Fine structure of interendothelial slits in the sinus wall of the human splenic red pulp. (Ai) Reticular meshwork in the human spleen red pulp made of cords (co) and sinus lumens (sl) on a Giemsa-stained histologic section (original magnification ×200). (Aii) Typical scanning electron micrograph of the abluminal side of a sinus wall in the red pulp, with periodic helical basal fibers (white arrowheads), normal RBC (stars), and a narrow interendothelial slit (horizontal thick arrow). (B) Squeezing of RBCs crossing an interendothelial slit, from the cord (co) to the sinus lumen (sl) on a Giemsa-stained histologic section (original magnification ×1000): (i black arrows), scanning (ii, white square), or transmission (iii, white square) electron micrograph of a human spleen. (C) Same pictures at original magnification ×4000. On electron micrographs (ii-iii), one part of the RBC (star) has emerged in the sinus lumen, although the other is still in the process of leaving the cord (white arrow). This illustrates that the interendothelial slits are not long enough (typically < 3.5 μm) to accommodate the whole elongated RBCs as they squeeze through. Bi-iii: bar represents 4 μm; Ci-iii: bar represents 1 μm.