The histology of epiphyseal union in mammals.

Epiphyses of man and dog in various stages of union are described. As union approaches, new cartilage is added to the epiphyseal surface of the cartilaginous epiphyseal plate by chondrification of the epiphyseal marrow. Before and during early union the cellular arrangement of the cartilaginous epiphyseal plate is well preserved, with good cartilage columns. Mineralization of the cartilage, demarcated by tide-lines, spreads from both surfaces, engulfing the cartilage columns. The first union of an epiphysis is by such mineralized tissue, a form of metaplastic bone. In the smaller and earlier-uniting epiphyses, the mineralized cartilage and the sheets of lamellar bone that cover its surfaces are first removed in one restricted area and replaced by new bone and marrow. The original perforation is later enlarged until all the cartilage is destroyed and union in complete, leaving no scar. In the larger and later-uniting epiphyses there are multiple perforations in the epiphyseal plate and remnants of epiphyseal bone often persist as an epiphyseal scar. In both types of union remnants of the peripheral parts of the plates may be found for a while with the cartilage columns set at right angles to the axis of the bone as a whole, an unexplained peculiarity. At full union all such "residual" cartilage is destroyed.