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MEDICAL
REHABILITATION
EMBRYOLOGICAL DEVELOPMENT OF
THE SKELETAL SYSTEM
INTRODUCTION TO : The Embryology
of the Skeletal System
• The skeletal system develops from
paraxial and lateral plate (somatic layer)
mesoderm and from neural crest.
Paraxial mesoderm forms a segmented
series of tissue blocks on each side of
the neural tube, known as
somitomeres in the head region and
somites from the occipital region
caudally.
Somites differentiate into a ventromedial
part, the sclerotome, and a dorsolateral part,
the dermomyotome. At the end of the fourth
week sclerotome cells become polymorphous
and form a loosely woven tissue, the
mesenchyme, or embryonic connective tissue.
It is characteristic for mesenchymal cells to
migrate and to differentiate in many ways.
They may become fibroblasts, chondroblasts,
or osteoblasts (bone-forming cells).
The bone-forming capacity of
mesenchyme is not restricted to cells of
the sclerotome, but occurs also in the
somatic mesoderm layer of the body
wall, which contributes mesoderm cells
for formation of the pelvic and
shoulder girdles and the long bones of
the limbs. Neural crest cells in the
head region also differentiate into
mesenchyme and participate in formation
of bones of the face and skull.
Occipital somites and somitomeres also
contribute to formation of the cranial
vault and base of the skull. In some
bones, such as the flat bones of the
skull, mesenchyme in the dermis
differentiates directly into bone, a process
known as intramembranous ossification.
In most bones, however, mesenchymal
cells first give rise to hyaline cartilage
models, which in turn become ossified by
endochondral ossification.
SKULL
The skull can be divided into two parts:
the neurocranium, which forms a
protective case around the brain, and the
viscerocranium, which forms the skeleton
of the face. The neurocranium is most
conveniently divided into two portions:
(a) the membranous part, consisting of
flat bones, which surround the brain as a
vault; and(b) the cartilaginous part, or
chondrocranium, which forms bones of the
base of the skull.
NEWBORN SKULL
At birth the flat bones of the skull are
separated from each other by narrow seams of
connective tissue, the sutures, which are also
derived from two sources: neural crest cells
(sagittal suture) and paraxial mesoderm (coronal
suture). At points where more than two bones
meet, sutures are wide and are called
fontanelles. The most prominent of these is
the anterior fontanelle, which is found where
the two parietal and two frontal bones meet.
Sutures and fontanelles allow the bones of
the skull to overlap (molding) during birth.
CLINICAL
IMPLICATIONS
• Cranial defects and skeletal dysplasias
• Neural crest cells- these originate in the
neuroectoerm from facial skeleton and most of
the skull.
• Craniosis- the cranial vault fails to form
cranioschsis meaning that the brain tissue
exposed to amniotic fluid degenerates (
anencephaly).
• Microcephaly- the brain fails to grow and the
skull fails to expand.
•
• Craniosynostosis and dwarfism
ACROMEGALY
THIS IS CAUSED BY HYPERPITUTARISM
AND PRODUCTION OF GROWTH
HORMONE
LIMB GROWTH AND THE
DEVELOPMENT OF THE FETUS
After four weeks of development, the buds for the
limbs become visible from the ventrolateral body
wall. It consists of a mesenchymal core that will
form the bones and connective tissues of the limb
which is covered by a layer of cuboidal ectoderm.
The ectoderm at the distal border of the limb then
becomes thicker which forms after the apical
ectodermal ridge (AER). As the limbs continue
growing, the cells that are farther away from, the
influence of AER begins to form into muscles and
cartilage.
5 WEEKS
6 WEEKS
8 WEEKS
THE LIMB BUDS DEVELOP
AS THE EMBRYO GROWS
The limb buds of the six
week embryo now become
flattened to form the hand
plates and foot plates which
is separated by a circular
constriction. Fingers and
toes are formed.
Development of the upper and lower limb is
similar except that the morphogenesis of the
lower limb is approximately one to two
days behind that of the upper limbs. During
the seventh week of gestation, the limbs
(both upper and lower) rotate in opposite
directions. The upper limbs rotates ninety
degrees laterally so that the extensor
muscles lie on the lateral posterior surface
and the thumbs lie laterally, meanwhile the
lower limbs rotates ninety degrees medially
placing the extensor muscles on an anterior
surface and the big toe medially.
