Molecular genetic control of axis patterning during early embryogenesis of vertebrates.

Formation of the axis and its subsequent patterning to establish the tube-within-a-tube body plan characteristic of vertebrates are initiated during gastrulation. In higher vertebrates (i.e., birds and mammals), gastrulation involves six key events: establishment of the rostrocaudal/mediolateral axis; formation and progression of the primitive streak and organizer; epiboly of the epiblast, ingression of prospective mesodermal and endodermal cells through the primitive streak, and migration of cells away from the primitive streak; regression of the primitive streak; establishment of the right-left axis; and formation of the tail bud. Over 50 years of study of these processes have provided a morphological framework for understanding how these events occur, and recent advances in imaging, microsurgical intervention, and cell tracking are beginning to elucidate the underlying cell behaviors that drive morphogenetic movements. Moreover, homotopic transplantation and dye microinjection studies are being used to generate high-resolution fate maps, and heterotopic transplantation studies are revealing the cell-cell interactions that are sufficient as well as required for mesodermal and ectodermal commitment. Additionally, the roles of the organizer and secondary signaling centers in establishing the body plan are being defined. With the advent of the molecular/genetic age, the molecular basis for axis formation is beginning to become understood. Thus, it is becoming clear that secreted growth factors/signaling molecules produced by localized signaling centers induce and pattern the axis, presumably through downstream activation of signal-transduction proteins and cascades of transcription factors.