An electron microscope study was made of the mode of lymphocyte migration across the endothelial layer of venules in the Peyer's patches of mice and rats. Single and serial sections were examined.

Of a total of about 800 lymphocytes observed in single sections, 91% were located between endothelial cells and 9% were surrounded by endothelial cytoplasm in the particular plane of section. 62% of the lymphocytes occurred in groups of two or more. In long sequences of serial sections through 21 endothelial cells, all lymphocytes were located external to the endothelial cells though some appeared "internal" at certain levels of sectioning.

The probability that a lymphocyte which appears to be surrounded by endothelial cell cytoplasm actually lies within the cell was analyzed with a mathematical model derived from data obtained from single sections. The results of this analysis suggested that at least 93–99% of lymphocytes (within 90% limits of confidence) take an intercellular path in their migration from blood to lymph.

It is concluded that lymphocytes migrate across the vascular endothelium by insinuating themselves between endothelial cells and not by passing through them. Rather than constituting an increased barrier to cell migration, the unusual height of the endothelial cells in these vessels is interpreted to be a special adaptation which allows sustained cell traffic without excessive fluid loss taking place concomitantly.

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