Embryonic start-up: mom's influence.

Embryonic start-up: Mom's influence

In the earliest stages of its developmental journey, an embryo depends entirely on substances originally packed into the egg cell by the mother. But even as the embryonic genes assume control, their activity pattern is directed by prepackaged maternal factors. In the frog Xenopus, embryonic genes first become active when the embryo is a blastula (a hollow sphere with a single layer of cells) about eight hours after fertilization. At this time, different genes are activated in specific regions of the embryo.

One hypothesis for the differential activation of embryonic genes is that information-carrying molecules, such as maternal messenger RNA molecules, are differentially distributed in the embryo. Douglas Melton of Harvard University now reports experimental support for this idea. He finds that a few types of maternal messenger RNA, making up less than 0.1 percent of the total, are localized to specific regions of an unfertilized egg and are subsequently distributed to different regions of the early embryo. These molecules may be responsible for the characteristic pattern of early gene activation--the first step in the sequence of events by which genetically identical cells develop into all the different tissues of the body.