Mechanisms of cartilage precursor replacement by bone in the mammalian skeleton

The initiation of the endochondral process in ontogenesis is preceded by structural and metabolic preparation of the cells of cartilage anlages of the skeleton as well as the satellite cells of the vascular terminations adjoining the cartilage. In the cartilage anlage, due to a progressive specific biosynthesis, the macromolecular chondroitin-protein complexes capable of binding a large amount of water, are accumulated. As a result the volume of a swelling chondrocyte increases twentyfold or more and a high hydrostatic tension is created within it. At the same time the metabolic situation within the cartilage favours a local concentration, near the hypertrophic chondrocytes, of the perivascular cells possessing an increased resorptive function. The resorption of the cartilage partition walls by these cells leads to the opening of swollen chondrocytes. The blood cells and the cells of the blood capillaries themselves are engulfed through the resulting crater into the emptied lacuna. In the lacuna the conditions of stasis are created which promote local redistribution of the supplied cells and reutilization of the cartilage destruction products. The perivascular cells concentrated along the remnants of the cartilaginous matrix are gradually differentiated into preosteoblasts and osteoblasts. The endosteal bony laminae are built on the remnants of the cartilaginous matrix. The perivascular cells of the apical zone of a lacuna retain their clastic function and continue spreading of the endochondral process on the adjacent chondrocytes. In the course of the successive destruction of hypertrophic chondrocytes, the system of vessels is formed in the endochondral centre, according to the sinusoid pattern.