MORPHOMETRIC STUDY OF OVARIAN FOLLICULAR-GROWTH DURING THE ESTROUS-CYCLE OF THE LABORATORY-MAINTAINED INDIAN DESERT GERBIL, MERIONES HURRIANAE (JERDON)

The pattern of changes in the population of healthy and regressing fol licles in relation to the estrous cycle, was studied in the recently d omesticated desert gerbil, Meriones hurrianae, which exhibits a longer cycle (proestrus: 1.5 days; estrus: 1 day; metestrus: 1.5 days; diest rus: 3 days). As the granulosa cell layers were not distinct, the foll icles were classified on the basis of their morphology and size (group I: small preantral follicles; group II: large preantral follicles; gr oup III: small antral follicles; group IV: medium-sized antral follicl es; group V: large-sized follicles; group VI: Graafian follicles). Reg ressing follicles were classified likewise. Five animals were used for each stage of the cycle. One ovary from each gerbil was serially cut and stained with hematoxylin-eosin. Healthy and regressing follicles w ere found throughout the cycle. The follicular development occurred ty pically in a biphasic manner. In phase A, in which follicles reach the antral stage, the growth of oocytes and follicles is linearly correla ted, whereas, in phase B, oocyte size remains stationary while follicl e size increases due to accumulation of antral fluid. However, oocyte growth and nucleus growth are positively and linearly correlated (r=0. 74). The healthy follicles under 260 mu m (groups I, II) do not fluctu ate in the ovary of cyclic gerbils, whereas follicles in group III (26 1-350 mu m) increase significantly (p<0.05) at estrus and those of gro up IV in diestrus. Further, healthy group III follicles are reduced in diestrus while regressing follicles in group III increase. These find ings thus suggest that the antral follicles are rescued from atresia a t estrus. Further selection of follicles for growth and ovulation, or their exit through atresia occurs in diestrus. The gerbil produces 3 o va per ovary during each cycle, and the growth of selected follicles i s completed in two cycles (14-15 days). Atresia in preantral follicles is characterized by the degeneration of oocytes followed by the granu losa cells, whereas initiation of atresia among antral follicles is du e to pyknosis of the granulosa followed by the degeneration of oocytes . Occasionally, pseudomaturation divisions occur in regressing follicl es prior to oocyte fragmentation. In addition to the commonly occurrin g types of atresia in the gerbil, occasionally intraovarian release of oocytes from the regressing follicles is encountered.