• VERTEBRAL COLUMN
• THE RIBS
• THE STERNUM
• THE SKULL
• FORMATION OF THE LIMS
• TIMETABLE OF SOME EVENTS
• The vertebral column
• is derived from the sclerotomes of somites.
• Each sclerotome divides into three parts:
• cranial, middle and caudal.
• A vertebra is formed by fusion of the
• caudal part of one sclerotome and the cranial part of the
next sclerotome, which therefore, intersegmental in
• The middle part of the sclerotome forms an intervertebral
disc, which is therefore segmental in position.
• The sternum is formed by fusion of right and left sternal bars.
• The SKULL develops from mesenchyme around the brain.
• Some skull bone are formed in membrane (e.g. parietal).
• Some partly in membrane and partly in cartilage (e.g. sphenoid).
• Few entirely in cartilage (e.g. ethmoid).
• The MANDIBLE is formed in membrane from the mesenchyme of
the mandibular process.
• The LIMBS are first seen as outgrowths ( limb buds) from the
side wall of the embryo.
• Each bud grows and gets subdivided to form parts of the limb.
• LIMB BONES develop from mesenchyme of the limb buds.
• JOINTS are formed in intervals between bone ends.
is made up of cells that can give
rise to cartilage, bone, muscle,
blood and connective tissue.
The Vertebral Column
• The vertebral column is formed from the sclerotomes of somites.
• The cells of each sclerotome converted into loose mesenchyme, this
migrate medially surrounds the notochord.
• Mesenchyme extends backwards on either side of the neural tube and
• Mesenchyme extend laterally ------- (future position of transverse processes).
• Mesenchyme extend ventrally in the body wall ------ (position of ribs).
• Mesenchymal cells are at first uniformly distributed.
• The cells soon become condensed in a region called the perichordal disc.
• Above and below the perichordal disc there are less condensed parts.
• The perichordal disc becomes the intervertebral disc.
• The fusion of the adjoining less condensed parts of two segments form the
body (centrum) of each vertebra.
Formation of the intersegmental
4th week- sclerotomal cells
migrate from adjacent somites
above and below each future
vertebra. (HOX gene)
A diagram of a human thoracic vertebra.
Notice the articulations for the ribs
Orientation of vertebral column
T3 is at level of medial part of
spine of scapula.
T7 is at inferior angle of the
L4 is at highest point of iliac crest.
S2 is at the level of posterior
superior iliac spine.
The Vertebral Column
• The neural arch, the transverse processes and the costal elements are
formed in the same way as the body.
• The interspinous and intertransverse ligaments are formed in the same manner
as the intervertebral disc.
• The notochord disappears in the region of the vertebral bodies .
• The notochord in the region of the intervertebral discs, becomes expanded and
forms the nucleus pulposus.
• We can summarize above notes as follows:
1. The vertebra is intersegmental structure made up from portions of two somites the
position of the somite is represented by intervertebral disc.
2. The transverse processes and the ribs are intersegmental structures. They
separate the muscles derived from two adjoining myotomes .
3. Spinal nerves are segmental structures. They emerge from between two
adjacent vertebrae and lie between two adjacent ribs.
4. The blood vessels supplying the structures derived from the
myotome are intersegmental like vertebrae. Therefore the
intercostal and lumbar arteries lie opposite the vertebral bodies .
Congenital Anomalies of the Vertebral column (skip)
1. Vertebrae may be absent.
2. Additional vertebra may be present.
3. Part of a vertebra may be missing:
a) # spina bifida/ spina bifida occulta: two halves fail to fuse in the midline.
b) # hemivertebra (usually associated with the absence of the
c) #anterior spina bifida.
4. Fusion of vertebrae : in the cervical region called (Klippel – Feil syndrome),
occipitalization of atlas. In the lumbosacral region, sacralization of the 5th
5. Separation of vertebrae : lumbarization of the 1st sacral vertebra. The
odontoid process may be separated from the rest of the axis vertebra.
