Human intervertebral disc cell morphology and cytoskeletal composition: a preliminary study of regional variations in health and disease

Classically, intervertebral disc cells have been described as fibrocytic in the anulus fibrosus and chondrocytic in the nucleus pulposus. Recent animal studies, however, have suggested that disc cell morphology may be more complex than previously considered. Here, by utilizing labelling of components of the cytoskeleton in combination with confocal microscopy, we have examined the detailed morphology of human intervertebral disc cells in pathological and non-pathological tissue. Filamentous-actin- and vimentin-positive cells that appeared either fibrocytic or chondrocytic were observed in all intervertebral discs. However, in localized areas of the disc, stellate cells that extended multiple, branching cytoplasmic processes into their surrounding matrix were also seen. This stellate appearance formed a marked feature of disc cells regionally in certain pathologies, i.e. in cells of the outer anulus fibrosus in scoliotic discs and in inner anulus/nucleus pulposus cells in one spondylolisthetic disc. We conclude that the phenotypic variation of human intervertebral disc cells should be extended to include cells with a stellate appearance, which may be more prevalent in tissue that has been subjected to abnormal load or tension.