Mechanisms of invasiveness of epithelial tumours: ultrastructure of the interactions of carcinoma cells with embryonic mesenchyme and epithelium.

Three different kinds of carcinoma cell, human breast tumours, mouse lung tumour and rat bladder tumours have been implanted into the developing chick wing to assay their invasive behaviour. We found that the majority of the carcinoma cells did not invade the mesenchyme but were positioned in the ectoderm. We examined the ultrasturcture of the interaction of the carcinoma cells with both the ectoderm and the mesenchyme to see if there are any differences. We found no specialized adhesive junctions between carcinoma cells and mesenchyme cells. In contrast, the carcinoma cells seem to be adhesive to the ectoderm cells; desmosomes between the two have been found and also frequent parallel alignment of membranes over long distances. This difference between the interaction of the carcinoma cells with the mesenchyme and the ectoderm may explain why the carcinoma cells can penetrate into the ectoderm but rarely do so into the mesenchyme. The carcinoma cells formed a smooth border with the mesenchyme even in the absence of a basal lamina. These results are discussed in relation to the mechanisms of invasion of epithelial tumours in vivo. In particular, the relative importance of the basal lamina, and cell adhesion within the tumour, in limiting invasiveness are assessed.