3. The periosteal bud enters the bone's internal cavities and spongy bone starts to form (pre-birth around 3 months).

The bud contains a nutrient arteriole and vein, nerve fibres, red marrow, osteoblasts, and osteoclasts (bone absorbing cells). The entering osteoblasts turn the remaining fragments of hyaline cartilage into spongy bone that gradually spreads throughout the inside of the bone.

4. Formation of the medullary cavity (around birth).

As the primary ossification centre expands, osteoclasts break down the newly formed spongy bone and open up a cavity (medullary cavity) in the centre of the shaft. This cavity is the final step in ossification of the shaft. Throughout the foetal period, the rapidly growing epiphyses consist only of cartilage and they continue to lengthen and elongate the shaft by division of cartilage cells. Bone formation or ossification literally "chases" cartilage formation along the length of the shaft.

5. Ossification of the epiphyses.

Around the time of birth most of our long bones have a bony diaphysis surrounding remnants of spongy bone, a widening medullary cavity and two cartilaginous epiphyses. At birth secondary ossification centres appear in one or both epiphyses. (Short bones - only the primary ossification centre is needed and most irregular bones develop from several ossification centres). The cartilage in the centre of the epiphysis calcifies and deteriorates, opening up cavities that allow entry of a periosteal bud. A bone matrix is then formed in the epiphysis from the remaining cartilage fragments by the entering osteoblasts.

Longitudinal Bone Growth

The growth in length of bones is much the same process as endochondral ossification.

Appositional Bone Growth

This is the process by which bones grow in thickness and diameter. Work by osteoblasts outweighs work by osteoclasts and bone material increases.