Scanning- and transmission electron-microscopic study of lymphatic vessels in the splenic white pulp of the macaque monkey.

The fine structure of the lymphatic vessels in splenic white pulp of the macaque monkey was studied by scanning and transmission electron microscopy. Lymphatic vessels were slit-like or widened channels which extended along central arteries and their large branches. The walls of the vessels were very thin in comparison with those of nearby arteries. They were composed only of a layer of endothelium supported by underlying reticular cells. Endothelial cells were mostly ribbon-like and extended along the long axis of the vessels. Perikarya of the endothelial cells were slightly protruded into the lumen. The thin peripheral cytoplasm showed smooth surfaces, except for some tiny processes, especially at boundaries between adjacent cells. The basal surface of the endothelial cells was attached to the lattice of reticular cell processes forming the framework of the white pulp. Basal laminae in strands were intercalated between endothelial cells and reticular cells. Perforations were often seen through the endothelial cell cytoplasm. Lymphocytes or processes of macrophages seen in the perforations were considered to be in migration. Large patent openings through the endothelium were not observed. The wall structure of the lymphatic vessels in the splenic white pulp suggests that lymphocytes in the white pulp may move directly into the lymph flow, in addition to moving into the blood flow via the vascular sinuses.