The temporal and spatial distribution of short chain skeletal (
Type X) collagen was immunohistochemically examined in the chick tibiotarsus from 6 days of embryonic development to 1 day posthatching. The
monoclonal antibody employed (AC9) was recently produced and characterized as being specific for an
epitope located within the helical domain of the
type X collagen molecule (T. M. Schmid and T. F. Linsenmayer, J. Cell Biol., in press). The earliest detectable appearance of
type X collagen was at 7.5 days, at which time it was restricted to a middiaphyseal location (i.e., in the primary center of ossification). This was in marked contrast to
type II collagen, which appears earlier and is distributed throughout the cartilaginous anlagen. With increasing embryonic age, the reactivity with the type X antibody progressively extended toward the epiphyses, lagging somewhat behind the progression of chondrocyte
hypertrophy. The anti-
type X collagen antibody also reacted with the bony matrix itself, but the immunofluorescent signal produced by this source was considerably less than that produced by cartilage. At 19 days of development, a new small site of type X deposition was initiated in an epiphyseal location, which subsequently enlarged in circumference. These results are consistent with our previous biochemical studies suggesting that, in cartilage,
type X collagen is specifically a product of that population of chondrocytes which have undergone
hypertrophy.