Immunolocalization of cytokeratin 8 (CK8) in tissue sections. Immunofluorescence staining was performed on frozen bovine (a-d) and porcine (e,f) nucleus pulposus (NP) tissue sections. Tissue sections were also imaged under classical phase-contrast microscopy (PHASE), and nuclei were visualized with DAPI (4'-6-diamidino-2-phenylindole) (scale bar = 100 μm). Bovine NP cells were CK8-positive only in cell clusters seen in a gelatinous region of the matrix (a,b). In other areas, no CK8-positive cells were seen (c,d). All cells in porcine notochordal discs (NCH) were CK8-positive (f).