RESUMEN
Rodents are distributed worldwide, playing important ecological roles regarding preservation of forest areas. Thus, the study of their reproductive biology is a key for conservation initiatives that prevent extinction and/or improve species management. The present study aimed to describe the spermatogenic dynamics of the spiny rat Kannabateomys amblyonyx, an endemic species from Atlantic Rainforest areas, in Brazil. The average body weight was 418.43g with a gonadosomatic index of 0.41%. The testicular parenchyma organization followed the pattern described for other rodent species, with a large amount of seminiferous tubules occupying 93.57% of the organ, in a total of 26.04m per testicle. Stage I of the seminiferous tubule cycle was the most frequent in K. amblyonyx, while stage IV the most scarce. Each tubular section in stage I showed 0.47 type-A spermatogonia, 11.78 primary spermatocytes in pre-leptotene, 3.81 in zygotene and 14.31 in pachytene, whereas 32.19 cells were round spermatids and 6.23 were Sertoli cells. From these results it was possible to determine the sperm reserve of 274.49×106 cells per gram of testis. The mitotic and meiotic indexes were 25.06 and 2.25 cells, respectively, whereas the spermatogenic yield was 69.73 cells. Those findings are significant since this is the first study regarding the reproductive aspects of the only Echimyidae species in Brazil, which shows a monogamous mating system.