By the sixth week of development of the embryo the first hyaline
cartilage models are formed by chondrocytes. Joints are now formed in
the cartilaginous condensations. Cells in this region then increase in
number and in density and join a cavity that is formed by cell death.
Ossification of the bones of the extremities, endochondral ossification
begins by the end of the embryonic period. The primary ossification
centers are present in all of the long bones of the limbs of the embryo
by the twelfth week of development.
At birth the diaphysis of the bones are usually
completely ossified but the two ends; the epiphyses
are still cartilaginous. A temporary a cartilage plate
remains between the diaphyseal epiphyseal
ossification centers. The epiphyseals plate plays an
important role in growth and length of the bones of
the embryo. When the bones has acquired its full
length, the epiphyseal plates disappear and the
epiphyses then unite with the shaft of the bone. In
long bones an epiphyseal plate is formed in each
extremity in smaller bones, such as the phalanges; it
is found only at one extremity and in irregular bones
such as the vertebrae one or more primary centers
of ossification.
Positioning of the limbs along the
cranio - caudal axis in the flank regions
of the embryo is regulated by the HOX
genes expressed along this axis.
Thesehomeoboxgenes are expressed
in overlapping patterns from head to
tail, with some having more cranial
limits than others. For example,
thecranial limit of expression of HOXB8
is at the cranial border of the
forelimb,andmisexpression of this gene
alters the position of these limbs
Although patterning genes for the limb axes have
been determined, it is the HOX genes that regulate
the types and shapes of the bones of the limb. Thus,
HOX gene expression occurs in phases in three
places in the limb that correspond to formation of the
proximal (stylopod), middle (zeugopod), and distal
(autopod) parts. Genes of the HOXA and HOXD
clusters are the primary determinants in the limb,
and variations in their combinatorial patterns of
expression may account for differences in forelimb
and hindlimbstructures. Just as in the craniocaudal
axis of the embryo, HOX genes are nested in
overlapping patterns of expression that somehow
regulate patterning.
CLINICALN IMPLICATION: Congenital hip
dislocation consists of underdevelopment of the
acetabulum and head of the femur. It is rather common
and occurs mostly in females. Although dislocation
usually occurs after birth, the abnormality of the bones
develops prenatally. Since many babies with congenital
hip dislocation are
breech deliveries, it has been thought that breech
posture may interfere with development of the hip joint. It
is frequently associated with laxity of the jointcapsule.
Vertebral Column
It is during the fourth week of development that cells of
the sclerotomes shift their position to surround both the
spinal cord and the notochord . This mesenchymal
column retains traces of its segmental origin, as the
sclerotomicblocks are separated by less dense areas
containing intersegmental arteries.
During further development the caudal portion of each
sclerotome segment proliferates extensively and
condenses.
This proliferation is so extensive that it proceeds into
the subjacent intersegmental tissue and binds the
caudal half of one sclerotome to the cephalic half of the
subjacent sclerotome. Hence, by incorporation of the
intersegmentaltissue into the precartilaginous vertebral
body, the body of the vertebra becomes
intersegmental. Patterning of the shapes of the
different vertebra is regulated by HOX genes.
Development of spinal cord in embryonic period
Mesenchymal cells between cephalic and caudal parts of
the original sclerotome segment do not proliferate but fill
the space between two precartilaginous vertebral bodies.
In this way they contribute to formation of the
intervertebral disc. Although the notochord regresses
entirely in the region of the vertebral bodies, it persists
and enlarges in the region of the intervertebral
disc. Here it contributes to the nucleus pulposus, which is
later surrounded by
circular fibers of the annulus fibrosus. Combined, these
two structures form The intervertebral disc
Rearrangement of sclerotomes into definitive vertebrae causes the
myotomes to bridge the intervertebral discs, and this alteration gives
them the capacity to move the spine. For the same reason,
intersegmental arteries, at first lying between the sclerotomes, now
pass midway over the vertebral bodies. Spinal nerves, however,
come to lie near the intervertebral discs and leave the vertebral
column through the intervertebral foramina
Vertebral
Development
Development of Ribs and Sternum
Ribs form from the costal processes of thoracic
vertebrae and are derived from The sclerotome portion
of paraxial mesoderm. The sternum develops
separately in somatic mesoderm in the ventral body
wall. Two sternal bands are formed on either side of the
midline, and these later fuse to form cartilaginous
models of the manubrium, sternebrae, and xiphoid
process
Illustration showing the manner in which each vertebral
centrum is developed from portions of two adjacent
segments. (Bartleby)