A study of fine structural changes in the cartilage-to-bone transition within the developing chick vertebra.

Abstract

AbstractForty‐eight chick embryos were killed at 9–16 days of incubation age. Tissue was obtained from the fourth or fifth cervical vertebra, immersed in Karnovsky's fluid, post‐fixed in osmium tetroxide, dehydrated in ethanol, stained “en bloc” with uranyl acetate in ethanol, and embedded in Epon 812. Vertebrae were oriented for cross‐sectional microtomy in cephalic to caudal sequence. Thin sections were stained with uranyl and lead salt solutions saturated with tribasic calcium triphosphate to prevent decalcification.Chondrocytes within the cartilaginous vertebral body occur in various stages of degeneration without orderly arrangement. Both reversible and irreversible stages are found at the cartilaginous resorption front. Electron‐lucent osteoid and mineralization appear in the intercellular matrix at about 12.5 days. Rapidly invading blood vessels form a highly variable resorption front and irregular marrow cavity. Capillaries with accompanying cells border on the front, but else‐where open capillaries allow blood elements to be in direct contact with cartilage. Chondroclasts are associated with small areas of calcified cartilage. At about 14 days trabeculae are formed at the resorption front by osteoblasts which deposit bone osteoid on uncalcified cartilaginous matrix. The matrix is eroded away. A free trabecula of bone without a core of calcified cartilaginous matrix remains.Basic differences between developmental growth processes in the epiphyseal plate and vertebral body may stem from the large amount of uncalcified cartilaginous matrix at the latter's resorption front.