Abstract
As the evolutionary descendants of the fresh water coelenterate mesogloea, basement membranes with similar chemical composition and physical characteristics to mesogloea are able to undergo deformation and flow. On the basis of studies on duodenal villous, skin, capillary and glomerular capillary basement membranes it is proposed that the function of basement membranes, whether in normal or pathological situations, can be explained as resulting from their thixotropic nature, that is, the capacity to undergo localized reduction in viscosity without change in temperature, followed by a comparatively slow recovery to their original consistency. The structural characteristics which provide the unique properties of biological thixotropes are probably similar to those of other thixotropic systems, in that they have an internal structure formed from anisometric particles. It is considered that biological thixotropes have an internal structure composed of rod or fibre-like units arranged into a comparatively loosely bonded anisometric lattice which is fully hydrated, with water as the dispersed phase. Reduction in viscosity occurs, when, as a result of the localized application of external energy, lattice units are displaced and fall free in the dispersion medium. Re-formation of the lattice and restoration of normal viscosity is achieved by the random redistribution of lattice units by Brownian movement. This time-dependent phase means that the deformed area of the lattice will exhibit reduced viscosity until lattice re-formation has been achieved. Thixotropic membranes with these features would permit small molecules and solutes to pass freely through the lattice. Larger molecules, motile cells and motile organisms would need to cause a localized deformation of the gel lattice in order to traverse the membrane. The energy needed to deform the gel could be obtained from increased intracapillary pressure, or by the expenditure of energy on the part of motile cells or organisms. Increased permeability of capillary basement membranes probably is due to the thixotropic nature of the membrane, the extent to which basement membrane is covered by endothelium and the level of intracapillary blood pressure. In other sites basement membranes are freely penetrated by cell processes, pseudopodia of motile cells and by motile organisms, all of which are able to exert localized pressure adequate to deform the membrane gel.