INCREASED CELL DIAMETER PRECEDES CHONDROCYTE TERMINAL DIFFERENTIATION, WHEREAS CELL-MATRIX ATTACHMENT COMPLEX PROTEINS APPEAR CONSTANT

Background: Chondrocytes in specific areas of chick sterna have differ ent developmental fates. Cephalic chondrocytes become hypertrophic and secrete type X collagen into the extracellular matrix, whereas middle and caudal chondrocytes remain cartilagenous throughout development, continuing to secrete collagen types II, IX, and XI. In this report, w e ask if the cell size and cytoarchitecture of chondrocytes differ in cephalic, middle, and caudal portions of whole sterna prior to and dur ing hypertrophy. In addition, what is the distribution of integrin sub units and actin associate proteins in differentiating chondrocytes? Me thods: Phalloidin was used to stain filamentous actin, and immunohisto chemistry was used to localize the distribution of collagen molecules, integrin receptor subunits, and actin-associated proteins. Results: C hondrocytes stained for filamentous actin demonstrated that on day 14 cephalic chondrocytes had a significantly larger diameter than middle and caudal chondrocytes. Day 17 chondrocytes in nonhypertrophic cephal ic and middle regions of sterna were significantly smaller than hypert rophic chondrocytes and significantly larger than caudal chondrocytes. In contrast to day 14 chondrocytes, day 17 chondrocytes in the hypert rophic region demonstrated similar diameters at all cartilagenous dept hs. The beta(1) integrin subunit appeared punctate and associated with cell membranes, allowing nonpolarized interactions with extracellular matrix molecules. The distribution of cr integrin subunits was simila r to the beta(1) integrin subunit, although alpha integrin subunits al so appeared cytoplasmic. Actin-associated proteins, vinculin, and alph a-actinin, were associated with F-actin, but vinculin was more specifi cally localized to the ends of the actin filaments. Focal adhesion kin ase was diffusely distributed throughout the cytoplasm but also demons trated areas of colocalization with vinculin. Zyxin and paxillin demon strated a punctate distribution, although paxillin was slightly more d iffuse. Using immunohistochemical detection, no difference in integrin subunit or actin associated protein distribution could be determined between chondrocytes and hypertrophic chondrocytes. Conclusions: The i ncreased chondrocyte diameter observed in cephalic regions of sterna o n day 14 suggests that intracellular changes may precede the specific hypertrophic marker, type X collagen, by several days. In addition, th e presence of integrin subunits, which are known to interact with coll agen and cytoskeletal proteins, suggests that communication may exist between chondrocytes and their extracellular matrix via these receptor molecules. (C) 1996 Wiley-Liss, Inc.