| Summary: | Many studies have been trying to establish standard protocols for animal experimentation, especially for animal species or strains, to master research variables with high precision. The main mouse strains used in the field of the biology of reproduction are Swiss, Balb/c, and C57BL/6. Since some of the strains show reproduction limitations, such as the size of the litter, the present study aimed to compare their spermatogenic processes to verify differences regarding the testicular parenchyma and germ cell populations, which could explain low offspring production. In addition, the present study provides additional information concerning the testicular parenchyma of such strains, which consequently would help researchers to choose the most suitable strain for reproductive studies. Six adult male mice were used for each of the strains. After euthanasia, the testes were weighed, fixated with Karnovsky fixative, embedded in methacrylate, sectioned, and stained with toluidine blue/sodium borate 1%. Morphometrical analyses from the testicular parenchyma (seminiferous tubules and interstitium) were made using the software ImageJ. Germ and Sertoli cells populations were counted in seminiferous tubules cross-sections at stage I of the seminiferous epithelium cycle. The lowest body and testicular weights were observed in C57BL/6 animals, followed by Balb/c and Swiss, however, the relative testes, parenchyma, and albuginea weights were significantly lower only in C57BL/6. Despite the seminiferous tubules and seminiferous epithelium proportions were lower in Swiss animals, their relative amount related to the body weight was the same among strains. The total number of germ cells was higher in Swiss animals, reflecting higher spermatogenic yield and daily sperm production. Due to the lower relative number of Sertoli cells, the Swiss animals showed the highest Sertoli cell index and support capacity. On the other hand, the lowest pathological indexes regarding the germ cells were observed in Balb/c animals, followed by Swiss and C57BL/6. In the interstitium, the proportion of blood vessels was lower in Swiss mice, while the lymphatic cell proportion was lower in C57BL/6 animals. Moreover, the highest proportions of Leydig cells and macrophages were noticed in Swiss mice, which may indicate increased testosterone levels. Altogether, such observations must be taken into account when choosing any of the studied strains for reproduction studies.
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