Live imaging experiments in zebrafish confirmed our computer model predictions that ectomesenchymal cells do not perform extensive individual migrations and predominantly divide in few orientation planes within a local microdomain. Moreover, similar to the situation in the developing limb (17), we discovered that ectomesenchymal cells execute large-scale, collective, coordinated morphogenetic movements, where the cellular arrangements of microdomains remain well preserved (few individual movement at single cell level).
Notice the yellow population (e.g.) in head, showing clear bondary of a single clone, suggesting a crow population movement. The situation in trunk is different, clones are more interminguled and mixed.
So, neural crest cells responsible for generating limbs and head are share a similar crowed movement for enlongation, whereas neural crest cells in trunk have different behaviors. This elongation may contribute from oriented cell divisions (and also crowed movement in population level).
This population behavior highly suggests there is a signal gradient controlling such sychronized cells behaviors.
The individual shape of clonal envelopes reflects the anisotropic (a perfect word to decribe 'oriented') growth of the structure (40, 41) following local orientation cues. Our results show that the cues that orient the plane of cell divisions in the face, at least in part, are represented by the gradient of Wnt5a (invovled in noncanonical wnt PCP pathway), which influences the allocation of daughter cells after mitosis and, through this, the general shape.
Both Wnt5a knock out and overexpression make the limbs and head become wide and short. This indicate the ingredient is important (since gene manipulation erase the gradient)
Neuroglial and mesenchymal directions of differentiation of neural crest cells are not within the same clone (suggest early fate restrictions in the cranial neural crest and the existence of an early choice between neuroglial and mesenchymal directions of differentiation)
Discussion (homology!)
a recent study demonstrated that cranial neural crest cells that give rise to mesenchymal derivatives in the head may undergo EMT from a neural fold domain that might not express neural markers. If true, it can be defined as a non-neural ectoderm and is possibly similar to analogous sites in regions of future limb buds. For instance, it is widely accepted that facial growth and patterning are regulated by the frontonasal ectodermal zone (FEZ), which includes SHH and FGF8 expression domains (44, 45). The roles of BMPs, endothelins, and other soluble factors in facial development and outgrowth have been thoroughly investigated (44, 46). Variation of signals that affect cartilage and bone development may also influence shaping programs at later stages. This is suggested, for example, by studies on BMP3 mutations associated with the size and varying geometry of the vertebrate skull (47). Apparently, these key signals, including SHH, FGFs, and BMPs, play critical instructive roles in both facial and limb development. It could be speculated that the apical ectodermal ridge secreting FGF8 and the zone of polarizing activity emitting SHH in limb buds might be considered to be deeply homologous to the FEZ in the face. Thus, a blueprint of the cellular and molecular logics that operate in the mesenchyme of the anterior head could become a starting point for the induction of appendages in the more posterior parts of the ancestral body. Some evidence suggest that the origin of paired appendages involved redeployment of genetic programs from the paraxial to the lateral mesoderm (48, 49). Our data highlight a great degree of similarity in clonal dynamics between neural crest– and paraxial mesoderm–derived mesenchyme in the face and branchial arches. Together, our results support a profound similarity between vertebrate face and limb development and, in turn, raise questions concerning a deep homology (50) between these seemingly unrelated structures.
https://www.science.org/doi/10.1126/sciadv.1600060
