Visualization of Transcription in early mouse embryos.

Transcriptional activity during early mammalian embryogenesis was examined in 2 cell, 4--8 cell and late morula/early blastocyst mouse embryos. Zona pellucida-free embryos were obtained after pronase digestion followed by a PMSF wash to inhibit proteolytic activity. The embryos were then lysed with NP40 detergent and spread for electron microscopy according to Miller and Bakken (1972). pre-mRNA transcription was observed at all stages. Comparison of growing RNP chain lengths revealed statistically significant differences in the distribution of shorter fibrils between the successive developmental stages examined. The number of shorter fibrils was lower in the 4--8 cell stage than in either of the two other stages. Transcription complexes of ribosomal type were detected only in 4--8 cell and morula/blastocyst embryos. Structures resembling replication loops were observed within chromatin from all stages. Similar loop-like structures as well as individually transcribed RNP fibrils were also occasionally found on fibres emerging from mitotic chromosomes. The results are discussed in the context of recent findings concerning genetic expression in early mouse embryos.