Fig. 2. Collagenase-isolated chick PSMs form somites in the absence of both ectoderm and endoderm. All explant types were either fixed immediately (0 hours, 0h) and labelled for fibronectin or cultured for 6 hours (6h) and processed for fibronectin and F-actin staining. (A-I) Pancreatin-isolated explants. Isolated PSMs are fibronectin-negative at 0 hours (A). At 6 hours, fibronectin staining is visible (B) but somites have not formed (C). In PSM+endoderm explants, some fibronectin immunoreactivity is present between endoderm and PSM at 0 hours (arrows in D). These explants show fibronectin immunoreactivity at 6 hours (E), but no somites have formed (arrow in F). PSM+ectoderm have fibronectin immunoreactivity between ectoderm and PSM (arrows in G), whereas the remaining PSM is negative (arrowheads in G). At 6 hours, clefts (asterisks in H,I) containing fibronectin immuoreactivity (H) and rings of F-actin staining (arrows in I) are observed. (J-R) Collagenase-isolated explants. All explants isolated with collagenase retain their endogenous, fibrillar (see insert in J) fibronectin matrix (J,M,P). At 6 hours, all explant types have clefts (asterisks in K,L,N,O,Q,R) containing fibronectin (K,N,Q) and several epithelialised somites (arrows in L,O,R). Anterior, right. psm, presomitic mesoderm; endo and en, endoderm; ecto and ec, ectoderm. Scale bars: 100 µm.