The human skeletal structure undergoes a significant transformation between infancy and adulthood. A newborn baby begins life with approximately 270 to 300 separate bony elements, nearly 100 more than the typical adult complement of 206 bones. This reduction occurs because many of these initial, smaller bones gradually merge together through a natural process called synostosis. This developmental change is a slow process that continues throughout childhood and adolescence.
The Necessity of Separate Bones at Birth
The greater number of unfused bones at birth is a functional adaptation with two primary purposes. A baby’s skull is made up of several separate bony plates connected by flexible, fibrous joints called sutures, which meet at spaces known as fontanelles. This loose arrangement allows the skull plates to temporarily overlap and mold their shape as the baby passes through the narrow birth canal.
Without this flexibility, the birthing process would be far more hazardous for both the mother and the infant. Separate plates also accommodate the rapid expansion of the brain that occurs after birth. The brain nearly triples its weight in the first year of life, and the open sutures permit the cranium to grow at a corresponding rate. The smaller posterior fontanelle closes first (within the first two to three months), while the larger anterior fontanelle remains open longer, closing between seven and nineteen months of age.
Major Fusion Points of the Central Skeleton
Beyond the rapid changes in the skull, some of the most significant bone mergers occur in the body’s central framework and the pelvis. The pelvic girdle, for example, is formed by a pair of large, irregularly shaped coxal bones, each of which begins as three distinct bones. These three bones—the ilium, ischium, and pubis—meet at the hip socket.
The fusion of the ischium and pubis begins relatively early, often between four and eight years of age. The ilium then joins the combined structure later in childhood, with the three components of the coxal bone fully uniting in the late teens to early twenties. This merger creates the strong, single hip bone required to support the entire upper body weight and transmit force to the legs.
Similarly, the base of the spine is defined by two structures that are the result of extensive fusion: the sacrum and the coccyx. The sacrum starts as five separate vertebrae which begin to merge around the time of puberty. This fusion proceeds gradually, uniting the intervertebral spaces, and is completed by the third decade of life. The coccyx consists of three to five smaller, rudimentary vertebrae that also fuse together, forming a single, small bone that serves as an attachment point for pelvic ligaments and muscles.
How Growth Plates Become Solid Bone
Another major form of fusion occurs in the long bones of the arms and legs, a process that determines final adult height. Longitudinal bone growth takes place at the epiphyseal plates, which are thin layers of hyaline cartilage located near the ends of the bone. These plates are responsible for adding length to the bone. Cartilage cells on the side of the plate closest to the end of the bone divide rapidly, while older cartilage cells on the opposite side are destroyed and systematically replaced by hard bone tissue.
This conversion process continues throughout childhood and adolescence. When a person reaches skeletal maturity in the late teens or early twenties, the rate of cartilage production slows and eventually stops. The cartilage of the growth plate is then entirely replaced by bone, and the epiphysis fuses completely with the diaphysis. This final merger, known as epiphyseal closure, marks the end of bone lengthening and the attainment of full adult height.