Basal lamina is not a barrier to neural crest cell emigration: documentation by TEM and by immunofluorescent and immunogold labelling

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Abstract
One of the factors proposed to control initiation of migration of neural crest (NC) cells is disruption of the basal lamina (BL) that is presumed to exist over the dorsal portion of the neural tube. Previously, we discovered that, in the mouse embryo, a continuous BL is not deposited over the dorsal portion of the neural tube until emigration of the NC cells is terminated. Here, we show that the pattern of BL deposition in chick embryos is similar, but not identical, to that in the mouse. In particular, (i) patches of BL are deposited on the premigratory NC cells in the chick but not in the mouse and (ii) BL is thicker and more interstitial matrix is deposited at the same stage of development in the chick. In addition, immunofluorescent and immunogold labelling of collagen IV, laminin and fibronectin show that (i) patches of young BL contain all three molecules; (ii) collagen IV and laminin are present in BL throughout neurulation but fibronectin either disappears or becomes masked in more mature BL and (iii) collagen IV and especially fibronectin are present in the interstitial matrix, but the relative abundance of fibronectin changes with time. The simultaneous use of immunolabelling for both light and TEM sections has allowed us to determine unambiguously that presence of a basement membrane (light microscopy) does not necessarily imply presence of basal lamina. We conclude that, as in mouse, the BL cannot be involved in the timing of the initiation of migration of NC cells. Our evidence in both the mouse and the chick, together with work in the axolotl, suggests that the basic pattern of BL deposition during neurulation may be a general phenomenon in embryonic development. Moreover, these results, in conjunction with the work of others, suggest that the critical step for initiation of migration of NC cells may be the loss of adhesions between cells.