Estrogen in peripheral plasma during postnatal development in gray short-tailed opossums.

Plasma samples obtained from gray short-tailed opossums (Monodelphis domestica) at selected ages through adulthood were assayed for estrogen (E). Levels of E in one mixed-sex plasma pool of animals aged postnatal day (pd) 4 and one of two mixed-sex plasma pools of animals aged pd 8 were over 300 pg/ml. On pd 16, E levels in males and females averaged 30 and 47 pg/ml, respectively. While no significant sex differences in E levels were seen on pd 30 or pd 60, mean E levels for animals on pd 30 were significantly higher (275 pg/ml in males and 181 pg/ml in females) than on pd 60 (78 pg/ml in males and 85 pg/ml in females) or pd 145 (adults). In adult animals, estrogen levels in females averaged 54 pg/ml; all adult male E levels were below the limit of sensitivity of the assay. Maternal E levels, which did not vary significantly by age of litter, averaged 10 pg/ml overall. These findings are discussed with respect to possible significance of high E levels in developing marsupials for sexual differentiation and general brain development.