Body axis determination during early development in amphibians

Abstract

The specification of the main axes of the body is a phenomenon based on cell communication and is among the early crucial events of embryonic development. Upon fertilization, the amphibian egg reorganizes its cytoplasmic content, leading to the establishment of the future dorsal–ventral axis of the body. Heterogeneous distribution of maternal components confers cellular regionalization after only a few mitoses. Development up to the 4000-cell stage proceeds almost entirely on maternal materials, and during this period there is remodeling of the chromatin to set up specific gene expression in various regions of the embryo. The zygote at this stage has already undertaken cellular interactions leading to mesoderm formation and regionalization. Dorsal mesodermal components then induce the formation of the Spemann's organizer, a structure directly involved in the specification of the anterior–posterior axis of the embryo (head to tail). Molecular analysis of these phenomena has allowed the identification of growth-factor-like and transcription-factor-like proteins that have characteristics typical of specification factors. We will review the recent advances on these molecules and will also discuss the putative role of retinoic acid as a posteriorizing agent.Key words: chromatin remodeling, cellular regionalization, homeobox, inducing factor, retinoic acid.