Human molecular embryogenesis: an overview

Molecular embryology is a rapidly evolving field of great complexity. This overview is primarily for the newcomer to this field, in an attempt to demystify the processes by which a human single-celled zygote eventually forms an embryo. Although all embryonic cells share the same genetic information, they differentiate according to the basic plan dictated not only by multiple families of transcription factors to silence some genes and activate others but also through DNA methylation, histone acetylation, and heterochromatinization. Regional expression of various transcription factors causes embryos to establish primary embryonic axes. Once the basic body plan is established, the region-specific diversity becomes progressively finer, and each cell eventually develops a "molecular address" characterized by the expression of specific genes. The overview is divided into two main parts: embryonic cell growth and morphogenesis. At the present time, more is known about the details of molecular regulation of the embryonic growth than about morphogenesis.