We characterized a novel extracellular matrix element that is present in the earliest developmental stages of Xenopus laevis, and is recognized by an mAb 3D7. Based on amino acid composition, breakdown patterns by bacterial collagenases, and the molecular weights of the components of the antigen (240, 200, and 140 kD), we found it very similar to mammalian collagen type VI. The antigen is evenly distributed in unfertilized eggs. Shortly after fertilization, it becomes localized intracellularly in the periphery of the cleaving embryo as well as in the extracellular spaces. During gastrulation, the antigen was localized in the cells lining the blastopore and in the extracellular space between the two cell layers, in the presumptive archenteron. When Fab elements of the 3D7 antibody were added to the culture medium, gastrulation was blocked, suggesting a role for the antigen in gastrulation movements.

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