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 immunofluorescence using specific antibodies against each protein. ECM proteins were well organized in the intramuscular connective tissue: type I, II, III collagen and fibronectin were localized primarily in the perimysium, type V and VI collagen in both the perimysium and endomysium, and type IV collagen and laminin were virtually confined to the endomysium. In the loose connective tissue holding the adipocytes together to form a tissue 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 demonstrated that a stromal-vascular preadipocyte cell line (BIP cell), derived from Japanese Black cattle, synthesized various ECMs in much the same way as fibroblasts. Exponentially growing BIP cells with a fibroblastic phenotype were found to produce type II, V, and VI collagens, in addition to the other previously identified connective tissue glycoproteins of mouse 3T3 preadipocytes. When confluent preadipocyte cultures were stimulated with adipogenic medium, a fibrillar network of ECM was observed to bridge the intercellular space and connect adjacent cell surfaces. During adipocyte differentiation, type III collagen and laminin were arranged in a non-fibrous structure, and type II collagen was only barely detected. These results are supported by the staining of the adipose tissue, where all ECM proteins studied except type II collagen were stained intensely. These data indicate that in vivo under conditions permissive for adipose conversion, the production and organization of ECM, accompanied by hyperplasia and hypertrophy of precursor cells, gives rise to adipose tissue in skeletal muscle with its own ECM products. These data further suggest that each ECM protein might have some role for the adipocytes in forming tissue.