6. Spondylolisthesis (slipping of vertebra).
7. Diastematomylia (division of vertebra + splitting of the spinal cord).
8. Chondro – osteodystrophy (short trunk + normal limbs length).
9. Sacrococcygeal teratoma.
Anomalies of the Vertebrae (skip)
• Anomalies of the Vertebrae are of particular importance in that:
I. Congenital scoliosis (the spine pent on itself). Congenital
II. Paralysis The spinal nerves, or even the spinal cord, may be
implicated. They may be subjected to abnormal pressure.
• The ribs are derived from ventral extension of the sclerotomic
mesenchyme that form the vertebral arches.
• These extension are present not only in the thoracic region but
also in the cervical, lumbar and sacral regions.
• They lie ventral to the mesenchymal basis of the transverse
• In the thoracic region the entire extension (primitive costal arch)
undergoes chondrification, and subsequent ossification, to form
• Some mesenchyme form the costotransverse joint (between
developing rib and developing transverse process).
• Costal element = the bone formed from the arch and fused
with the transverse process.
• The sternum is formed by fusion of two sternal bars
(plates) that develop on either side of the midline .
• Mesenchymal condensations become cartilaginous In
the 7th week of intrauterine life.
• Laterally, the sternal bars are continuous with ribs .
• The fusion of two sternal bars occurs at their cranial end
(manubrium) and extend caudally.
• The manubrium and the body of the sternum are
• The xiphoid process ossifies only late in life.
Anomalies of the Sternum and Ribs (skip)
1. Missing ribs. Unilateral absence of a rib is often
associated with hemivertebra.
2. Accessory ribs. Cervical rib (7th cervical vertebra). Lumbar
rib (1st lumbar vertebra).
3. Cleft, parietal, transverse or even complete midline cleft
occurs when the fusion of two bars is faulty. Minor dergee
of non-fusion may result in a bifid xiphoid process or
4. Funnel chest, the lower part of the sternum and the
attached ribs are drawn into thorax. The primary defect
(short central tendon of the diaphragm).
5. Pigeon chest, the upper part of the sternum and related
costal cartilages may project forwards.
• The skull is developed from the mesenchyme
surrounding the brain.
• The following structures contribute to the development of the
1) Four occipital somites.
2) Otic and nasal capsule.
3) Mandibular and maxillary processes.
Some bones of the skull are formed in membrane,
some in cartilage, and some partly in
membrane and partly in cartilage, as follows;
Bones that Completely Formed in Membrane
1) Frontal and parietal bones are formed in relation
to mesenchyme covering the developing brain.
2) The maxilla (excluding the premaxilla),
zygomatic and palatine bones, and part of the
temporal bones are formed by intramembranous
ossification of the mesenchyme of the maxillary
3) The nasal, lacrimal and vomer bones are ossified
in membrane covering the nasal bone.
Bones that are Completely Formed in Cartilage
• The ethmoid bone and the inferior nasal concha
are derived from the cartilage of the nasal capsule.
• The septal and alar cartilages of the nose represent
parts of the capsule that do not undergo ossification.
• Hyoid bone
Smaller cornu of hyoid bone and Superior part of body of
hyoid bone Second Arch derivatives.
Greater cornu of hyoid bone and lower part of the body
of hyoid bone are 3rd arch derivatives.
Bones that are Partly Formed in Cartilage and Partly in
1. Occipital bone. Interparietal part is formed in membrane,
the rest is formed by endochondral ossification .
2. Sphenoid. The lateral part of the greater wing and the
pterygoid laminae are formed in membrane; the rest is
3. Temporal bone. The squamous and tympanic parts are
formed in membrane. The petrous and mastoid parts are
formed by ossification of the otic capsule. The styloid process
(2nd branchial arch cartilage).
4. Mandible. Most of the bone is formed in membrane in the
mesenchyme of the mandibular process . The ventral part
of Meckel’s cartilage gets embedded in the bone .
