A morphometric study of early ovarian development in pouch young of the brushtail possum (Trichosurus vulpecula).
BACKGROUND: Although the pattern of gonadal differentiation in marsupials is well documented, there is little information available on germ cell initiation and development. Furthermore, neither parameter has been well described for the brushtail possum (Trichosurus vulpecula). METHODS: A quantitative study of ovarian germ cell proliferation and the onset of folliculogenesis was undertaken in pouch young of the brushtail possum. Pouch-young age was estimated from measurements of head length and crown-rump length. Estimates of total ovary volume, the number of primary and meiotic germ cells, and the numbers of primordial follicles were made by using Cavalieri's principle and the optical disector. In addition, the age at which follicle growth began was determined. RESULTS: The estimated ages of the pouch young in the study ranged from 26 to 207 days postpartum. Body weight, ovary weight, and ovary volume were all highly correlated with estimated age. Germ cell meiotic activity was well established by day (d) 26 of pouch life. Germ cell numbers reached a maximum (691 x 10(3)/ovary) at d67 postpartum and then declined with increasing age. Primordial follicles were first evident at d67. Primary follicles were first seen at d97, secondary follicles at d105, and antral follicles at d155. CONCLUSIONS: This first quantitative study of ovarian follicle development in the female pouch young of the brushtail possum provides a basis for the temporal definition of ovarian maturation in this marsupial species. The pattern of germ cell proliferation and development is similar to that seen in eutharian mammals.