TRANSPORT OF MACROMOLECULES THROUGH MICROVASCULAR WALLS

This review addresses the long-standing controversy over the principal mechanisms of transport of macromolecules through the endothelium of microvessels of 'normal' permeability. Two types of mechanism have bee n proposed: convective transport through 'large pores' in the endothel ium; transport via vesicles (transcytosis). The different techniques f or estimating microvascular permeability to macromolecules are describ ed and values for microvascular permeability to serum albumin in diffe rent tissues are tabulated. Whereas the evidence for convective transp ort when obtained from experiments on perfused microvascular beds rema ins convincing, attention is drawn to recent measurements using the tr acer uptake technique which suggest that transport in the intact circu lation from blood to tissues may not be coupled to fluid movement. Dir ect evidence for the involvement of endothelial vesicles in transendot helial transport has been reported relatively recently but the mechani sms whereby macromolecules are conveyed through the vesicular system h ave yet to be established. The possibility of convective transport thr ough transient transendothelial channels formed by the fusion of vesic les is discussed.