A subpopulation of bovine nucleus pulposus (NP)-derived cells stains positive for cytokeratin 8 (CK8). (a) Immunofluorescence staining of CK8 performed on HepG2 cells, used here as a positive control, and on freshly isolated NP cells, annulus fibrosus cells, and articular cartilage cells. Cells were also imaged under classical phase-contrast microscopy (PHASE), and nuclei were visualized with DAPI (4'-6-diamidino-2-phenylindole) (scale bar = 20 μm). Note that some of the NP CK8-positive cells were single cells of appearance similar to the CK8-negative cells shown here, whereas others were in clusters of two or more cells (not shown). Insets show the boxed NP CK8-positive cell at a higher magnification. (b) Histogram of CK8-positive cell counts in formalin-fixed and in methanol-fixed freshly isolated NP cells. (c) Size distribution of CK8-positive and -negative cells on a surface area basis, analyzed using IMOD (Image MODeling) software.