Cell movements driving neuruiation in avian embryos

Neuruiation, formation of the neural tube, a crucial event of early embryogenesis, is believed to be driven by the coordination of a number of diverse morphogenetic cell behaviors. Such behaviors include changes in cell number (division, death), cell shape and size (wedging, palisading and spreading), cell position (rearrangement or intercalation) and cell–cell and cell–matrix associations (including inductive interactions). The focus of this essay is on epiblast cell movements and their role in shaping and bending of the neural plate. Neuruiation is a multifactorial process requiring both intrinsic (within the neural plate) and extrinsic (outside the neural plate) forces. The origin and movements of three populations of epiblast cells have been studied in avian embryos by constructing quail/chick transplantation chimeras and by labeling cells in situ with identifiable, heritable markers. MHP (median hinge-point neurepithelial) cells originate principally from a midline epiblast area rostral to and overlapping Hensen's node. In addition, a few caudal MHP cells originate from paranodal epiblast areas. MHP cells stream down the length of the midline neuraxis in the wake of the regressing Hensen's node. This streaming occurs as a result of cell division (presumably oriented so that daughter cells are placed into the longitudinal plane rather than into the transverse plane) and rearrangement (intercalation), resulting in a narrowing of the width of the MHP region with a concomitant increase in its length. L (lateral neurepithelial) cells originate from paired epiblast areas flanking the rostral portion of the primitive streak, and they stream down the length of the lateral neuraxis concomitant with regression of Hensen's node. They do so both by oriented cell division and by intercalation. SE (surface epithelial) cells originate from the epiblast of the area pellucida, as far lateral as near the area pellucida area opaca border. From this area they stream medially, toward the forming lateral margins of the neural plate. Collectively, movements of the three populations of epiblast cells generate the convergent-extension movements characteristic of the epiblast during neuruiation. Heterotopic grafting has been used to assess the relationship between cell position and cell fate and to determine whether transplanted heterotopic cells can adopt the behaviors typical of the new site. For example, SE cells can replace L cells, changing their fate and adopting L-cell behavior. Similarly, prospective MHP and L cells both can change their fate and adopt the behavior of SE cells. L cells, when placed into prospective MHP-cell territory, move out of this territory by intermingling with adjacent host L cells. Likewise, prospective MHP cells placed into L-cell territory, move out of this territory by intermingling with host MHP cells. Collectively, these results suggest that cell fate is determined principally by the ultimate position of cells, and that adjacent, different cell populations are restricted from intermingling with one another. How positional information is specified, the nature of restriction of intermingling and the guidance cues used for cell navigation during streaming remain to be elucidated.