This study was performed in order to understand the reasons why the metastasis of malignancy into the disc is rare in contrast to the commonly seen vertebral metastasis. By histopathologic studies of 14 vertebral columns with vertebral metastases of various malignant tumors and 616 disc specimens, the authors found three pathways for an intravertebral tumor to invade into the adjacent disc: 1) direct infiltration from the rim of the vertebral body not covered by the cartilagenous plate, 2) infiltration from the side of the vertebral body close to the vertebral corner, through the subspace beneath the longitudinal ligament, and 3) hematogenous invasion via small vessels in the subspace beneath the longitudinal ligament. The authors concluded that the cartilagenous plate between the vertebral body and the disc as well as high intradiscal pressure would be acting as a barrier against tumor invasion from the vertebral body into the disc, and suggests that gradual increase of capillaries in the disc with age may enhance hematogenous invasion into the disc in rare occasions.