Distribution of type I and type II collagen gene expression during the development of human long bones

The temporal and spatial gene expression of collagen type I and type II during the development of the human long bones was studied by the technique of in situ hybridization covering the period from the cartilagenous bone anlage to the formation of a regular growth plate in the newborn. Analysis of the early stages around the seventh week of gestation revealed for type II collagen a strong hybridization signal limited to the chondrogenic tissue. The surrounding connective tissue and the perichondrium showed weak type I collagen expression, while the zones of desmal ossification like the clavicle gave a strong signal. Beginning with the eighth week of gestation, type I collagen mRNA was detectable in newly formed osteoblasts at the diaphysis and appeared along with the formation bone marrow, in the areas of enchondral ossification. Parallel to the development of the different zones of cartilage differentiation, a specific pattern of type II expression could be observed: type II was mainly found in the chondrocytes of the hypertrophic zone and to a lesser degree in the zone of proliferation, while the resting zone and the zone of provisional calcification showed little activity. This segregation of type II expression was most pronounced in the early stages of cartilage calcification and in the growth plate of the newborn.