Osteogenesis 3 Development of a Long Bone

Osteogenesis 3 - Development of a Long Bone, is a short summary of the histological  development of a long bone and surrounding structures. The story starts  at ~ 6 weeks in the limb bud of the embryo when the bone starts as a cartilage model.  The cartilage cells follow their cell cycle leading to the formation of calcified cartilage internally, and a woven bone collar externally at the primary center of ossification.  The woven bone collar becomes the cortex, bone that is appied externa to the model. The cortex in all bones, thus, forms through membranous bone formation, not enchondral bone formation (EBF).  EBF is the mechanism whereby bone is formed internally in the cartilage model.  It forms primary trabeculae, in which bone is applied around the calcified cartlige that remains after waves of chondroclasts have deleted most.  EBF forms the initial trabecular bone during growth. The cortex through one year of age is fibrolamellar bone tissue.  The increasing activity of the child transmits mechanical forces to the bone, and stress induced remodeing begins through the activity of the bone remodeling unit (BRU), or bone remodeling unit (BMU),  which creates a resorption cavity by osteoclasts, and refills the space with lamellar bone organized a cable-like functional unit called a secondary osteon. The bone over time reflects the best structural fit for the applied force pattern. The story ends at the end of adolescence when adult bone structure has been achieved.