Abstract
The embryonic development and changes of the uterine tissue during pregnancy were examined morphologically to reveal causes of the age-related decline in litter size in the golden hamster. Uteri were obtained from young (9-12-week old, n=48) and aged hamsters (43-53-week old, n=47) at estrus and on Days 2 to 16 of pregnancy. Viability of conceptuses were examined in half of implants from Day 6 onwards by a dissecting microscope. The remaining implants were evaluated histologically. The number of resorbing implants was significantly larger in the aged hamsters than in the young females on Day 6 (4.4 vs. 1.2, p<0.05), although no difference was found in the overall mean number of ova ovulated (14.8 vs. 14.2, respectively). Delays in the embryonic development and uterine response were observed in all aged hamsters throughout gestation. Resorbing implants noted in the aged hamsters were accompanied with stunted decidual cells having pale cytoplasm (incomplete decidualization) and heavy infiltration of endometrial granulocytes. It is suggested that the embryonic wastage may occur much crucially during the peri-implantaion period rather than late gestational period, due to the retarded development of embryos and less-developed decidualization, and that endometrial granulocytes may function in resorption of greatly retarded embryos in the aged hamsters.
