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The bone collar is a structure that appears during endochondral bone development to support the growing bone and help it retain its shape. In endochondral development, the body creates a cartilage model and it ossifies over time, replacing the flexible cartilage with more rigid bone. This is the formation process seen with the long and short bones, like the femur and the bones in the fingers. It starts in fetal development as the growing fetus starts to develop a network of bones that will eventually develop into a skeleton, fully maturing around age 21.
In this model of bone development, a small patch of ossification starts in the middle of the bone to replace the cartilage. This causes the surrounding cartilage to weaken, forcing the bone to develop a bone collar. The bone collar surrounds the shaft of the bone, providing support with rigid bone tissue while the inside of the bone slowly fills in. The ends of the bone do not fully ossify, allowing room for the shaft to grow so the person can continue developing.
Blood vessels work their way in from the outside to supply the tissue with ample blood and provide a mechanism for carrying away cellular wastes, while a matrix of bone cells builds up inside. The bone collar maintains the shape and size of the bone during this process, acting like scaffolding on a house under construction. When the bone is fully ossified, the former bone collar will be integrated with the rest of the bone.
Errors in bone development can result in bones that are unstable or do not have the right shape. Sometimes people develop bones of unequal length because of errors with bone development or injuries that interfere with the bone. Surgery is available to adjust the length of the bones if the condition does not resolve. This can address issues like gait abnormalities. It is also possible for patients to develop overgrowths of bone, where the cells multiply too quickly and can make bones oversized or create lesions on the surface that may cause pain or interfere with the function of the bone.
Numerous studies of bone development provide a wealth of information on the development of different bones. Forensic anthropologists can analyze fetal bones to determine length of time that has passed since conception, and this can sometimes be useful in forensic identification. They can also look at bones of children and young adults and make age estimates on the basis of how much development has occurred. In older adults, wear on the bones provides important aging clues.