The fusion of Parallel long bones and the formation of secondary cartilage.

Abstract

The formation of cartilage is described in the repair of the fractured fibula of the guinea pig. Cartilage and bone developed from a common blastema and transitions occurred between them. No convincing evidence was found that cartilage was ever directly transformed into bone. In some guinea pigs, whether with the fibula fractured or not, the tibia and fibula came into contact near the distal end, and the apposed periostea fused and chondrified. Later the cartilage was resorbed from below and replaced by bone. Thus, the tibia and fibula were united by a bony connection near their distal ends. In the chondrification of the periostea, the fibrous layer was involved as well as the cambial layer. In some specimens the periostea chondrified without fusing, making a structure resembling a nearthrosis. In the normal development of the rat, the tibia and fibula become fused near their distal ends within a few days after birth. The fusion process begins in the periostea, which unite. The cambial layers then chondrify, and the chondrification process spreads through the fused fibrous layers so that the two bones are united by a cartilaginous pad. This pad is later replaced by bone from below so that a bony union is established. In the embryo chick, metatarsals 2, 3, and 4 are at first separate cartilages. Periosteal bone develops around each cartilage and at first there is no connection between the three bones. Later, bony trabeculae form on the anterior and posterior aspects, connecting pairs of adjacent bones, close beneath the common fibrous periosteum. The fusion thus occurs without the formation of cartilage. In the frog, the tibia and fibula, and the radius and ulna, become fused during metamorphosis. No cartilage is formed during the fusion, the two bones becoming enclosed in a common sheath of periosteal bone which develops beneath the common fibrous periosteal layer. In a discussion of the induction of cartilage formation it is concluded: (1) That presumptive cartilage cells of the embryo chondrify independently of mechanical stimulation, the determining agent being presumably humoral: (2) That cells which do not normally chondrify, but which belong to the skeletal group, form cartilage if they are subjected to certain mechanical conditions, and that these conditions include pressure and shear; (3) That cells of the non-skeletal connective tissue group can form cartilage only if they undergo an induction, presumably by a humoral agent, and that no further mechanical stimulation seems then to be required.