Bone formation (osteogenesis) begins during prenatal development and persists throughout adulthood. The bones of infants and children are softer than in adults because it has not yet been ossified (the process of synthesizing cartilage into bone). There are two ways in which osteogenesis occurs: intramembranous ossification and endochondral ossification. Both types form by replacing existing cartilage however differ in the method they go about doing it. Two types of cells that are of great importance in the process are osteoblasts and osteoclasts. Osteoblasts, used mainly in intramembranous ossification, are the specialized cells in bone tissue that deposit calcium into the protein matrix of bone (collagen). Osteoclasts, used in endochondral ossification, dissolve calcium previously stored away in bone and carry it to tissues whenever needed. One third of all of the bone's components is collagen; a flexible, gelatin-like matrix. Bones formed during intramembranous formation are called membranous bones, or occasionally dermal bone, and bones formed during endochondral formation are called cartilage bone.
As seen under a microscope, membranous bones first appear as flat, membrane-like layers of early connective tissue. These layers are provided with a constant flow of nutrient blood supply by networks of blood vessels formed in between the layers. Early connective tissue cells first arrange themselves among the layers and then differentiate into bone-forming cells called osteoblasts. The osteoblasts then remove calcium from the blood and deposit it among the bone matrix (the cartilage). As a result, layers of spongy bone are formed around the original cartilage. Later in development, spaces among the spongy bone are filled with bone matrix and become compact bone.
Osteoblasts continue to deposit calcium supplements into the matrix until it is totally surrounded by it. Once this occurs the osteoblasts are considered to be encased in a lacunae and now called osteocytes. The original connective cells first formed around the network of blood vessels are now called the periosteum. Osteoblasts still not isolated in a lacunae can emerge from beneath the layer of compact bone and form layers of spongy bone over compact bone. Examples of intramembranous ossification are certain broad, flat skull bones.
Endochondral ossification forms bone by replacing a cartilaginous model, or precursor, that appeared there earlier in embryonic development. The cartilaginous models first undergo quick changes as the connective tissue cells enlarge which in turn destroys the surrounding matrix. Soon after, the connective tissue cells die. While the cells disintegrate, a periosteum is formed on the outside of the developing structure (a membrane with many blood vessels). Next blood vessels and undifferentiated cells raid on into the disintegrating tissue. Certain connective tissue cells differentiate and form spongy bone around the previous template of cartilage.
Further growth in cartilage bones occurs once a significant amount of spongy bone has formed. Thickness in cartilage bones is accomplished by intramembranous ossification. Just beneath the layer of periosteum yet above the newly developed spongy bone, compact bone is formed and hardened with the help of osteoblasts filling portions of the porous spongy bone with calcium phosphate crystals (apatite). Sometimes compact bone is formed on the surfaces of existing bone tissue and they must be eroded away by specialized cells called osteoblasts. The crystals of apatite extracted from the bone tissue are delivered to blood and tissues on demand.
Growth in the length of bones continues until about the age of 25. This is possible by an epiphyseal disk. The epiphyseal disk is found on a portion of the bone that remains cartilaginous. This portion of the bone, called the epiphysis, are both ends of the bone that continue to grow throughout development (lifelong) while the centers of premature bones undergo ossification. The full length of bone is attained by the deposition of calcium on the epiphyses.
One of the most widely accepted myths about bone is that it is dead, unchanging matter. However, your bone is constantly changing every second of the day as new bone cells replace old cells, just as your dead skin cells are brushed off but are continually replaced.
The healing process of broken bones is also very similar to the healing process of skin. As soon as a bone breaks, a jacket of cells forms around the fracture (a broken bone) called a callus, in the same way skin forms a scab jacket of new cells around broken skin. Unfortunately, the callus only provides for protection from infections and such but further damage can be done if not kept in a cast. A cast is put on to keep the bone straight while healing and also it is said that by applying pressure on it the bone heals quicker. Many times the cast will be put on in such a way that you are still able to use it. This is because bone, like muscle, grows in thickness and endurance when used.
Although bone cells reproduce faster when repairing yet it still takes a long time to fully heal. In young children, while bones are still developing, repairs and healing are done fairly quickly. However as you age, healing tends to take longer. By adolescence, bones are hardened to an extent so bones are more complete. Mending in teenagers requires 6-8 weeks in a cast while after you're 60 or over breaks occur much more easily. Sometimes just the slightest stress on a bone can break it, and then repairs are made slowly and sometimes imperfectly. However, when fractures in bone are healed with the right amount of time and nutrients to aid in it, the break or breaks in the bone cannot even be seen with an x-ray.