Fig. 1. Apoptosis is induced at the graft border in heterografts. Control (24 h old) homografts of H. oligactis/H. oligactis (A) and H. vulgaris/H. vulgaris (E), and heterografts H. oligactis/H. vulgaris (B) and H. vulgaris/H. oligactis (C) vitally stained with Acridine Orange. (D) H. vulgaris/H. oligactis heterograft stained by the TUNEL method. (F—H) Graft sites indicated by the frames in B—D, respectively, shown at higher magnification. O, H. oligactis tissue; V, H. vulgaris tissue. Broken lines indicate the graft border. Arrows show Acridine Orange- or TUNEL-positive cells undergoing apoptosis. The cells in focus are localized in the ectodermal epithelium. Note that most of them are apparently restricted to the H. vulgaris tissue adjacent to the graft border.