Biophysical aspects of connective tissue

Issue title: Microcirculation, Interstitium, Lymph, Pathophysiology and Disease. Proceedings of the International Symposium, Villa La Principessa, Lucca, Italy, June 19–20, 1981

Guest editors: Siegfried Witte

Article type: Research Article

Authors: Silberberg, A.

Affiliations: Weizmann Institute of Science, Rehovot, Israel

Abstract: Mechanical and chemical communication between cells depends upon the structure and biophysical character of the connective tissue space. From the point of view of the microcirculation (mainly its extravascular part and lymph formation) the permeability, selective and unselective, of the connective tissue matrix is of most obvious interest. It turns out, however, that the mechanical properties of the tissue are of no less importance, since they help to determine the gradients in chemical potential which exist between the blood stream and the tissue interstitium on the one hand and the interstitium and the lymph collecting system on the other. These biophysical features of the connective tissue space, the transport and the mechanical properties, depend upon connective tissue structure and composition. It is convenient to divide the materials composing connective tissue into two groups: Immobilized, or essentially immobilized, components and diffusible components. The former involve mainly the fiber network system. This provides the mechanical integrity both of the connective tissue matrix and of the cells which are embedded in it or are attached to it. The fiber system is composed mainly of highly organized collagen fibers but also involves fibers of elastin and a network of structural glycoprotein. The ultimate strength of the tissue is that of the collagen system. The mechanical response to smaller deformation, however, is determined both by collagen fiber bending and elastin fiber rubber-like extensibility. The structural glycoprotein seems to have a modifying role. While these components are truly immobilized they constitute only a minority volume fraction in the tissue space. The voids are filled with diffusible species (mainly water), but also with a system of extremely high molecular weight, hydrophilic proteoglycans/hyaluronic acid associates. The chains of these have the tendency to coil freely through space being charged and largely carbohydrate in nature. Their large size traps them in the fiber environment though they do not really seem to be linked to the fiber system, even by secondary chemical interactions. Because of their intimate contact with water and of the charges they carry, they strongly affect the chemical potential of water and thus determine the tendency of tissue to absorb water. The extent of swelling attained is limited by a pressure rise imposed by mechanical stress induced in the fiber network. In this respect the connective tissue space is behaving like any gel-like system. Of interest here is that the mechanical and concentration effects on water chemical potential are physically separated and arise in two distinctly different immobilized structural components. The diffusible species, which besides water fill the interstitium can also be graded into roughly two groups: A group of low molecular weight components, mainly sodium chloride, which cross the blood/tissue barrier, essentially without hindrance, along with water. A group of high molecular weight species, mainly serum albumin whose passage is severely restricted and which occur, therefore, in the interstitium at a concentration very different to their concentration in blood. The chemical potential of water in tissues thus differs from that in blood because of this concentration difference, because of the presence of the proteoglycan/hyaluronic acid system and because of a difference in hydrostatic pressure. Since the connective tissue space can support only small gradients (due to much faster diffusion than convection of the diffusible species) and the chemical potential of water in blood decreases as the hydrostatic pressure drops from the arterial to the venous end of the microcirculation, the driving force on water will be outward at the arterial end and inward at the venous end. This is the basis of the Starling balance. In addition, water (and diffusible protein) can be cleared from the tissue spaces as lymph. This is necessary since protein driving forces do not permit a direct return to the blood stream by reabsorption.

Keywords: Connective tissue, collagen, proteoglycan, lymph, diffusion

DOI: 10.3233/CH-1982-25-608

Journal: Clinical Hemorheology and Microcirculation, vol. 2, no. 5-6, pp. 497-508, 1982