Dynamics and mechanisms of posterior axis elongation in the vertebrate embryo

Abstract

During development, the vertebrate embryo undergoes significant morphological changes which lead to its future body form and functioning organs. One of these noticeable changes is the extension of the body shape along the antero-posterior (A-P) axis. This A-P extension, while taking place in multiple embryonic tissues of the vertebrate body, involves the same basic cellular behaviors: cell proliferation, cell migration (of new progenitors from a posterior stem zone), and cell rearrangements. However, the nature and the relative contribution of these different cellular behaviors to A-P extension appear to vary depending upon the tissue in which they take place and on the stage of embryonic development. By focusing on what is known in the neural and mesodermal tissues of the bird embryo, I review the influences of cellular behaviors in posterior tissue extension. In this context, I discuss how changes in distinct cell behaviors can be coordinated at the tissue level (and between tissues) to synergize, build, and elongate the posterior part of the embryonic body. This multi-tissue framework does not only concern axis elongation, as it could also be generalized to morphogenesis of any developing organs.

Keywords: Axis elongation; Bird embryo; Live imaging; Morphogenesis; Multi-tissue; PSM; Proliferation; Tissue deformations.

Fig. 1

Schema of posterior tissue morphologies and organization during posterior elongation. Dorsal view (top) and transverse sections (bottom) of the posterior part of higher vertebrate embryo during different times of axis elongation (from left to right: equivalent to stage 9HH, 11HH, and 15HH in chicken embryo). Paraxial mesoderm is in purple, neural tissue in green, notochord in red, lateral mesoderm in pink, ectoderm in blue, and endoderm in dark green, progenitor domain in purple and green. Ectoderm is not represented on the dorsal views. Note that tissues have a higher width posteriorly than anteriorly in the early phase (black arrows), whereas they are becoming more straight during later phases. General tissue organization remains conserved throughout the stages (transverse section); A anterior, P Posterior

Fig. 2

Schema of the main cell and tissue movements during posterior elongation. Dorsal view of a schematized vertebrate embryo during posterior axial elongation. Local cellular movements on the left schema, relative tissue movements on the right schema. Note that tissue movements are shown relative to the last formed somite. Note that for a matter of simplicity, the elongation phase represented is a transition phase in which early and late phase movements are both present (equivalent of stage 10–11HH in chicken embryo)