Skeletal System
Time period:
day 27 to birth
Introduction
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).
Neurocranium
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.
Viscerocranium
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).
Clinical relevance
Cranium
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).
Vertebrae
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.
