Some structural aspects of precapillary vessels.

According to the present state of the art, the former Chambers-Zweifach conception is no longer tenable as the ubiquitous module of microvascular organization. Any attempt to define the consecutive segments of precapillary vessels in a random collection of sectioned tissues is impaired by the extreme scatter of variations and the lack of additional criteria necessary for a distinct classification. With these inadequacies in mind, arterioles, terminal arterioles, and their precapillary segments (sphincters?) are identified by applying the following parameters: luminal diameter, composition of the subendothelial space, and thickness as well as completeness of the media. Endothelial components of particular functional significance are different types of cytoplasmic filaments and myoendothelial junctions. The former serve as a cytoskeleton and may provide a mechanism to adapt endothelial tensile strength to variations of shearing forces. The latter appear to be sites of firm intercellular attachment and of electrotonic coupling of the endothelium to the underlying smooth-muscle cells. The former unanimously accepted helicoidal arrangement of vascular smooth muscle (VSM) must be modified according to most recent findings, and the tridimensional shape of VSM and its transition to pericyte-like elements remain to be further elucidated. Future effects, from which solutions to the many open problems may be expected, must therefore aim at combined vital-, light-, and electron-microscopic studies of the methodically accessible microvascular beds. This necessity has already been recognized, and first promising steps have been taken.