Bone Development: Endochondral Ossification

Endochondral ossification is the other majory embryonic process of bone formation. This is a process that uses hyaline cartilage as the model for long bone formation. A good way to remember the difference between endochondral and intramembranous ossification is to pay attention to their names. Remember the Greek word root for cartilage is chondr/o.

Here are the events that occur during this process in short; you can get a more detailed explanation in the video below:

1) Collar formation: periosteum forms around a piece of hyaline cartilage. Then the osteogenic cells (stem cells) differentiate (specialize) into osteoblasts. The osteoblasts then secrete their organic fibers (proteins) on the outside of the cartilage. Remember that these condensed fibers are called the osteoid. The end result of this is the formation of a bony collar on the outside of the cartilage.

2) Cavity formation: while step #1 occurs the cartilage in the center of the model begins to ossify (form into bone). This centered cartilage is called the primary ossification center. The calcification/hardening of the center makes the inner cartilage impermeable to the diffusion of nutrients. As a result the inner cartilage starts to deteriorate and cavities begin to form.

3) Vascular invasion: eventually the vessels that are within the periosteum will pass through the lamellar (compact bone) bony collar and invade the inner cavity of the cartilage model. The hole that the vessels poke through are called the nutrient foramen. Many other components enter through the nutrient foramen such as nerves, lymphatics, osteoclasts, osteoblasts, nutrients etc. The remaining cartilage is broken down by osteoclasts and the osteoblasts secrete their osteoid forming trabaculae (spongy bone).

4) Elongation: as blood vessels, osteoclasts, and osteocytes continue to invade the bone the shaft (diaphysis) will start to elongate. As a result the medullary cavity (hollow inner cavity) is formed and the diaphysis will slowly continue to lengthen during embryonic development. Also, vessels bud into the hyaline cartilage at the ends (epiphysis) of the long bones forming what are called secondary ossification centers.

5) Epiphyseal ossification: the events here are almost identical to #3 except instead of forming compact bone spongy bone is formed. Also, hyaline cartilage is left on the ends of the bones (called articular cartilage) and the epiphyseal plates (growth plates) are also formed. The articular cartilage and epiphyseal plates are the only remains of the original hyaline cartilage model.