Postcapillary venule in rabbit tonsil and entry of lymphocytes into its endothelium: a scanning and transmission electron microscope study.

Postcapillary venules in rabbit tonsils were studied by scanning (SEM) and transmission electron microscopy (TEM). Tonsils were perfused with Ringer solution and a glutaraldehyde fixative and blood cells were drained away leaving the migrating lymphocytes in the vascular wall. Specimens were freeze-cracked and the luminal surface of the venules was exposed for examination by SEM. The postcapillary venules under the SEM were lined by endothelial cells conspicuously and irregularly bulging into the lumen. The endothelial cell possessed columnar marginal processes, which were interdigitated at the base of the endothelium with those of adjacent cells, while they often formed intercellular bridges on the luminal surface. The endothelium thus was more intricate in construction than previously regarded. Migrating lymphocytes observed on the endothelium by SEM were located on the endothelial cell boundary region or plugged between the adjacent endothelial cells. Lymphocytes incorporated into the endothelial wall were found in the intercellular space by TEM. Careful analysis of SEM and TEM images led to the conclusion that they pass through the intercellular space in the endothelial wall. Migrating lymphocytes on the endothelium were all small lymphocytes and classified into three types on the basis of their surface architecture (occurrence of each type given