Abstract
The large Japanese field mouse (Apodemus speciosus ) is a seasonal breeder, and its spermatogenic activity changes
throughout the year. In some mammal species, testicular regression results from increased apoptosis of the male germ
cell. However, no study has investigated the association between apoptosis and temporal testicular regression using
closely classified spermatogenic phases in seasonal breeders. Using terminal deoxynucleotidyl transferase-mediated
dUTP nick end-labeling staining, we immunohistochemically investigated male germ cell apoptosis and its relation
to temporal testicular regulation and circannual cycles of activation and regression in this species. Furthermore, we
classified spermatogenic activity into 9 phases (pre-increase, early-increasing, mid-increasing, late-increasing, peak,
early-decreasing, mid-decreasing, late-decreasing, and post-decreasing) using testicular weight and the histological
observation of spermatogenesis, and investigated the morphological temporal changes of seminiferous tubules and
epididymides. Germ cell apoptosis occurred at low levels around the peak phase, but was observed at high rates during
the pre-increase, mid to late-decreasing, and post-decreasing phases. It should be noted that relatively many apoptotic
cells were observed even in the early to mid-increasing phases. The primary type of apoptotic germ cell observed during
all phases (except the mid-decreasing phase) was the spermatocyte. It is considered that apoptosis would be the main
cause of cell loss, and live cell desquamation would accelerate testicular regression. The histological description of
seminiferous tubules and epididymides will support future testicular histological evaluations of A. speciosus and provide
guidelines for comparison with other species.