Dady, A. | bioRxiv Science

bioRxiv2021

A lateral protrusion latticework connects neuroepithelial cells and is regulated during neurogenesis

Dynamic contacts between cells within the developing neuroepithelium are poorly understood but play important roles in cell and tissue morphology and cell signalling. Here, using live-cell imaging and electron microscopy we reveal multiple protrusive structures in neuroepithelial apical endfeet of the chick embryonic spinal cord, including sub-apical protrusions that extend laterally within the tissue, and observe similar structures in human neuroepithelium. We characterise the dynamics, shape, and cytoskeleton of these lateral protrusions and distinguish these structures from cytonemes/filopodia and tunnelling nanotubes. We demonstrate that lateral protrusions form a latticework of membrane contacts between non-adjacent cells, depend on actin but not microtubule dynamics and provide a lamellipodial-like platform for further extending fine actin-dependent filipodia. We find that lateral protrusions depend on the actin-binding protein WAVE1: mutant-WAVE1 misexpression attenuated protrusion and generated a round-ended apical endfoot morphology. However, this did not alter apico-basal cell polarity nor reduce tissue integrity. During normal neuronal delamination sub-apical protrusions were withdrawn, but mutant-WAVE1-induced precocious protrusion loss was insufficient to trigger neurogenesis. This study uncovers a new form of cell-cell contact within the developing neuroepithelium regulation of which prefigures neuronal delamination.

developmental biology↗

bioRxiv2021

Human spinal cord differentiation proceeds rapidly in vitro and only initially maintains differentiation pace in a heterologous environment

Species-specific differentiation pace in in vitro assays indicates that some aspects of neural differentiation are driven by cell-autonomous processes. Here we describe a novel in vitro human neural rosette assay that recapitulates the temporal sequence of dorsal spinal cord differentiation but proceeds more rapidly than in the human embryonic spinal cord, suggesting that in vitro conditions lack endogenous signalling dynamics. To test the extent to which this in vitro assay represents a cell intrinsic differentiation programme, human iPSC-derived neural rosettes were homo-chronically grafted into the faster differentiating chicken embryonic neural tube. Strikingly, in vitro human differentiation pace was not accelerated, even in single host-integrated cells. Moreover, rosette differentiation eventually stalled in a neural progenitor cell state. These findings demonstrate the requirement for timely extrinsic signalling to accurately recapitulate human neural differentiation tempo, and suggest that while intrinsic properties limit differentiation pace, such signals are also required to maintain differentiation progression.

developmental biology↗