Biochemistry of differentiation of mouse trophoblast: delta 5, 3 beta-hydroxysteroid dehydrogenase.

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Of all embryonic and extraembryonic tissues in midgestation mouse conceptuses, only trophoblast has significant levels of Δ5,3β-hydroxysteroid dehydrogenase (3β-HSD) activity as measured by progesterone production by sensitive biochemical assays. Because of the importance of progesterone in the maintenance of pregnancy, we have carried out a study of 3β-HSD activity during trophoblast development in vivo and in vitro. Enzyme activity is first detected in trophoblast homogenates on the ninth day of gestation, peaks on the eleventh day, and then falls on the twelfth day. The enzyme activity is almost totally restricted to that fraction of the trophoblast layer containing giant cells. 3β-HSD can be detected in growths containing trophoblast cells developing from blastocysts implanted in extrauterine sites. Enzyme activity is also present in cultured blastocysts, and is associated mainly with trophoblast-containing monolayers rather than with vesicles which are devoid of trophoblast cells. However, compared to trophoblast developing in utero, 3β-HSD activity is low in ectopic trophoblast, and very low in trophoblast from blastocyst cultures. Blastocyst cultures are capable of converting pregnenolone added to the medium to progesterone. In this way, 3β-HSD activity can be measured in small numbers of blastocysts. Using this technique, we have found that 3β-HSD is detectable in trophoblast cells developing in culture at least as early in gestation as it is observed in uterine trophoblast homogenates.