The progress zone is a layer of

mesodermal The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Embry ...

cells immediately beneath the

apical ectodermal ridge The apical ectodermal ridge (AER) is a structure that forms from the ectodermal cells at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb. After the limb bud induces AER formation, the AE ...

in the developing

limb bud The limb bud is a structure formed early in vertebrate limb development. As a result of interactions between the ectoderm and underlying mesoderm, formation occurs roughly around the fourth week of development. In the development of the human ...

. The fate of the mesodermal cells is thought to be patterned by the length of time spent in the progress zone during limb outgrowth. However, some recent evidence using microinjected embryos suggests that the cells are prespecified early in limb bud development. The progress zone acts as positional information to tell which cells to develop into the limb. If cells spend a very short time in this area as they receive signals from the apical ectodermal ridge, then more proximal limb structures are not able to develop even if distal ones can.

References

Developmental biology {{Developmental-biology-stub