Abstract
Wild large Japanese field mice (Apodemus speciosus) responses to cyclic seasonal changes are associated with physiological and behavioral changes. However, the detailed regulation of oogenesis in the ovary during the seasonal reproductive cycle in wild large Japanese field mice has not been studied. We assessed the dynamics and changes in ovarian morphology and hormone concentrations associated with reproductive seasonality throughout the year. The stages of the ovarian morphological breeding cycle of wild large Japanese field mice were classified as breeding, transition, and non-breeding periods during the annual reproductive cycle. Measurement of blood estradiol concentrations throughout the year showed that the levels in September and October were higher than those in other months. It is presumed that follicle development starts from a blood estradiol concentration of 38.4 ± 27.1 pg/mL, which marks a shift from the transitional season to the breeding season, followed by the transition to the non-breeding season at 26.1 ± 11.6 pg/mL. These results suggest that seasonal follicle development in wild rodents is correlated with estradiol regulation. We consider this species to be an alternative animal model for studying seasonal reproductive changes and the effects of environmental changes.
Highlights
Many mammals are seasonal breeders and respond to seasonal changes by adjusting their physiology and behavior (Fuentes et al, 1991)
After classifying follicles into five types according to the hematoxylin and eosin (HE)-stained images, it was observed that Type 1–3 and 5 follicles were present in the ovaries of mice caught in April (Table 3)
In the ovaries of mice collected from May to June, 2.5 ± 1.5 and 2.0 ± 1.0 Type 4 follicles were observed and accounted for approximately 18–24% of the total number of follicles
Summary
Many mammals are seasonal breeders and respond to seasonal changes by adjusting their physiology and behavior (Fuentes et al, 1991). Seasonal reproduction is a common adaptive strategy among mammals that allows breeding to occur at times of the year when it is most advantageous for the subsequent survival and growth of offspring. The switching on and off of reproductive function during the annual breeding cycle is the most striking example of spermatogenesis- and/or oogenesis-induced processes. Detailed studies on seasonal reproductive changes in wild animals have.
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