ADIPOSE-TISSUE EXTRACELLULAR-MATRIX - NEWLY ORGANIZED BY ADIPOCYTES DURING DIFFERENTIATION

The distribution of eight types of extracellular matrix (ECM) proteins (type I-VI) collagen, laminin and fibronectin) in the skeletal muscle of Japanese Black cattle was determined by indirect immunofluorescenc e using specific antibodies against each protein. ECM proteins were we ll organized in the intramuscular connective tissue: type I, II, III c ollagen and fibronectin were localized primarily in the perimysium, ty pe V and VI collagen in both the perimysium and endomysium, and type I V collagen and laminin were virtually confined to the endomysium. In t he loose connective tissue holding the adipocytes together to form a t issue mass between the muscular bundles, seven of the ECM proteins not type II collagen were relatively abundant in a disordered arrangement . Further analysis by in vitro immunocytochemical staining also demons trated that a stromal-vascular preadipocyte cell line (BIP cell), deri ved from Japanese Black cattle, synthesized various ECMs in much the s ame way as fibroblasts. Exponentially growing BIP cells with a Fibrobl astic phenotype were found to produce type II, V, and VI collagens, in addition to the other previously identified connective tissue glycopr oteins of mouse 3T3 preadipocytes. When confluent preadipocyte culture s were stimulated with adipogenic medium, a fibrillar network of ECM w as observed to bridge the intercellular space and connect adjacent cel l surfaces. During adipocyte differentiation, type III collagen and la minin were arranged in a non-fibrous structure, and type II collagen w as only barely detected. These results are supported by the staining o f the adipose tissue, where all ECM proteins studied except type II co llagen were stained intensely. These data indicate that in vivo under conditions permissive for adipose conversion, the production and organ ization of ECM, accompanied by hyperplasia and hypertrophy of precurso r cells, gives rise to adipose tissue in skeletal muscle with its own ECM products. These data further suggest that each ECM protein might h ave some role for the adipocytes in forming tissue.