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 demy stify 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 multip le families of transcription factors to silence some genes and activate oth ers but also through DNA methylation, histone acetylation, and heterochroma tinization. Regional expression of various transcription factors causes emb ryos to establish primary embryonic axes. Once the basic body plan is estab lished, the region-specific diversity becomes progressively finer, and each cell eventually develops a "molecular address" characterized by the expres sion of specific genes. The overview is divided into two main parts: embryo nic cell growth and morphogenesis. At the present time, more is known about the details of molecular regulation of the embryonic growth than about mor phogenesis.