Labelling of basement membrane constituents in the living chick embryo during gastrulation.

The basement membrane of the living chick embryo epiblast has been labelled with ultrastructural markers in order to study the movement and turnover of this structure during gastrulation. Two problems were addressed in these experiments. Firstly, to what extent does the basement membrane move medially with the epiblast during morphogenesis? Secondly, what is the relationship to the basement membrane of the so-called interstitial bodies? The ultrastructural markers used were concanavalin A conjugated to ferritin and fibronectin antibodies conjugated to peroxidase. Embryos were cultured using the technique of New, and the label was applied to the periphery of the basal surface of the epiblast through a hole in the endoblast at the early primitive streak stage of development. The embryos were then allowed to develop to the full primitive streak stage in the presence of the label. When the position of the label was determined after incubation, it was found to have accumulated in large amounts at the edge of the primitive streak at the point where the basement membrane is disrupted. This indicates that constituents of the basement membrane are transported medially with the epiblast cells and are sloughed off as the latter pass through the primitive streak. This movement of basement membrane constituents is counter to the direction of migration of the underlying mesoderm cells. When embryos are exposed to label for only 1 h, then washed and incubated for a further three hours, the marker was found in the interstitial bodies and not distributed throughout the basement membrane itself. This suggests that the interstitial bodies, which have been implicated in influencing the migration of the mesoderm cells, are turnover products of the basement membrane to which they are attached.