The development of the skull begins around 5 weeks after the fertilization. It begins as a series of cartilaginous plates that form beneath the developing brain and around the ears, nose, and eyes. These cartilage plates enlarge and fuse at about 8 weeks of development to form the chondrocranium. This cartilaginous saddle cradles the brain and sense organs; it does not cover over the brain.
At about 9 weeks of fetal development isolated ossification centers begin to form in and around the chondrocranium. The rostralmost ossification centers will form the, ethmoid, lacrimal, nasal, zygomatic and maxillary bones, the caudalmost centers will form part of the occipital bone, and in between the ossification centers for the sphenoid, and part of the temporal bones form. Also, at about this time ossification centers for the bones that will make up the cranium (the frontal, parietal, and parts of the occipital, and temporal bones) begin to form in the connective tissue layers of the dermis overlying the brain. These cranial bones will eventually form the roof of the skull. The mandible also begins to from at about this time.
From about the 10th –12th week of development these ossification centers continue to expand and form sutures. Additional ossification centers form in the roof of the oral cavity. These ossification centers fuse to form the maxillary part of the hard palate and the palatine bones. At the time of birth, the bones of the face (nasal, lacrimal, maxillary, zygomatic, and vomer) have formed extensive sutures. The bones comprising the face are particularly sensitive to retinoids and alcohol, which can potentially cause facial clefting and other facial defects that are associated with fetal alcohol syndrome.
Furthermore, at birth, the bones of the cranium, and particularly the frontal, parietal, sphenoid and occipitals remain incomplete and membrane covered spaces called fontanelles remain between these bones. These fontanelles allow deformation of the skull during the birthing processes. If the skull were fully formed at the time of birth, it would be difficult, if not impossible, for the head of the infant to pass through the birth canal. The fontanelles also allow for the growth of the brain that occurs during infancy. Normally, these fontanelles close within the first two years after birth. However, all bones are continuously remodeled throughout a person’s lifetime, so bones continue to adapt to stresses that might be put on them and to hormonal changes that occur during life.
Developmental Anatomy, 7th edition, by L.B. Arey, W.B. Saunders Company, Philadelphia, Pennsylvania.
Human Anatomy, 5th edition, by K. Van De Graaff, McGraw Hill, Boston, Massachusetts.
Human Anatomy, 4th edition, by F.H. Martini, M.J. Timmons, and R.B. Tallitsch, Prentice Hall, Upper Saddle River, New Jersey.