Quantification of healthy and atretic germ cells and follicles in the developing and post-natal ovary of the South American plains vizcacha, Lagostomus maximus: evidence of continuous rise of the germinal reserve

Abstract

Female germ line in mammals is subjected to massive cell death that eliminates 60-85% of the germinal reserve by birth, and continues from birth to adult life until the exhaustion of the germinal pool. Germ cell demise occurs mainly through apoptosis by means of a biased expression in favour of pro-apoptotic members of the BCL2 gene family. In contrast, the South American plains vizcacha, Lagostomus maximus, shows as sustained expression of the anti-apoptotic BCL2 gene throughout gestation and a low incidence of apoptotic germ cells. This led to propose that, in the absence of death mechanisms other than apoptosis, female germ line should continuously increase from foetal life until after birth. We quantified all healthy germ cells and follicles in L. maximus ovaries from early foetal life to day 60 after birth using unbiased stereological methods and labelling with TUNEL assay to detect apoptosis. Healthy germ cell population increased continuously from early developing ovary reaching a 50 times higher population number by the end of gestation. TUNEL positive germ cells were <0.5% of germ cell number except at mid-gestation (3.62%). Mitotic proliferation, entrance to prophase I and primordial follicle formation occurred as overlapped processes from early pregnancy to birth. Germ cell number remained constant in early postnatal life but a remnant population of non-follicular VASA and PCNA positive germ cells still persisted at postnatal day 60. L. maximus is the first mammal so far described in which female germ line develops in the absence of constitutive massive germ cell elimination.