Lymphocyte migration through the wall of postcapillary venules in the fetal development of human palatine tonsil.

Migration of lymphoid cells through the wall of postcapillary venules was studied in the palatine tonsils of five human fetuses. The direction of the migration of lymphoid cells as observed in the early fetal period seems to be mostly from the lumen of the venule into the tonsillar parenchyma. Migration starts with the adherence of the lymphocyte by means of a cytoplasmic projection to the cellular surface of a high-endothelial cell. The projection slips between two neighbouring endothelial cells and extends to settle on the basal membrane of the endothelium. After the disruption of the basal lamina the lymphocyte migrates into the subendothelial connective tissue and further into the extrafollicular tissue of the tonsil. The possibility of transendothelial migration of lymphoid cell through the wall of the postcapillary venule is discussed. Even in case, when the lymphocyte appears to be completely enveloped by the cytoplasm of the endothelial cell it may be in reality the intercellular migration--the lymphoid cell is pressed into the cytoplasm of an endothelial cell.