Intervertebral disc tissue engineering I: characterization of the nucleus pulposus.

The characteristics of the nucleus pulposus cells from adult rabbits maintained in in vitro cultures were described in another study. Herein, the authors provide a parallel profile of adult rabbit nucleus pulposus in situ, therefore allowing direct comparisons between in vitro and in situ investigations. Nucleus pulposus specimens from adult rabbits were evaluated using biochemical and immunohistochemical morphologic techniques. The nucleus pulposus from adult rabbits contained cell clusters embedded in proteoglycan-collagen matrix. The cells exhibited a well-defined Golgi system, an extensive endoplasmic reticulum, and a complex vesicular system filled with beaded structures (proteoglycans). Neither necrotic nor apoptotic cells were evident. There was a lack of mitochondria. The extensive extracellular matrix contained amorphous, beaded, and fibrillar components. The fibrillar banding was indicative of Type VI collagen. The nucleus pulposus of adult rabbits expressed aggrecan, collagen Type I and Type II, and CD44, but not collagen Type X and displayed low alkaline phosphatase activity.