Extracellular matrix in development of the intervertebral disc

Intervertebral discs allow bending and twisting of the spine whilst resisti ng compression from gravity and muscle action, and are composite structures of the peripheral annulus fibrosus enclosing the nucleus pulposus. Their d evelopment is complex, involving several different connective tissue types, yet little is known of the developing extracellular matrix (ECM). We repor t the ECM composition of foetal rat discs from their first appearance to bi rth. The earliest collagen detected was type III, which was subsequently re placed by type II in the cartilaginous inner annulus and joined by type I i n the fibrous outer annulus. Type IV collagen appeared in outer annulus, as sociated with myofibroblast-like cells of the orienting collagenous lamella e. Laminin and fibronectin co-distributed here in later stages, although ov erall they had a wider distribution. Aggrecan ocurred in early nucleus pulp osus and then appeared in the inner annulus, in association with cartilage differentiation. Versican appeared later in the inner annulus, and also in the dorsal region of the outer annulus. Comparisons of glycosaminoglycan an d proteoglycan label allowed extrapolations to be made as to likely glycosa minoglycan components of the large proteoglycans, and of other proteoglycan s that may be present - thus differential distribution of aggrecan and kera tan sulfate label suggested the presence of fibromodulin and/or lumican. Fu nctionally aggrecan would confer compression resistance to cartilaginous st ructures. Versican may also contribute, but along with the small proteoglyc ans is likely to be associated with various stages of control of cell diffe rentiation, tissue morphogenesis and collagen fibre formation in the assemb ly of the annulus fibrosus. (C) 2001 Elsevier Science B.V./International So ciety of Matrix Biology. All rights reserved.