Development of the frontal bone and cranial meninges in the embryonic chick: an experimental study of tissue interactions.

The frontal region of the embryonic chick was studied to determine whether epithelial influences are necessary for frontal bone development. The frontal bone is a membrane bone, of neural crest and head mesodermal origin, which develops within mesenchyme sandwiched between two epithelia, neural ectoderm and epidermis. Rudiments were treated enzymatically to separate epithelial and mesenchymal tissues. Frontal mesenchyme then was grown as chorioallantoic membrane grafts either in the presence or absence of neural ectoderm and/or epidermis. The results indicate that neural ectoderm, though required during early stages of development to induce frontal bone development (Schowing, 1968), is not required during later stages (HH 22-30, the stages tested in this study) for osteogenesis. Epidermis, however, was shown to be required for frontal bone development during the stages tested. Frontal mesenchyme formed bone when epidermis was present on the outer aspect of the mesenchyme, and did not form bone when the epidermis had been removed prior to grafting, whether or not neural ectoderm was present. This dependence upon epidermis continues beyond the onset of meningeal differentiation. Once the outer ectomeninx-dermis is distinguishable from the inner endomeninx, osteogenic capabilities are confined to the ectomeninx-dermis layer. Furthermore, the ectomeninx-dermis layer attached to epidermis is able to form membrane bone in the absence of the endomeninx and neural ectoderm. The endomeninx, though normally nonchondrogenic, was shown to be capable of forming cartilage when the neural ectoderm is removed. Neural ectoderm, therefore, may have an inhibitory effect on chondrogenesis in the endomeninx.