Labyrinthine structure of arterial terminals in the human spleen, with special reference to "closed circulation." A scanning electron microscope study.

Spleens of normal structure were obtained in surgery on nine patients with gastric cancer. The freeze-cracked surfaces of the organ as well as the vascular casts of methacrylate resin were examined by scanning electron microscopy. Penicillar arteries were confirmed to terminate in the cords of Billroth, representing the open circulation. Labyrinthine channels of arterial capillaries were found in restricted regions in the red pulp neighboring pulp arteries and veins. They characteristically possessed in their lumen spanning trabecullae covered with endothelial cells. In some places, the flat endothelium of a channel continued to the lattice-like endothelium of a thin sinus, representing closed circulation. The occurrence and distribution of the arteriolar labyrinths were confirmed by SEM observation of the vascular casts. Their continuation to the thin sinuses was also demonstrated in the casts. The present study evidences that in the human spleen, specialized arteriolar terminals provide a closed circulation route in restricted regions, besides the hitherto known, predominant route of open circulation.