FIBRONECTIN FIBRIL GROWTH IN THE EXTRACELLULAR-MATRIX OF THE XENOPUS EMBRYO

We show that the mechanism of fibronectin fibril formation on the blas tocoel roof of the Xenopus embryo is comparable to that in other syste ms, Fibril assembly is inhibited by RGD peptide, by an amino-terminal fragment of fibronectin, and by cytochalasin B. When added exogeneousl y, intact fibronectin, but not a 110 kDa cell binding fragment of fibr onectin, is incorporated into fibrils, Thus, the blastocoel roof of Xe nopus represents a valid model system for the study of fibronectin fib ril formation in situ, Moreover, we show that fibril formation can be induced experimentally in this system, Examination of fibril elongatio n by double-labelling experiments reveals that individual, unbranched fibronectin fibrils grow only at one end, i.e. in a unipolar fashion. The rate of elongation is 4.7 mu m/min. Most fibrils grow only for a s hort time, and the increase in total fibril length per cell is driven by the repeated initiation of new fibrils, Assembly of fibronectin int o fibrils precedes cross-linking of fibronectin into multimers in this system.