Lymphatic Programing and Specialization in Hybrid Vessels

Abstract

Building on a large body of existing blood vascular research, advances in lymphatic research have helped kindle broader investigations into vascular diversity and endothelial plasticity. While the endothelium of blood and lymphatic vessels can be distinguished by a variety of molecular markers, the endothelia of uniquely diverse vascular beds can possess distinctly heterogeneous or hybrid expression patterns. These expression patterns can then provide further insight on the development of these vessels and how they perform their specialized function. In this review we examine five highly specialized hybrid vessel beds that adopt partial lymphatic programing for their specialized vascular functions: the high endothelial venules of secondary lymphoid organs, the liver sinusoid, the Schlemm's canal of the eye, the renal ascending vasa recta, and the remodeled placental spiral artery. We summarize the morphology and endothelial expression pattern of these vessels, compare them to each other, and interrogate their specialized functions within the broader blood and lymphatic vascular systems.

Keywords: endothelium; high endothelial venule; hybrid vessel; liver sinusoid; lymphatic.

FIGURE 1

Comparative wall structure of hybrid vessels. (A) The endothelial cells of high endothelial venules are tall and cuboidal and are associated with pericytes and a thick basement membrane. (B) The endothelium of the liver sinusoid is fenestrated, lacks mural cells, is supported by a thin basement membrane, and is associated with intraluminal Kupffer cells. The Disse space separates the liver sinusoid endothelium from hepatocytes and is inhabited by hepatic stellate cells. (C) The Schlemm’s canal has a discontinuous basement membrane, lacks mural cells, and the inner wall endothelium forms large vacuoles in response to aqueous humor flow from the trabecular meshwork. (D) The endothelium of the ascending vasa recta is fenestrated, lacks mural cells, and takes up water from the descending loop of Henle. (E) The endothelium of the remodeled spiral artery has minimal smooth muscle coverage, a discontinuous basement membrane, and closely associates with uterine NK cells.