Fig. 3. Immunohistochemistry on a composite block of longissimus dorsi (Ld) and rectus lateralis (Rl) muscles. Serial sections are stained with anti-neonatal MHC (a), anti-2B MHC (b) and anti-extraocular MHC (c) antibodies. In one skeletal muscle (Ld), all fibres are negative whereas in extraocular muscle (Rl) some fibres are positive to the antibodies tested. The `hybrid' fibres are numerous and positive either to all antibodies (asterisk) or to two of them (circle); only a few fibres are `pure' and positive with anti-2B MHC antibody (hash mark). Scale bar, 100 µm.