The condylar and coronoid processes are ossified
from secondary cartilages that appear in these
Anomalies of the Skull (skip)
1) Anencephaly .
2) Cleidocranial dystosis
7) Congenital hydrocephaly.
8) Hand-Schuller-Christian disease, large defects are seen in
the skull bones. (the primary defect is in the
reticuloendothelial system; the changes in the bones are
9) Occipito-Atlas fusion.
10) Mandibulo-Facial dysostosis.
Formation of the Limbs
• The bones of the limbs, including the bones of the shoulder
and pelvic girdles, are formed from mesenchyme of the
• With the Exception the clavicle (which is a membrane
bone), they are all formed by endochondral
• The limb buds arise at he beginning of 2nd month of
• Each bud is a mass of mesenchyme covered by ectoderm.
• At the tip of each limb bud, the ectoderm is thickened to
form the apical ectodermal ridge.
Formation of the Limbs (continue)
• The forelimb buds appear earlier than the hindlimb.
• As each forelimb bud grows, it becomes subdivided by
constrictions into arm, forearm and hand.
• The hand shows outline of digits.
• The interdigital areas show cell death (the digits separate
from each other).
• Similar changes occur in the hindlimb.
Formation of the Limbs (continue)
• The limb bud are at first directed forward and laterally
from the body of embryo.
• Each bud has a preaxial (cranial) border ------- lateral border.
• And postaxial border ----------------------- medial border.
• The radius is the preaxial bone of the forearm.
• The original ventral surface form the anterior surface of
the arm, forearm and hand.
• In the lower limb the tibia is the preaxial bone of the leg.
• The thumb and great toe are formed on the preaxial border.
• Adduction of the limb is accompanied by medial
rotation with the result that the great toe and
tibia come to lie on the medial side.
Formation of the Limbs (continue)
• The original ventral surface of the hindlimb is represented
by the inguinal region, the medial side of the lower part of
the thigh, the popliteal surface of the knee, the back of the
leg and the sole of the foot.
• The forelimb is derived from the part of the body
wall belonging to the segments
• C4, C5, C6, C7, C8, T1 and T2.
• The hindlimb bud is formed opposite the
• L2, L3, L4, L5, S1 and S2.
• The tissues of the joints are derived from mesenchyme
intervening between developing bone ends.
• This mesenchyme may differentiate into fibrous tissue,
• Fibrous joint (syndesmosis).
• Cartilaginous joint.
• In some cartilaginous joints (synchondrosis or primary cartilaginous
joints) the cartilage connecting the bones is later ossified, this is
seen between diaphysis and epiphysis of long bones .
• Synovial joint ////the mesenchyme is usually seen in three layers,
outer two are continuous with the perichondrium.
Anomalies of the Limbs (skip)
1. Phocomelia / Amelia.
2. Club foot (talips equinavarus).
3. Congenital strictures, amputations, or contractures.
4. Syndactyly / synphalangia.
5. Abnormal digital shape :
A. Macrodactyly X Brachydactyly
B. Arachydactyly (spider finger).
6. Polydactyly (thumb).
7. Lobester claw (deep longitudinal cleft.
8. Achondroplasia .
9. Abnormal joints :
1. Congenital dysplasia.
2. Congenital dislocation.
Timetable of some events mentioned in this lecture
Age Developmental events
4th week (26th day) Forelimb bud appear
4th week (28th day) Hindlimb bud appear
5th week Limbs become paddle shaped
6th week(36th day) Formation of future digits can be seen.
Cartilaginous models of bone start forming
7th week Rotation of limb occurs
8th week (50th day) The elbow and knee established,
and fingers and toes are free
12th week Primary centres of ossification
are seen in all the long bones
Note ///????!!!!!!/ teratogens!!!
• The extremities are most susceptible to
teratogens during the 4th to 7th weeks;
and slightly less susceptible in the 8th week.