[Polyovular follicles]

Abstract

Folliculogenesis covers the sequential steps in the development of a follicle, from primordial to preovulatory. Most of the time, one follicle contains a single oocyte, but some follicles are polyovular in that they contain several. These follicles were considered earlier as pathological, but they are, actually, fairly common in several species, from mice to humans. The frequency of polyovular follicles (number of polyovular/total number of follicles) varies among species, <0.1% to 14% in the dog, and with age (more polyovular follicles during the prepubertal period). More than 20 oocytes (and even more than 100 in the marsupials like the opossum) may be present in a single follicle. These follicles may form during the earliest stages of follicle formation, due to an imbalance between somatic and germinal cells, which induces an incomplete germ cell cyst breakdown. In polyovular follicles, the quality (size and maturity) of the various oocytes is often heterogeneous. Numerous authors reported that polyovular follicles are able to reach ovulation and ovulate. Polyovular follicles, naturally found in several species, may also be induced by exposure to therapy or agents in the environment, especially with estrogen activity such as pesticides or diethylstilbestrol/DES. Polyovular follicles are also observed in the ovaries of mutated rodents.