Describe the process of endochondral ossification before birth

- chondrocytes (which have differentiated from mesoderm) begin to deposit a hyaline cartilage model of the bone that is going to be formed. -Cells at the edge of the model, called the perichondrium, then differentiate into osteoblasts and begin to secrete an osteoid collar around the bone. -Chondrocytes trapped inside the collar then become hypertrophic, get calcified and die. -Following the death of the chondrocytes, blood vessels penetrate the degenerating cartilage model and deposit osteoprogenitor cells -Shortly thereafter, these cells differentiate into osteoblasts and start forming the primary ossification center in the center of the spongy bone of the diaphysis (shaft). -Ossification proceeds outwards from the primary ossification center leaving some space around the blood vessels where the medullary cavity of the bone is formed