Guidance of filopodial extension by fibronectin-rich extracellular matrix fibrils during avian gastrulation. A study using confocal microscopy.

We have used double-label confocal microscopy to examine the relationships between the orientation of filopodial extension in mesoderm cells and the orientation of fibronectin-rich extracellular matrix fibrils during chick embryo gastrulation. We fluorescently labeled mesoderm tissue dissected from donor embryos by immersion in carboxyfluorescein and then grafted it into unlabeled host embryos at the same stage of gastrulation. After further incubation, the host embryos were fixed, the endoderm removed, and the extracellular matrix was immunostained with antibodies to fibronectin conjugated to Texas Red. We found that both the general shape of the mesoderm cells and the orientation of filopodial extension were influenced by the surrounding matrix fibrils. Elongated shape was associated with individual fibrils which impinge on only one side of the cell. Similarly, filopodial extension followed a single fibronectin-rich fibril, although filopodia were also observed to be channeled between pairs of parallel fibrils. Cells attached to non-aligned regions of substratum showed no polarity. The mesoderm cells themselves apparently synthesize their own fibronectin, and deposit this on the cell surface not attached to the substratum. We conclude that individual fibronectin-rich substratum fibrils, in the size range 0.7-2.8 microns, are able to exert contact guidance on the mesoderm cells, despite the production of endogenous fibronectin by the cells themselves. These results support the contention that contact guidance is a physiological mechanism influencing the orientation and directionality of cells during the morphogenetic movements of embryogenesis.