Fig. 2. Cell death in the developing chick retina. (A,B) Detection of DNA fragmentation in apoptotic nuclei by TUNEL staining of retinal sections, showing the optic nerve region of E6 chick retinae. TUNEL-positive nuclei (arrowheads) are interspersed in the pseudostratified neuroepithelium. Many more apoptotic nuclei were labeled in control sections (A) compared with sections from anti-TGF-ß-treated embryos (B). (C-E) Quantification of cell death. Apoptotic levels were quantified in retinal sections using the TUNEL assay (C,E) or in retinal extracts using the nucleosome assay (ELISA; D; see Materials and Methods). Apoptosis was reduced in retinae obtained from anti-TGF-ß-, anti-NGF- and anti-TGF-ß/anti-NGF-treated embryos when compared with untreated controls. Co-treatment with TGF-ß neutralizing antibody and recombinant NGF (E) did not block the anti-TGF-ß effect, resulting in an equivalent decrease in apoptosis. Values are mean±s.e.m. *P<0.01; **P<0.005; ***P<0.001; unpaired Student’s t-test. on, optic nerve; ov, optic ventricle. Scale bar, 50 µm.