Rats with bilateral small testes were found in a Wistar-Imamichi derived inbred strain in 1986 at the Institute for Animal Reproduction. TT was established from these mutant rats as known as aspermia rats.
Their phenotype is caused by autosomal recessive mutation, as (aspermia). Homozygous mutant males show arrest of spermatogenesis, mainly at the pachytene stage. Testis weight of mutants was about one-third of that of normal males. A large basophilic inclusion-like body existed in the cytoplasm of late pachytene spermatocytes. The result of spermatogonial transplantation suggested that defective spermatogenesis is caused by defects of both germ cell itself and somatic cell component such as blood-testis barrier. Heterozygous males and homozygous females are fertile. aswas revealed to be caused by a genomic deletion including exon 8 of Fkbp6 gene on rat chromosome 12.