A Global Pictorial Assessment of the Intervertebral Disc Cell in Several Species Reveals a Remarkable Biodiversity in this Cell Type which should be taken into Account in Experimental Studies on Intervertebral Disc Repair.

In this study we undertook a global assessment of the morphological characteristics of disc cells in several species. Morphological cellular characteristics in mouse, deer, human foetal and ovine intervertebral disc (IVD) were assessed using histological and immunohistological techniques. Notochordal cells of differing morphologies were identified in these species. Progenitor cell nests were identified in mice in the outer AF at its juncture with the vertebral growth plate. The NP of adult ovine IVDs also contained distinctive colonies of dividing chondroid-like cells within a well defined basophilic aggrecan rich but versican negative matrix. These cells were of a different morphology to authentic disc fibrochondrocytes in the surrounding NP in a fibrous versican rich matrix. HA was also identified as a component of the chondroid cell-nests in the NP. These may represent a progenitor cell niche, HA has been shown to be associated with stem cell niches in a number of tissues where it has a role in the regulation of stem cell quiescence and maintenance of stem cell viability. Chondroid cell nests were also identified in IVDs in which disc degeneration had been induced by an annular lesion, these may be a source of cells for repair processes. The chondroid-disc cell population in the central adult ovine NP differed from all other cell types in foetal and newborn ovine, murine, or cervine IVDs and may represent resident adult progenitor cell population worthy of further study. Similar cellular arrangements have recently been identified in human IVDs, isolated and cultured and shown to express stem cell markers in monolayers and by flow cytometry. The animal species examined in the present study are commonly used in IVD repair studies, the remarkable biodiversity of disc cells observed across species indicate caution should be made in any extrapolations to the human IVD