Fine structure of cartilage canal and vascular buds in the rabbit vertebral endplate

Abstract

Object The vascular terminations (vascular buds) in the bone–disc junction area are structurally very similar to cartilage. In all previous studies to date, however, the roles of cartilage canals and vascular buds were mainly discussed using histological and transparent sections but not electron microscopic sections. The purpose of this study was to clarify the ultrastructure of the vascular bud seen in the bone–disc junction in comparison to the cartilage canal. Methods Japanese white rabbits from 2 days to 6 months of age were used in this study. The bone–disc junctions were examined by microangiogram and light and electron microscopy, and morphological changes and their association with the age of the animals were noted. Results The fine structure of the vascular bud was similar to that of the cartilage canal that nourished the growing cartilage. They were composed of arteries, veins, capillaries, cells resembling fibroblasts, and macrophages. The capillaries in the cartilage canal were all the fenestrated type. Vascular buds were seen over the entire bone–cartilage interface, with maximum density in the area related to the nucleus pulposus. They projected into the bone–disc junction area from the vertebral body contacting the cartilaginous endplate directly. Conclusions The results of this study clarify the formation process and ultrastructure of the vascular bud seen in the bone–disc junction. The authors found a strong structural resemblance between the vascular bud and the cartilage canal and hypothesize that the immature cells seen surrounding the cartilage canal and vascular bud represent a common precursor for the 3 main types of connective tissue cells seen during early vertebral development.