The oviduct as a barrier to exogenous thymidine in the early development of the mouse embryo.

Mouse embryos are able to use exogenous thymidine during cleavage stages in vivo if the thymidine is injected directly into the oviduct. DNA synthesis during cleavage occurs asynchronously and the intensity of DNA synthesis in the blastomeres of the same embryo varies. At all stages, the label in some nuclei is distributed unevenly, suggesting that some chromosomes may be heterochromatinized very early in development.