Role of Mesoderm and Ectoderm in Limb Morphogenesis

A series of very interesting experiments on wing and leg buds of chick embryos have clarified the respective roles of mesoderm and ectoderm and interactions between them in limb development in vertebrates. Many of these experiments have involved separating the two components of early and later embryonic stages of the same or different limb types or even species and studying the development after recombining them with each other or with cells from non-limb regions in various ways. It is established that the presumptive limb mesoderm which is localized very early in the presumptive limb area is essential for Sib development. This is demonstrated by the following experiments:

1) When limb mesoderm is removed from under the ectoderm of limb area or replaced by mesoderm from some non-limb region, no limb forms.

2) The type of limb (wing or leg) that develops is determined by the mesoderm of limb site. Thus, when leg bud mesoderm is combined with wing bud ectoderm a leg is formed and vice versa. It means the mesoderm of each limb field is already determined as to what limb type it will form and determination is species specific.

3) When wing mesoderm is grafted under the ectoderm at a non-wing site in early embryo, a wing is formed at that site.

4) When limb bud ectoderm is grafted at a non-limb site, no limb is formed, but if limb bud ectoderm is replaced by non-limb ectoderm on the limb mesoderm. A normal limb forms. The ectoderm of any region up to a certain stage chick embryo (and also amphibians) is able to participate in limb development if associated with limb mesoderm.