Abstract
We reconstructed serial sections of a representative adult human spleen to clarify the unknown arrangement of the splenic microvasculature, such as terminal arterioles, sheathed capillaries, the red pulp capillary network and venules. The resulting 3D model was evaluated in virtual reality (VR). Capillary sheaths often occurred after the second or third branching of a terminal arteriole and covered its capillary side or end branches. The sheaths started directly after the final smooth muscle cells of the arteriole and consisted of cuboidal CD271++ stromal sheath cells surrounded and infiltrated by B lymphocytes and macrophages. Some sheaths covered up to four sequential capillary bifurcations thus forming bizarre elongated structures. Each sheath had a unique form. Apart from symmetric dichotomous branchings inside the sheath, sheathed capillaries also gave off side branches, which crossed the sheath and freely ended at its surface. These side branches are likely to distribute materials from the incoming blood to sheath-associated B lymphocytes and macrophages and thus represent the first location for recognition of blood-borne antigens in the spleen. A few non-sheathed bypasses from terminal arterioles to the red pulp capillary network also exist. Red pulp venules are primarily supplied by sinuses, but they also exhibit a few connections to the capillary network. Thus, the human splenic red pulp harbors a primarily open microcirculation with a very minor closed part.
Highlights
The red pulp of the human spleen exhibits a unique microvasculature (Steiniger and Barth 2000; Steiniger 2015)
After having discovered that strong expression of CD271 could be used to identify stromal capillary sheath cells, we demonstrated sheaths as rather long structures which started in a direct post-arteriolar position and covered up to three branchings of the respective capillary
Four regions of interest (ROIs) were arbitrarily chosen in the first serial section (Fig. 1) of a representative adult human spleen and were processed for 3D reconstruction
Summary
The red pulp of the human spleen exhibits a unique microvasculature (Steiniger and Barth 2000; Steiniger 2015). The blood passes a reticular connective tissue without endothelia and, enters venous sinuses which join and form venules. The blood-containing spaces in the reticular connective tissue of splenic cords represent the open component of the splenic circulatory system. This special construction makes the splenic red pulp cords the most effective phagocytic compartment of the human body. It provides a storage compartment for platelets (Kotzé et al 1986). It is an unsolved question how blood coagulation is prevented in the absence of endothelial cells
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