Fig. 4

Fig. 4 Autoradiographs of labeled (A) and unlabeled (B) M-cells after injections of [³H]thymidine. The yolk sacs of embryos were injected 9 (A) or 15 hr (B) after fertilization (25.5°C) with about 5 nl of isotope ([methyl-³H]thymidine, 1.0 mCi/ml, 6.7 Ci/mmole, New England Nuclear). After development for 1 week, the animals were fixed overnight with an aldehyde mixture [4% formaldehyde, 2% glutaraldehyde, 8 mM CaCl₂, 0.15 M cacodylate-HCl (pH 7.0)]. They were washed thoroughly, dehydrated through ethanols, and embedded in Epon-Araldite. Serial 5-μm transverse sections were taken and coated with Kodak NTB 2 emulsion. After exposure times of about 9 days, the slides were developed and stained with 1% toluidine blue. In all cases neurons near the M-cells showed specific labeling over nuclei demonstrating that the label was available and that incorporation was specific. Dimension marker = 10 μm.