An in vitro analogue of early chick limb bud outgrowth.

Our culture system appears to represent an in vitro analogue of early chick limb morphogenesis. Organized mesodermal cell accumulations resembling limb buds were derived from a monolayer of limb mesoderm cells when covered by limb ectoderm which included the apical ectodermal ridge (AER). The ridge retained its normal configuration when grown over a limb mesoderm monolayer and the mesoderm cells accumulated under the ridge to form a multilayered structure (10-25 cells in thickness) with the characteristic shape of a limb bud. Ectoderm which did not include the ridge failed to promote the formation of limb-like mesodermal accumulations thus the action of the ridge appears to be specific. The AER-elicited expression of mesodermal cell behaviour leading to early limb outgrowth is discussed in terms of possible morphogenetic mechanisms involved i.e. differential mitosis, cell migration, changes in cell shape and especially the adhesive properties of the cells.