Fig. 1. Initial morphological events within 1 hour of decapitation as monitored by whole-mount immunofluorescence using antibody to hydra laminin ß1 chain (LM) (A-C) and hydra type I collagen (Col) (D-F), light microscopy (G-I) and transmission electron microscopy (TEM) (J). As shown in A,D,G, the ECM (arrow) is continuous along the head pole. Immediately after decapitation, the epithelial bilayer is separated into two halves and as indicated by the arrows in B,D,H, the ECM is contained within each half. The cut edge of the ECM can be visualized in whole mounts by immunofluorescent staining of both LM (localized to basal lamina) (B) and Col (localized to the interstitial matrix) (E). One hour after decapitation, the two separated halves of the bilayer have fused (I) creating a closed head pole that lacks the morphological features of an adult polyp (no hypostome or tentacles). The arrow in I indicates that the ECMs of each epithelial bilayer half are still not fused at this time as shown by TEM analysis in J (region indicated by the box in I). As also shown in J, the cut edge of the ECM is thickened (white arrow in J) when compared with the normal thickness of the ECM more distal to the cut edge (white arrowhead in J). The thickened cut edge of the ECM 1 hour after decapitation is seen in whole-mount immunofluorescence as a bright circular signal (white arrows in C and F) at the apical pole of the body column as monitored by staining for LM (C) or Col (F). The epithelium at the apical pole that has fused, but lacks an ECM, is flattened (arrowhead in I) when compared with the epithelium that is associated with an ECM (epithelium in the left half of the box shown in I). Scale bars: in F, 250 µm for A-F; in I, 100 µm for G-I.