Silver staining of DNA synthesizing cells in the neural tube of chick embryos

I report a study by light microscopy of the spinal cord of early chick embryos stained with the ammoniacal silver carbonate solution of Del Rio Hortega. Cell nuclei are stained in a selective fashion and two classes of nuclei - dark and pale - can be distinguished in the neuroepithelium. Neuronal nuclei also show a characteristic staining pattern. A radioautographic study after [3H] thymidine incorporation has shown that it is the dark neuroepithelial nuclei that are engaged in DNA synthesis. Dark nuclei disappear after administration of cytosine arabinoside, supporting the association between DNA synthesis and silver staining of neuroepithelial nuclei.