Expression of exogenous DNA during the early development of the chick embryo. 

A plasmid construct containing the reporter gene,lacZ, under the control of the cytomegalovirus immediate early promoter, was injected into the germinal disc of fertilised chick ova. The distribution of cells expressing β-galactosidase was examined in the embryos after periods of from 3 h to 7 days in culture. β-galacto-sidase-positive cells were first observed at mid-cleavage (250–500 closed cells) in the centre of the blastodisc. After one day, they were prominent in large segments of the blastoderm and, at later stages, in proportionately smaller segments of the extra-embryonic membranes, notably in the endodermal layer. In the embryonic regions, positive cells were scattered in the vicinity of the primitive streak of most cultures, but after gastrulation they were present in the embryonic tissue of only 7% of surviving embryos. The results provide supportive evidence for transcriptional activity during the cleavage stages of avian development. They also confirm previous findings on the loss of exogenous DNA during the early development of the chick.