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Little is known about the cytoarchitecture of growth fronts in developing mammalian nerves. We report here the first quantitative, ultrastructural analysis of growth cones (GCs) and their immediate cellular and tissue environment at tips of growing nerves that are nearing their targets in fore limbs of E14 rat embryos. Schwann cell precursor (SCP) marker, p75 neurotrophin receptor, and growth cone marker, SCG10, were used to identify nerve fronts, respectively. Using confocal 3D reconstructions and immunoelectron microscopy, we found that growth cone and Schwann cell precursor migrate together at the nerve front, where growth cone contact adjacent growth cone and Schwann cell precursor with similar frequency. Schwann cell precursor are extensively connected by adherens junctions and form elaborate scaffolds that enmantle growth cone at nerve fronts, so that 80% of the nerve front surface is covered by …