Question

3rd set of experiments You inject one veg1 cell from a 64 cell stage sea urchin embryo with a fluorescent dye in each of 10 embryos. In embryos at the pluteus stage both ectoderm and gut endoderm cells fluoresce in all embryos.

Questions Are veg1 cells in 64-cell stage sea urchin embryos specified to become endoderm only? What is the evidence?

Answer 1

ans - no, veg1 cells in 64-cell stage sea urchin embryos are not specified to become endoderm only.

Studies have shown that by the 60-cell stage, most of the embryonic cell fates are specified, but that the cells are not irreversibly committed. In other words, particular blastomeres consistently produce the same cell types in each embryo, but these cells remain pluripotent and can give rise to other cell types if experimentally placed in a different part of the embryo.

In a fate map of the 60-cell sea urchin embryo- The animal half of the embryo consistently gives rise to the ectoderm—the larval skin and its neurons. The veg1 layer produces cells that can enter into either the ectodermal or endodermal organs. The veg2 layer gives rise to cells that can populate three different structures—the endoderm, the coelom (body wall), and secondary mesenchyme (pigment cells, immunocytes, and muscle cells). The first tier of micromeres produces the primary mesenchyme cells that form the larval skeleton, while the second tier of micromeres contributes cells to the coelom.

Most of these fates are achieved by conditional specification. if for example skeletogenic micromeres micromeres are transplanted into the animal region of the blastula, not only will their descendants form skeletal spicules, but the transplanted micromeres will alter the fates of nearby cells by inducing a secondary site for gastrulation. Cells that would normally have produced ectodermal skin cells will be respecified as endoderm and will produce a secondary gut. The micromeres appear to produce a signal that tells the cells adjacent to them to become endoderm and induces them to invaginate into the embryo. Their ability to reorganize the embryonic cells is so pronounced that if the isolated micromeres are recombined with an isolated animal cap (the top two animal tiers), the animal cap cells will generate endoderm, and a more or less normal larva will develop.