Chromatin structure and gene expression in the preimplantation mammalian embryo.

The preimplantation period of mammalian development encompasses the formation of the zygote, the activation of the embryonic genome, the first cellular polarizations and the beginning of cellular differentiation. It is a period of transitions; protamines are replaced by histones, maternal control of development is succeeded by zygotic control, the highly methylated haploid parental genomes form a diploid genome which undergoes a wave of demethylation. It is also a period of manipulation of embryos for improvements in animal production; embryos are transferred, cryopreserved, cloned, microinjected with transgene constructs. This paper reviews the role that dynamic changes in chromatin structure and nuclear architecture play in the biological transitions of the preimplantation embryo. Data on the nature of these changes is presented, avenues of research that are opening up are discussed and potential applications in animal production are proposed.