Practice Questions

TLS Online TPP Program

#Question id: 7099

A fate map of a Xenopus blastula, just before gastrulation begins, shows that the top portion of the embryo will become ectoderm (skin and nerve), the central portion will become mesoderm (bone, muscle, and blood), and the lowest portion will become endoderm (gut). How is it that the endoderm and mesoderm, shown on the outside in the fate map, end up on the inside in the embryo after gastrulation?

#Unit 5. Developmental Biology

The endoderm ends up inside through a process that can be visualized as if one pokes their finger into the bottom of a soft ball until the lowest endodermal portion ends up deep inside, the marginal zone mesoderm also ends up inside, and the ectoderm now encloses the entire outer surface.
The endodermal cells begin first to move into the embryo through the blastopore, displacing the blastocoel and forming a gut; as gastrulation proceeds, the blastopore spreads sideways and the mesoderm follows the endoderm in, ending up between the endoderm and the ectoderm.
The ectodermal cells divide and spread down over the rest of the embryo, so that the endoderm ends up on the bottom, ventral, surface of the embryo, and the mesoderm ends up around the middle, just as is shown in the fate map.
The three germ layers adopt their final positions through, first, the growth of the mesodermal marginal zone down over the endoderm, followed by, secondly, the growth of ectoderm down and over the mesoderm.