Development of osteogenic and chondrogenic potentials along the mediolateral axis of the embryonic chick mandible.

Various regions of the mandibular process were tested for these potentials to determine whether regional differences exist and vary with embryonic age. Mandibular processes from HH stages 17-21.5 were cultured and grafted intact, or were subdivided into medial, mediolateral and lateral fragments and the separate regions cultured or grafted. The intact mandible from all these stages can form cartilage and membrane bone, but the 3 regions are not equally osteogenic and chondrogenic. The lateral region from all stages could form cartilage and membrane bone; the mediolateral region could form cartilage and membrane bone but, in mediolateral fragments from HH stage 17, membrane bone was formed only in scant amounts. The medial region from HH stages 17 and 18 formed cartilage in only 50 per cent of cases and never formed membrane bone. By HH stage 20, the medial region could form membrane bone, but only in scant amounts. Medial fragments from HH stage 21.5 formed extensive membrane bone and cartilage. The acquisition of these potentials, therefore, proceeds in a lateral-to-medial sequence, and the acquisition of an osteogenic potential lags slightly behind that of a chondrogenic potential. These findings do not indicate the mechanisms by which the two subpopulations of chondrogenic and osteogenic cells are distinguished from one another, but they give the temporal and spatial sequence in which this determination must occur.