Apical cell escape from the neuroepithelium and cell transformation during terminal lip fusion in the house shrew embryo.

The house shrew embryo has many cells in the ventricular lumen and on the luminal surface of the fusing terminal lip of the cephalic neural tube. The origin and fate of these cells were studied by means of light and electron microscopy, and by DiI labeling in a whole-embryo culture system. The cells appeared at stage 11A and persisted until stage 12A. Most of the cells seemed to originate from the neuroepithelium, as shown by frequent observations of epithelial cell escape and DiI labeling analysis. The cells on the luminal surface sometimes showed apoptotic features, but were not subjected to phagocytosis. Some of the escaping cells seemed to migrate to the ventral part of the prosencephalic neuropore and insert themselves into it. Others separated from the luminal surface and floated into the lumen. It seems likely that the floating cells either become autolyzed, or else change into macrophage-like cells, the latter alternative being supported by the results of DiI labeling. The macrophage-like cells actively phagocytosed the other degenerating cells and apoptotic bodies. These observations suggest that the apical escape of cells may play an important role in the remodeling of the neural fold during the terminal lip fusion, and that early neuroepithelial cells may have the potential to become cells with vigorous phagocytic activity, like macrophages.