Cellular contribution

The origin of bone can be followed back to the first three weeks of human development, the Pre-embryonic period. During this period, the cells that will eventually give rise to all structures of the body differentiate into three germ layers. These specialized layers of cells are ectoderm (forming all nerve and some epithelial tissue), mesoderm (forming all connective, muscle and some epithelial tissue) and endoderm (forming some epithelial tissue) (see Fig. 1). These cells undergo rapid regional development in the embryo, producing the precursors of adult structures, many of which become increasingly recognizable internally and externally.

Of interest during skull development, are primarily ectoderm and mesoderm. At the end of Week 3, ectoderm differentiates into neuroectoderm and epidermis. The latter covers the outside of the body. Neuroectoderm forms the neural tube (eventually becoming the brain and spinal cord) and neural crest. The neural crest is first defined as a region. However, neural crest cells are migratory and begin leaving the neural crest at about Week 5 to reach various target areas where further specialization occurs (see Fig. 3).

Mesoderm is divided into regions, named for their postion relative to the embryo's midline. That portion of meosderm that migrates to the periphery of the embryo during the second month is called lateral plate mesoderm (see Fig. 2).

As the development of the head progresses, neural crest cells and lateral plate mesoderm both migrate into rapidly forming pharyngeal arches, a series of bump-like structures on both sides of the embryonic head (see Fig. 2, Fig. 3). These arches surround the endoderm tube, the beginnings of the mouth and digestive system.

Neural crest cells, in addition to forming nerve tissue, produce the bones of the cranial vault, part of the neurocranium. Within the pharyngeal arches, neural crest cells and lateral plate mesoderm give rise to bones of the jaw and lower face, the viscerocranium (see Fig. 4). Lateral plate mesoderm also contributes to the formation of the cartilages of the larynx (Larsen, 1997, Carlson, 1999, Sweeney, 1998).

Types of Ossification

The creation of bone in all living organisms is known as ossification. Bone throughout the body is formed by one of two types of ossification processes: endochondral ossification, and intramembranous ossification. Individual bones can form from one of the other of these two types of ossification or as a combination of the two. The final adult bones, although produced by two different methods, result in bone material that has the same properties and structure. In the skull, different bones within the neurocranium and the viscerocranium are formed by one of the two types of ossification (see Fig. 4).

 Endochondral ossification is a bone formation process that begins within (endo) cartilage (chondral). Bone tissue is derived from mesenchyme, specialized mesoderm cells. These mesenchymal cells first form cartilaginous centers in the location where bone will eventually form. within the developing embryo starting at Week 5. These cartilage structures are then modified and transformed into bone through the work of osteoblasts (bone forming cells) and osteoclasts (bone cells that break down previously formed bone). These cells migrate into ossification centers, where bone will begin to be produced.
      As osteoblasts secrete a calcified bony matrix, osteoclasts remodel the growing bone by destroying or resorbing sections. Osteoclasts and osteoblasts continue to work in conjunction throughout craniofacial growth and development to modify the shape of growing bones. (England, 1996). The ossification of cartilaginous models of facial bones begins at approximately the third month of development, during the beginning of the Fetal period. The cranial base region of the neurocranium and select portions of the viscerocranium are formed by using this method of bone formation.

 Intramembranous, or direct, ossification is a process whereby bone is formed directly without first going through a cartilage stage. The cells destined to become bone derive from specialized mesoderm cells that form membranous sheets, loci of which diferentiate into osteoblasts and osteoclasts. This type of ossification takes place in several regions within the skull. Both the cranial vault portion of the neurocranium, and select portions of the viscerocranium utilize this method of bone formation. (Sweeney, 1998, England, 1996).

The bones of the skull are created as these regions of ossification merge. A single bone can therefore be made up of many smaller bones which fuse together during ossification. Single bones, once ossified, are united at their borders by a series of joints or articulations. The 22 bones of the adult skull are joined by two types of joints: amphiarthroses (slightly movable)and synarthroses (immovable). The bones of the cranial base joined by synchondroses, cartilaginous joints (Cohen, 1986) An important synchodrosis is the basioccipital synchondrosis. The bones of the cranial vault are united by sutures, a type of synarthosis. Sutures are relatively immovable, but are also active sites of bone deposition and resorption, allowing changes in size, shape and reorientation of other craniofacial elements. If sutures close prematurely this is known as synostosis , which hinders normal skull and potentially brain development. (see our Skull Vault section in Disorders of Development)

Significant modification and growth of the skull occurs during the first years of life. The skull as a whole grows most rapidly from birth to the 7th year. The cranial vault increases the most during the first year, corresponding with the rapid growth of the brain during that period. The neonate face at birth is comprises only approximately one-eighth of the total cranium, a ratio which, by adulthood becomes one-half. (see Fig.5).