Phenotypic characteristics of rabbit intervertebral disc cells. Comparison with cartilage cells from the same animals.

STUDY DESIGN: Intervertebral disc cells were extracted from the surrounding matrix, and their metabolic activities and phenotypes were studied.

OBJECTIVES: To compare the metabolic activities and phenotypes of cell populations extracted from the intervertebral discs of young rabbits with those of articular and growth plate chondrocytes from the same animals.

SUMMARY OF BACKGROUND DATA: The phenotype of intervertebral disc cells has been poorly studied and still is debated.

METHODS: The intervertebral discs as well as articular and vertebral growth plate cartilage of rabbits were digested enzymatically. The morphology of freshly isolated cells was examined. Their contents of collagen II and X mRNAs were determined by Northern blot analysis, and their sulfation activity by 35S-sulfate incorporation as chondrocytic markers. Cells were cultured at high density or low density and grown in primary culture. The stability of their phenotype was monitored by evaluating the collagen I and II mRNA ratio. The proteoglycans newly synthesized by the cells also were quantified, and their elution profile analyzed on Sepharose 2B columns.

RESULTS: The anulus fibrosus cells were morphologically undistinguishable from articular chondrocytes. The nucleus pulposus contained mainly large vacuolated cells and a few smaller cells. All freshly extracted cells expressed different levels of collagen II mRNA. Anulus fibrosus and nucleus pulposus cells contained, respectively, 22% and 8% of collagen II mRNA compared with that found in articular or growth plate chondrocytes from the same animal. Only growth plate chondrocytes expressed collagen X. When anulus fibrosus cells were incubated for 48 hours at high density, they had collagen II mRNA contents similar to those of articular and growth plate chondrocytes, but synthesized five to six times fewer sulfated proteoglycans. When seeded at low density, anulus fibrosus cells divided more slowly than articular chondrocytes and incorporated four times fewer 35S-sulfate into proteoglycans. Their collagen II mRNA content was 2.75-fold lower than that of chondrocytes, and the procollagen alpha 1II/alpha 1I mRNA ratio was 3.1 for anulus fibrosus cells and 7 for chondrocytes. No collagen X mRNA was detected. When incubated for 48 hours at high density, the nucleus pulposus giant cells had four times less collagen II mRNA content than cartilage cells but synthesized the same amounts of sulfated proteoglycans. They did not divide during 21 days in culture and still contained collagen II mRNA but no collagen X mRNA.

CONCLUSIONS: Findings showed that intervertebral disc cells all express cartilage-specific matrix proteins with quantitative differences, depending on their anatomic situation. It is suggested that anulus fibrosus cells are chondrocytic cells at a different stage of differentiation than articular and growth plate chondrocytes. The phenotype of nucleus pulposus cells still is unclear. They could be chondrocytic or notochordal. A definitive answer to this important question requires differentiating markers of notochordal cells.