Role of actin stress fibres in the development of the intervertebral disc:Cytoskeletal control of extracellular matrix assembly

Orientation of collagen fibrils is a key event in the development of many t issues. In the intervertebral disc, the outer annulus fibrosus comprises la mellae of parallel collagen fibres, the direction of orientation of the lon g axis of which alternates in angle between lamellae. In development, this organisation is preceded by the formation of sheets of oriented fibroblasts , which then deposit the oriented lamellae. Here, using fluorescent labelli ng, confocal and electron microscopic techniques on developmental series, w e show that the orientation of cells in lamellae is associated with the for mation of adherens junctions intercellularly, involving cadherins and vincu lin, and longitudinal stress fibres (label for filamentous actin and tropom yosin) intracellularly. The stress fibres direct the initial elongation of cells and control the deposition of oriented extracellular matrix via junct ional complexes with the matrix involving vinculin and alpha 5 beta 1 integ rins, which in turn promote the formation of oriented fibronectin at the ce ll surface; oriented collagen is deposited between cells at the same stages . Shortly after birth, the stress fibres disappear, probably because cells now gain orientational cues from the matrix, and are undergoing differentia tion-related changes to form fibrocartilage cells. (C) 1999 Wiley-Liss, Inc .