IDENTIFICATION OF DISTINCT METABOLIC POOLS OF AGGRECAN AND THEIR RELATIONSHIP TO TYPE-VI COLLAGEN IN THE CHONDRONS OF MATURE BOVINE ARTICULAR-CARTILAGE EXPLANTS

The metabolism and distribution of newly synthesized aggrecan present in the extracellular matrix of intact explant cultures of mature artic ular cartilage was investigated with respect to type VI collagen-stain ed chondrons. Using biochemical, autoradiographical and novel confocal immunohistochemical techniques it was shown that aggrecan exists as a number of distinct pools that are located within the extracellular ma trix of the tissue. The first was identified as a pool of high specifi c radioactivity, much of which appeared in the medium one day after in cubation with radiolabeled sulfate. Of the radiolabeled aggrecan remai ning within the extracellular matrix, three pools were differentiated on the basis of time and location within the extracellular matrix. One pool was resident within the pericellular microenvironment associated with the chondron, one migrated into the territorial matrix adjacent to the chondron and one was sequestered long term in the interterritor ial matrix. Analysis of the kinetics of loss of radiolabeled aggrecan macromolecules present in the region of matrix defined by the chondron suggests that this pool rapidly turns over and is a precursor to the pools of aggrecan present in the territorial and interterritorial matr ix. There were marked differences in the distribution of newly synthes ized aggrecan present in these regions of the extracellular matrix in explant cultures maintained with or without fetal calf serum. In the a bsence of serum, more of the newly synthesized aggrecan moved into the territorial and interterritorial matrix indicating that the presence of serum in the culture medium influenced the tissue distribution of a ggrecan. This work indicates that the pericellular microenvironment of the chondron plays an important role in the retention and maturation of aggrecan prior to the sequestration of aggrecan complexes into the functional load bearing matrices of adult articular cartilage.