Determinants of spatial polarity in the growth plate

Growth of long bones occurs at the growth plate, a layer of cartilage that separates the epiphysis from the metaphysis. Growth plate exhibits spatial polarity. Proliferative chondrocytes undergo terminal differentiation when they approach the metaphyseal, but not the epiphyseal, border of the growth plate. The adjacent bone also exhibits spatial polarity. Metaphyseal, but not epiphyseal, blood vessels and bone cells invade the adjacent growth pla te, remodeling it into bone. As a result, the metaphysis, but not the epiph ysis, elongates over time. To determine whether cartilage polarity determin es bone polarity and/or whether bone polarity determines cartilage polarity , rabbit distal ulnar growth plates were excised, inverted, and reimplanted in their original beds. Thus, cartilage polarity was inverted relative to bone polarity. Histological examination showed that the inverted cartilage polarity was maintained over time. In contrast, the polarity of the adjacen t bone reversed after surgery, to match that of the cartilage. Blood vessel and bone cell invasion ceased in the metaphysis and arose in the epiphysis . Longitudinal bone growth (measured with weekly radiographs) occurred at t he epiphyseal, not at the metaphyseal, surface of the growth plate. We conc lude that the polarity of growth plate cartilage is determined by intrinsic factors. The cartilage polarity then determines the polarity of the adjace nt bone and, consequently, the functional polarity of longitudinal bone gro wth.