Cell fractions from rat rib growth cartilage. Biochemical characterization of matrix molecules.

In an attempt to isolate and characterize the putative target cells for growth hormone, chondrocytes were isolated from rat rib growth cartilage and fractionated by centrifugation in a discontinuous Percoll gradient. This resulted in three cell fractions with differing properties. The fraction with the lowest density consisted mainly of large, lipid-containing cells which became flattened in subsequent culture. The cells in this fraction were fair collagen producers but synthesized only minor amounts of proteoglycans and apparently no proteoglycan aggregates. These cells probably originate in the hypertrophy zone of the growth plate. The fraction with highest density, on the other hand, consisted of small cells which upon cell culture became polygonal and surrounded with refractile matrix. They synthesized less collagen, but more proteoglycans than the low-density fraction. The proportion of proteoglycan aggregates was rather low (10-20%) but otherwise the proteoglycans were similar to those synthesized by other chondrocytes. This cell fraction was tentatively identified as cells from the upper part of the growth plate. Finally, the middle fraction was heterogeneous, consisting of cells of varying shape. This fraction synthesized large amounts of both collagen and proteoglycans. In all three cell fractions, type II collagen predominated. There were also minor amounts of (1a, 2a, 3a) collagen, and, in the two denser fractions, of type I collagen.