Morphological and morphometric changes in the ovaries of white-footed mice (Peromyscus leucopus) following exposure to long or short photoperiod.

Two experiments were carried out with a total of 36 white-footed mice (Peromyscus leucopus) exposed to either long (LP) or short (SP) photoperiod for six weeks. Ovaries and uteri were weighed. Serial sections of the ovaries were processed for light microscopy. The total number of the interstitial gland cells, preantral and antral follicles with early and advanced stages of atresia as well as corpora lutea, was determined. Uteri from LP mice weighed significantly more than those from SP mice in both experiments. In contrast, the weight of ovaries from LP and SP animals only differed significantly in one experiment; this experiment also produced no obvious changes in the total number of corpora lutea. While interstitial gland cells of LP ovaries were hypertrophic and appeared mature, the interstitial gland cells of SP ovaries were of the degenerating cell type. The total number of follicles as well as their mean follicular diameters were higher in LP animals in comparison with SP animals. In addition, the percentage of antral follicles were increased and there were fewer signs of advanced stages of atresia in the LP group. It is suggested that the changes of the interstitial gland cell morphology combined with alterations of follicular growth during LP or SP appear to support intensified follicular growth.