Fig. 1. Distribution of fibronectin in the chick PSM and surrounding tissues. (A,B) Longitudinal (A) and sagittal (B) sections reveal short strands of fibronectin immunoreactivity within the PSM (arrowheads in A,B), and an extensive fibronectin immunoreactivity surrounding the anterior PSM. This immunoreactivity is distinguishable from that of the neural tube, ectoderm and endoderm (arrows in A,B). (C) Non-enzymatic removal of ectoderm exposes fibrillar fibronectin immunoreactivity on the PSM. (D,E) By contrast, PSMs isolated with pancreatin (D) or dispase (E) are completely negative. (F) When collagenase is used an intact fibronectin matrix is maintained. Polyclonal anti-fibronectin (A,B) and monoclonal anti-cellular fibronectin (C-F) antibodies were used. fn, fibronectin. Scale bars: 100 µm in A,B; 25 µm in C-F.