Invasion of a basement membrane matrix by chick embryo primitive streak cells in vitro

At the time of gastrulation in the chick embryo the upper epiblast layer penetrates its own basement membrane at the primitive streak so that its cells may invade the underlying tissue space. In so forming the primary mesoderm, the cells undergo a concomitant epithelial-to-mesenchymal transform-ation. In this study, epiblast tissue has been explanted onto a basement membrane gel in order to examine its invasive potential. Fully ingressed primary mesoderm cells were able to penetrate the gel as individual cells, during the course of which the texture of the gel was disrupted. By contrast, epiblast tissue taken from the immediate vicinity of the primitive streak penetrated the gel, but only as a coherent tongue of cells and without gel disruption. These tongues of cells did not undergo the epi-thelial-to-mesenchymal transformation, and conse-quently spread as a epithelial sheet when replated on glass. Thus, the absence of gel disruption corre-lated with the failure of transformation, suggesting that these two events may be linked and that they may require in situ cell interactions for their mani-festation. Tissue from the lateral epiblast failed to penetrate the gel. Instead, this tissue either spread on the gel surface or rounded up into a hollow sphere with the basal surface of the cells innermost. In the former case, despite the cell spreading, no lamina densa 'was organized beneath the sheet, but in the latter case polarity reversal occurred with the formation of a new lamina densa at the cell-gel interface. These results demonstrate that epiblast cells from the primitive streak region tend to show some inherent invasiveness, while more lateral cells do not. However, although this in vitro system may model such embryonic invasive potential, it cannot elicit a prompt epithelial-to-mesenchymal transformation. Key words: chick embryo, gastrulation, invasiveness, basement membrane