Basement membrane.

Basement membrane has attracted less attention than the other components of connective tissue. But, like the other extracellular fibres and matrix materials, it can profoundly influence both the structure and function of advanced life forms. Usually taking the shape of thin, structureless cementing material between epithelial and connective tissue, basement membranes are widely distributed throughout the body. They are found as extracellular components of blood capillaries, renal glomeruli and tubules, alveoli, retina, lens capsule, muscle parasarcolemma, sweat glands, Schwann cells, and breast ducts. Although sharing similar morphological, biochemical, and antigenic properties, basement membrane can be developed for different functions in special situations. In the lens capsule of the eye, for example, it provides elastic support while in the glomerulus it is the foundation of selective filtration. Elastic support and filtration represent the essence of basement membrane function. Because structure and function can be examined in detail in the glomerulus, it is an ideal model system in which to study the characteristics of basement membrane (Fig. 1). Moreover, the glomerulus is especially suited to detailed examination because cross-sectional measurements of basement membrane thickness are more precise in glomerular than in muscle or other capillaries (Cameron et al., 1975). Many of the recent biochemical studies have been of glomerular capillaries (Spiro, 1976). Further insight derives from examining the defects associated with glomerular capillary disease. In particular, the metabolic changes in the diabetic glomerulus show how alteration in the biochemical configuration of basement membrane not only modifies filtration but leads to the deposition of macromolecules such as fibrin. Mechanisms leading to fibrin or antigen deposition are obviously relevant to the development of connective tissue disease. Accordingly, this review will focus on the glomerular model, examining normal basement membrane turnover and biochemical structure. The metabolic changes in diabetes that lead to altered basement membrane porosity and fibrin deposition 59 will be reviewed with the aim of providing new insight into the function of basement membrane in health and disease.