Differential screening of a cDNA library constructed using PCR amplification techniques from RNA isolated from the distal portion (embryonic ectoderm, mesoderm and visceral endoderm) of 7.5 days post coitum (dpc) mouse embryos led to the isolation of two cDNA clones

A variety of experimental strategies have been utilized to isolate genes whose expression is important for early postimplantation mouse development. These strategies include the isolation of mouse homologs of genes known to be important for development in other organisms and the production of developmental mutations by insertional mutagenesis (for reviews, see Gridley, 1991; Kessel and Gruss, 1990). Another approach to the identification of genes important for early postimplantation mouse development is the isolation of genes differentially expressed during these stages. Recent developments in the construction of cDNA libraries from small numbers of cells using PCR techniques (Belyavsky et al., 1989; Brady et al., 1990; Welsh et al., 1990) have made feasible the production of cDNA libraries from defined regions of early postimplantation mouse embryos. One developmental stage of particular interest is the 7.5 dpc mouse embryo. At this time, the embryonic axis is established and mesoderm formation is well underway (Rugh, 1990; Theiler, 1989). Over the succeeding 24 hours, extensive changes will take place in the developing embryo. The heart primordium and the foregut begin to develop, the neural plate forms, the neural folds start to close and somites begin to condense in the paraxial mesoderm. In order to examine genes that might be differentially expressed at this important developmental stage, we have used PCR amplification techniques to construct a cDNA library from the distal portion (embryonic ectoderm, mesoderm and visceral endoderm) of 7.5 dpc mouse embryos. This library was differentially screened with labelled cDNA from 12.5 dpc embryos. Two clones were isolated that were 555 Development 116, 555-561 (1992) Printed in Great Britain © The Company of Biologists Limited 1992