Fibrous-Membrane Formation (Intramembranous)

The bones of the cranium are formed from a fibrous membrane. In various portions of the membrane, ossification centers develop. From these centers, tiny calcium spines radiate in all directions; thus, a compact network of bone is formed centrally with peripheral areas being much less compact. Between the radiating calcium columns, the osteoblasts (bone-forming cells) construct the bone. Eventually, the periosteum is formed from the membrane; the subperiosteal osteoblasts form the inner and outer tables of compact bone; and the endosteal osteoblasts form the cancellous bone. The bone marrow occupies the spaces within the cancellous bone (diploe). The bones of the cranium are not completely ossified at birth and the membranous areas are called fontanelles.

Cartilaginous (Endochondral) Formation

The tones of the rest of the skeleton are preformed in cartilage. Ossification proceeds from an ossification center toward the extremities, which remain cartilaginous for some time. Subsequently, a similar process begins in one or more places in the extremities and gradually proceeds toward the center. An area of cartilaginous tissue persists after birth for various periods of time. In infants and children, this area affords growth in length. It is called the epiphyseal zone (the suffix “physeal” means “growth”).

Cranial Growth

Growth of the cranial bones is affected in formative steps, which are modifications of intramembranous formation. Their development entails ossification of the membranous fontanelles that becomes complete when the child is approximately 2 years old.

Growth of Other Bones

The remaining bones of the skeleton undergo changes similar to those of a long bone during growth, that is, an increase in diameter and length. The periosteum that covers the bone contains osteoblasts that progressively deposit layers of bone to form the external portion of the bone. Correlated with this growth in diameter externally, osteoclasts (bone-destroying cells) in the endosteum destroy some of the bone internally, thereby enlarging the internal (medullary) canal. Growth in length takes place in the epiphyseal zones. The shaft is called the diaphysis and the end of the bone is called the epiphysis.

Maturity Rates

The time required for the bones to reach full development varies for different parts of the skeleton. The skeleton matures somewhat earlier in the female than in the male. The appearance of the epiphyseal centers of ossification and their development can be followed by radiographic examination. Some appear at birth while others appear from time to time during the first 15 years of life. Later, the epiphysis and shaft of the bone unite. Ordinarily, all have united by the age of 25.

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