Plasma progesterone concentrations during pregnancy in the dasyurid marsupial, Antechinus stuartii: relationship with differentiation of the embryo.

Antechinus stuartii females were sacrificed at various stages of pregnancy timed from the first day that epithelial cell numbers declined in the urine (day 0). Embryos were recovered and their developmental stage assessed, corpora lutea (CL) were fixed for histological sections and the plasma was taken for progesterone determination. The rate of development of the embryo appeared to parallel the formation of the CL and the plasma progesterone concentrations: cleavage (less than day 6) and the expansion of the unilaminar blastocyst (less than day 14), were periods of slow development, during which the formation of the CL was incomplete and plasma progesterone was low [less than 6 ng mL-1 (19.1 nmol L-1)] though steadily increasing. After day 14 the CL reached its maximum size and plasma progesterone was maximally elevated [greater than or equal to 15 ng mL-1 (47.7 nmol L-1)] until parturition on day 27. Rapid differentiation and growth of the embryo occurred between day 22 and day 27.