[Nature and evolutive aptitudes of cells of the mitotic compartment of the secondary cartilages of the mandible and maxilla of the young rat. Experience with cytotypic culture and homotransplantation].

1. Two cell types are found in the mitotic compartment of a secondary cartilage of the mandible (condylar, angular and coronoid cartilages) and of the maxillary (cartilage of the sagital suture): - the "skeletoblast", which bears a spontaneous tendancy to differentiate into an osteoblast, but which differentiates in vivo into a chondroblast; - the prechondroblast, originating from the skeletoblast and differentiating only into a chondroblast. 2. After homotransplantation of the condylar cartilage the skeletoblast no longer differentiates into a prechondroblast. The preexisting prechondroblast continues to mature into chondroblasts until exhaustion of the supply. The chondroblastic zone is soon replaced by "endochondral" bone. Mitosis of prechondroblasts soon stop in the presence of chondroblasts, probably by a chalone-like mitotic inhibitor. But, if the disappearing of the cartilage is more rapid than the transformation of prechondroblasts into chondroblasts, the prechondroblasts then resume mitosis for some time. The skeletoblast differentiates into an osteoblast when prechondroblasts have transformed into chondroblasts; only then, does it lay down periosteal-like bone.