively the establishment of spermatogenesis. Season of birth (spring or fall) did not influence (P>.05) either testis weight at castration or the establishment of spermatogenesis. With age held constant, scrotal circumference and testis weight were correlated (.79; P<.05). Testis weight increased from 9 ± 1 g at 12 weeks to 117 ± 10 g at 32 weeks of age. Seminiferous tubules occupied 44% of the testicular parenchyma at 12 weeks and 81% at 32 weeks. Differentiation of basal indifferent supporting cells to Sertoli cells started at 20 weeks, and formation of Sertoli cells was completed near 28 weeks. Gonocytes predominated at 12 weeks, but by 20 weeks most had been replaced by prespermatogonia and A-spermatogonia. Thus, transformation from a prepubertal testis to a pubertal testis containing Sertoli cells and A-spermatogonia occurred between 16 and 24 weeks (testis weight = 18 to 42 g). Preleptotene to diplotene spermatocytes appeared sequentially between 16 and 28 weeks and their numbers increased through 32 weeks. Significant numbers of pachytene spermatocytes and spermatids were not present before 28 and 32 weeks, respectively. Establishment of spermatogenesis was completed by 32 weeks (testis weight 
80 g). Testes weighing less than 80 g were producing few if any sperm. Thus, the establishment of spermatogenesis was progressive, associated with more than a fourfold increase in testis weight, and occurred over a period of about 16 weeks.
1 Authorized for publication as Paper No. 6103 in the Journal Ser. of the Pennsylvania Agr. Exp. Sta.
2 Appreciation is expressed to Dr. L. C. Griel, Jr., for performing castrations and providing veterinary services, and to the personnel of the Dairy Breeding Research Center, University Park, PA, for their assistance.
4 Dairy Breeding Research Center, Dept. of Dairy and Anim. Sci.
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