Histohematic Barriers

The aim of this review was to analyze the morphofunctional characteristics of histohematic barriers. The development of the concept of histohematic barriers is associated with the name of Academician Lina Solomonovna Shtern (1878–1969). Histohematic barriers are defined as the barriers between the blood in the vessels of the microvasculature and the cellular elements of the parenchyma of a given organ. They maintain the balance of metabolic processes between the blood and tissues and regulate the selective transport of substances between the blood and specific enclosed spaces of various organs. The number of structural-functional systems classified as histohematic barriers currently amounts to several dozen. Among the most extensively studied histohematic barriers are the blood-brain, blood-testis, and placental barriers. The components of any histohematic barrier are: endothelial cells of the blood capillaries, the endothelial basement membrane, the interstitial matrix, and the organ parenchyma (which, in most histohematic barriers, is an epithelium resting on a basement membrane). A crucial feature of histohematic barriers is the presence of a large number of different types of tight junctions between the cellular elements of the vascular and parenchymal parts of the barrier. An important role in the functioning of tight junctions is played by transmembrane proteins, primarily occludins and claudins, as well as a number of other proteins (cadherins, catenins, scaffolding proteins, and various adhesion molecules). The most complex histohematic barrier is the blood-testis barrier. It consists of the following components (in the direction from the blood to the cells of the spermatogenic epithelium): the endothelium of the capillaries located in the testicular interstitium, the endothelial basement membrane, a layer of fibroblast-like cells, the basement membrane and 2–3 layers of myoid cells situated on it, the basement membrane of the spermatogenic epithelium, and the basal portion of the sustentocytes (Sertoli cells). A universal classification of histohematic barriers is still lacking. The interpretation of some barrier systems as histohematic barriers is a subject of debate. It is evident that only those barriers which impede the penetration of certain substances from the blood into the body's tissues can be considered histohematic barrier systems.