Embryology
Lec: 11 Prof Dr. Al-Hubaity
Development of Skull:
The skull consists of neurocranium (calveria and base of the skull) and
viscerocranium (the small bones of the face).
The neurocranium includes:
1. Calvaria which develop by membranes ossification, where
mesenchymal cells are deposited from the blood vessels on the
surface of the meninges at certain sites and these will convert directly
to osteoblast (bony) without passing through the cartilaginous stage.
2. Base of the skull which is formed by endochondral ossification i.e the
mesenchyme will converts into cartilaginous model which in turn
converts to bone i.e endochondral ossification.
The base of the skull is formed from the following items:
1. Parachordal plate which fuses with the caudal 3 pairs of occipital
sclerotomes forming the base of the occipital bone, followed by
dorsal expansion of this occipital base behind and around the
neural tube forming the basi occiput of the occipital bone
formation is completed.
2. Hypophyseal plate will form the body of sphenoid bone.
3. Trabeculae cranae will form the body of ethmoid.
4. Alla orbitalis will form the lesser wing of the sphenoid.
5. Alla temporalis will form the greater wing of sphenoid.
The two wings are sperated by the superior orbital fissure.
During the formation of neurocrainum six fontanells appears (anterior,
posterior and 2 lateral on each side) .the posterior fontanel usually ossify
3-6 months after birth while anterior fontanel disappears within 12-18
months after birth and its persistence after that is an indication of
Rickets. Also sutures are formed between the bones of calvaria.
Congenital Anomalies during Development of Skull:
1. Microcephaly: in this case the skull fails to grow and expand when
the brain fails to grow and stays small.
2. Craniostenosis: due to premature closure of one or more sutures.
3. Scaphocephally: is due to premature closure of the sagittal suture
results in frontal and occipital expansion.
4. Acrocephally: is due to premature closure of coronal and lambdoid
suture on one side only.
Development of Vertebral Column:
It passes through 3 stages
1- Precartilagenous stage: here the sclerotome of each somite divides
into 2 parts (halves), cephalic and caudal. The caudal half of the
first sclerotome fuses with the cephalic half of the next sclerotome
to form the precartilagenous stage, where mesenchymal cells in the
sclerotome is seen in 3 areas :
a) Around the notochord → to form mesenchymal centrum.
b) Around the neural tube → will form the neural arch.
c) In the body wall → costal process cribs.
The mesenchymal centrum will form the vertebral body. Now
arteries lie on each side of the vertebral body (i.e close to the
intervertebral discs), and the remnant of the notochord forms the
nucleus pulposus which is later surrounded by annulus fibrosus to
form the intervertebral disc.
2- Cartilaginous stage which starts at 6 weeks of intrauterine life, here
2 chondrification centers appear which by the end of the embryonic
period fuse and form the cartilaginous centrum and at the
meantime, the centers in the vertebral arches fuse with each other
thus chondrification spreads till a cartilaginous vertebral column is
formed.
3- Bony stage. By the end embryonic period 3 primary ossification
centers are seen, one in the centrum and one in each half of the
vertebral arch. Those of the 5 arch are fused by the first 3-5 years of
the life.
After puberty 5 secondary ossification centers appear, one at spinous
process, one at the tip of each transverse process and 2 ossification
centers related to the vertebral body (anular epiphyses). All the
secondary ossification centers unite with the rest of vertebrae around
25 years of life.
Congenital anomalies
1. Fusion of two adjacent vertebrae.
2. Very rarely extra number of vertebra or less number
3. Spina bifida due to failure of the 2 halves of the vertebral arch
to fuse, it could be:
a) A minor one (spina bifida occulta) with no complains.
b) Amajor one (spina bifida cystica) with involvement of spinal
cord meninges (some neurological signs) it could be of 3
forms:
With meningocele, the sac of meninges contain only
C.S.F its incidence is 1:1000.
Meningomyelocele with mild involvement of sic.
Neuro anencephaly with varying degrees of neurological
defect.
These can be diagnosed during pregnancy by amniocentesis where
alphafeto-protein in the amniotic fluid is high.