Human babies are born with considerably more bones than they will have as older children and as adults. Depending upon the development of the individual baby, he or she will be born with approximately 270 bones as compared to 206 for adults. Even this number is not written in stone due to the variations in genetic makeup, dietary and nutritional intake, and a host of other biological and physiological factors.
What happens to the extra bones?
After a baby's birth, the extra bones don't literally disappear; they fuse with other bones to become one larger bone. A classic example is the human skull. At birth, five distinct bones comprise the baby's skull; later, these fuse into a complete skull. The five separate bones allow for the elasticity and malleability necessary for the baby to pass through the birth canal. Another example would be the tailbone, or coccyx. In an adult it is one bone, but in a newborn it is five separate bones.
The development of bones
To understand why a baby is born with more bones than an adult, it is necessary to understand where bones originate and how they form. In the developing fetus, all bones start as cartilage-a relatively soft, gelatinous, collagen-based material. Beginning at about the 12th week of gestation, calcium deposits form over the frame of collagen and start to form bone. This process is called ossification. There are two types of ossification: Intramembranous ossification and endochondral ossification.
Overview of ossification types
Intramembranous ossification starts out with flat, membrane-like layers of cartilage. The layers receive nutrients and blood via a fine network of microscopic vessels between the layers of membranes. Early in gestation, the membranes arrange themselves and then differentiate into bone-forming cells called osteoblasts. The osteoblasts extract calcium from the blood supply and deposit it into the collagen framework, forming a bone matrix. With this process, spongy bone forms around the original cartilage. Spaces in the spongy bone continue to fill with bony matrix over time; that becomes the cortical or compact bone layer. Bones that are formed as a result of intramembranous ossification include the flat, broad skull bones and the scapula (shoulder blades).
Endochondral ossification is similar to intramembranous ossification in that this type also starts out with cartilage. In this type of ossification, the connective tissue cells enlarge to the point where they destroy the surrounding matrix. As the connective tissue cells begin to die out and disintegrate, a bony outer layer called periosteum forms around the remaining inner membranes that are rich in blood supply and undifferentiated cells. This will become what we call the medullary or inner bone canal. The undifferentiated cells in the inner membrane then begin to differentiate and start forming spongy bone between the inner membrane and the periosteum. Then, through the addition of the intramembranous ossification process, more compact bone is produced between the spongy bone and the periosteal layer. Typical bones produced by endochondral ossification include long bones like the femur, tibia and humerus.
In a human baby, the ossification process may be under way in any part of what will evenutally become bones. Each of these patches of bone will be connected with cartilage that will later become bone; however, until the process is complete, they are each distinct bones.