Download 12-AXIAL SKEL(1)..

AXIAL SKELETON
SKULL. 
VERTEBRAL COLUMN.
STERNUM. 
RIBS. 

PARAAXIAL MESODERM
It is longtudinal •
columns on each side
of the notochord.
At the end of the 3rd •
week, it is
differentiated into:
Somitomers (in the •
head region).
Somites (from the •
occipital region
downward).
SOMITES
They are differentiated
into :
Sclerotome •
(ventromedial).
It forms the vertebrae
and ribs.
Dermomyotome •
(dorsolateral).
It forms the muscles •
and dermis of the
skin.
•
•
LATERAL PLATE MESODERM
It is the Somatic 
mesodermal
layer of the body
wall
It is responsible for 
the formation of :
1. Pectoral and 
Pelvic girdles.
2. Long bones of 
the limbs.
MESENCHYME
At the end of the 4th week, •
the mesodermal cells form
the polymorphous embryonic
connective tissue
(MESENCHYME).
The mesenchymal cells can 
migrate to different locations
and are able to differentiate
into :
Fibroblasts, 
Chondroblasts and
Osteoblasts.
NEURAL CREST CELLS
In the head region
they differentiate
into mesenchyme
that share in the
formation of the
bones of the face
and skull.

MODE OF OSSIFICATION
(1)
Membranous
In some •
bones,such as the
flat bones of the
skull ,
mesenchyme
differentiate
directly into bone.
.
•
•
MODE OF OSSIFICATION
(2) Endochondral
In most bones, •
mesenchymal cells
first give rise to
hyaline cartilage
models, which in
turn ossify .
•
THE VERTEBRAL COLUMN
1) MESENCHYMAL 
(Precartilagenous)
Stage
In the 4th week, the 
mesenchymal cells from
sclerotomes shift their
position to be condensed
around
1. Notochord ( the 
structure around which
the vertebrae develop).
MESENCHYMAL
(Precartilagenous) Stage
2. Neural tube 
(primordium of spinal
cord).
3. Body wall. 
SCLEROTOME
Each sclerotome consists
of loosely arranged cells
cranially and densely
packed cells caudally.
Some densely packed 
cells move cranially
opposite the center of
the myotome, where
they form the
Intervertebral disc.

CENTRUM
It is the primordium of 
the body of the vertebra.
It is formed from the 
remaining densely packed
cells which fuse with the
loosely arranged of the
immediately caudal
sclerotomes.
It is an intersegmental 
structure.
CENTRUM
Nerves lie in close
relation to the inter
vertebral discs.
Arteries lie on •
each side of the
vertebral body.
The dorsal •
intersegmental
arteries become
the intercostal
arteries.
•
VERTEBRAL (NEURAL) ARCH
It is formed from
the mesenchymal
cells around the
neural tube.

INTERVERTEBRAL DISC
The notochord 
degenerates, between
the vertebrae, it expands
to form the Nucleus
pulposus.
Later this nucleus is 
surrounded by the
Anulus fibrosus.
These together constitute
the intervertebral disc.


THE CARTILAGENOUS
STAGE
It begins in the 6th 
week.
Two chondrification •
centers appear in
each centrum .
They fuse with each •
other and with the
centers of the
vertebral arch.
THE CARTILAGENOUS STAGE
Spinous and •
Transverse
processes are formed
from
extensions of •
Chondrification centers
in the vertebral
arch.
A cartilagenous •
vertebral column is
formed.
BONY VERTEBRAL COLUMN
Ossification begins
during the
embryonic period.
It ends at the age
of 25 years.
•
•
PRIMARY OSSIFICATION
CENTERS
They are Three in •
number
One for the centrum •
(dorsal and ventral
centers that fuse to
form one)
One in each half of the
vertebral arch.
Ossification is evident in
the arch in the 8th
week.
•
•
PRIMARY OSSIFICATION
CENTERS
The bony halves of •
the neural arch fuse
with each other in
the first (3- 5)
years.
The union starts first
in the lumbar
region then it
progresses cranially.
•
NEUROCENTRALJOINTS
They are cartilagenous •
joints between the
vertebral arch and the
centrum.
They permit growth of •
the vertebral arches as
the spinal cord expands.
These articulations •
disappear during the
(3rd- 6th) years of age.
SECONDARY OSSIFICATION
CENTERS
They are five in number. •
They appear after puberty.
They are : •
One for the tip of the •
spinous process.
One for the tip of each •
transverse process.
Two Anular Epiphyses on
the superior and inferior
rami of the vertebral body.
•
•
SECONDARY OSSIFICATION
CENTERS
All secondary centers
unite with the rest of
the vertebra around
the age of 25 years.
EXCEPTIONS •
C1 (Atlas) •
C2(Axis) •
C7 •
Lumbar,Sacrum and
Coccyx.
•
•
THEVETEBRAL BODY
It is a composite of the 
anular epiphyses and
the mass of bone
between them.
It includes : 
Centrum 
Parts of the vertebral 
arch
Facets for the heads of 
the ribs.
RIBS
They are formed from 
mesenchymal costal
processes of the
thoracic vertebrae.
They are united with the
vertebrae at the
CostoVertebral joints.
They become 
cartilagenous and
ossified before birth.

SPINA BIFIDA
It is failure of fusion of
the two halves of the
vertebral arch.

SPINA BIFIDA
It occurs more 
frequently in girls
than boys.
SPINA BIFIDA OCCULTA
A minor, 
insignificant
anomaly of the
vertebral column
that usually causes
no clinical
symptoms.
The skin over the 
bifid arch is intact
.

SPINA BIFIDA OCCULTA
There may be no 
external evidence
of the vertebral
defect.
Sometimes the 
anomaly is
indicated by a tuft
of hair.
The spinal cord and
spinal nerves are
usually normal.

SPINA BIFIDA CYSTICA
It is a severe type of 
spina bifida.
The spinal cord and 
meninges are
involved.
Associated with 
neurologic symptoms.