Time period: day 27 to birth
Cells for the developing skeleton come from a variety of sources. We have described the development of the somites, and the subdivision of the sclerotome (see Chapter 22). Those cells are joined by contributions from the somatic mesoderm and migrating neural crest cells.
Development of the skeleton can be split into two parts: the axial skeleton consisting of the cranium, vertebral column, ribs and sternum; and the appendicular skeleton of the limbs.
The skull can be divided into another two parts: the neurocranium (encasing the brain) and the viscerocranium (of the face).
The bones at the base of the skull begin to develop from cells origi- nating in the occipital somites (paraxial mesoderm) and neural crest cells that surround the developing brain. These cartilaginous plates fuse and ossify (endochondral ossification) forming the sphenoid, ethmoid and occipital bones and the petrous part of the temporal bone (Figure 24.1).
A membranous part originates from the same source and forms the frontal and parietal bones (Figure 24.2). These plates ossify into flat bones (through intramembranous ossification) and are connected by connective tissue sutures.
Where more than two bones meet in the foetal skull a fontanelle is present (Figure 24.3). The anterior fontanelle is the most prominent, occurring where the frontal and parietal bones meet. Fontanelles allow considerable movement of the cranial bones, enabling the calvaria (upper cranium) to change shape and pass through the birth canal.
Cells responsible for the formation of the facial skeleton originate from the pharyngeal arches (see Chapters 40–43), and the viscerocranium also has cartilaginous and membranous parts during development. The cartilaginous viscerocranium forms the stapes, malleus and incus bones of the middle ear, and the hyoid bone and laryngeal cartilages. The squamous part of the temporal bone (later part of the neurocranium), the maxilla, mandible and zygomatic bones develop from the membranous viscerocranium (Figure 24.4).
In week 4, cells of the sclerotome migrate to surround the notochord. Undergoing reorganisation they split into cranial and caudal parts (Figure 24.5).
The cranial half contains loosely packed cells, whereas the caudal cells are tightly condensed. The caudal section of one sclerotome joins the cranial section of the next sclerotome. This creates vertebrae that are ‘out of phase’ with the segmental muscles that reach across the intervertebral joint. When these muscles contract they induce movements of the vertebral column.
Ribs also form from the sclerotome; specifically, the proximal ribs from the ventromedial part and the distal ribs from the ventrolateral part (Figure 22.4). The sternum develops from somatic mesoderm and starts as two separate bands of cartilage that come together and fuse in the midline.
Endochondral ossification of the long bones begins at the end of week 7. The primary centre of ossification is the diaphysis and by week 12 primary centres of ossification appear in all limb long bones (Figure 24.6).
The beginning of ossification of the long bones marks the end of the embryonic period. Ossification of the diaphysis of most long bones is completed by birth, and secondary centres of ossification appear in the first few years of life within the epiphyses (Figure 24.6).
Between the ossified epiphysis and diaphysis the cartilaginous growth plate (or epiphyseal plate) remains as a region of continuing endochondral ossification. New bone is laid down here, extending the length of growing bones.
At around 20 years after birth the growth plate also ossifies, allowing no further growth and connecting the diaphysis and epiphysis (Figure 24.6).
Craniosynostosis is the early closure of cranial sutures, causing an abnormally shaped head. This is a feature of over 100 genetic syndromes including forms of dwarfism. It may also result in under- development of the facial area.
Neural crest cells are often associated with cardiac defects and facial deformations due to failed migration or proliferation. Neural crest cells are also vulnerable to teratogens. Examples of cranial skeletal malformations include: Treacher Collins syndrome (mandibulofacial dysotosis), which describes underdeveloped zygomatic bones, mandible and external ears; Robin sequence of underdeveloped mandible, cleft palate and posteriorly placed tongue; DiGeorge syndrome (small mouth, widely spaced down‐slanting eyes, high arched or cleft palate, malar flatness, cupped low‐set ears and absent thymus and parathyroid glands).
Spina bifida is the failure of the vertebral arches to fuse in the lumbosacral region. There are two types. Spina bifida occulta affects only the bony vertebrae. The spinal cord remains unaffected but is covered with skin and an isolated patch of hair. This can be treated surgically. Spina bifida cystica (meningocoele and myelomeningocoele) occurs with varying degrees of severity. The neural tube fails to close leaving meninges and neural tissue exposed. Surgery is possible in most cases but, because of the increased severity of cystica, continuous follow‐up evaluations are necessary and paralysis may occur. It is currently possible to detect spina bifida using ultrasound and foetal blood alpha‐fetoprotein levels.
Pregnant women and those trying to be come pregnant are advised to take 0.4 mg/day folic acid as it significantly reduces the risk of spina bifida. Folates have an important role in DNA, RNA and protein synthesis.
Scoliosis is a condition of a lateral curvature of the spine that may be caused by fusion of vertebrae, or by malformed vertebrae. The range of treatments for congenital scoliosis includes physiotherapy and surgery. Klippel Feil syndrome is a disease where e fuse. Common signs include a short neck and estricted movement of the upper spine.