Light microscopy of developing fetal vertebral column. In the fetus, vertebral bodies are formed of cartilage (blue stained) which is gradually replaced by new bone starting in the centre of the cartilage (ossification centres, pink/light blue) and moving outwards. The replacement of cartilage with new bone formation is called endochondral ossification. Elliptical-shaped structures, each known as a nucleus pulposus, are the remnants of the embryonic body axis and will form the centres of the intervertebral discs. Magnification x90 when narrow width printed at 10 cm.