EMBRYOLOGYBY

Dr. THAAER MOHAMMED DAHER ALSAADSPECIALIST IN GENERAL SURGERY

M.B.Ch.B. (MBBS) F.I.B.M.S. )Ph.D.)SENIOR LECTURER

IMS MSU

THE SKELETON

•VERTEBRAL COLUMN

•THE RIBS

•THE STERNUM

•THE SKULL

•FORMATION OF THE LIMS

•TIMETABLE OF SOME EVENTS

TOPICS

Highlights

• 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 position.

• 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.

Highlights

Mesenchyme :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 surrounds it.

• 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 vertebrae

4th week- sclerotomal cells migrate from adjacent somites above and below each future vertebra. (HOX gene)

Ob

liqu

e view o

f cervical vertebrae

A diagram of a human thoracic vertebra. Notice the articulations for the ribs

Orientation of vertebral column on surface. T3 is at level of medial part of spine of scapula.T7 is at inferior angle of the scapula. L4 is at highest point of iliac crest. S2 is at the level of posterior superior iliac spine.

http://upload.wikimedia.org/wikipedia/commons/f/f8/Illu_vertebral_column.jpg

• 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 .

The Vertebral Column

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 corresponding rib.

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

lumbar vertebra.

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

torticolis.

II. Paralysis The spinal nerves, or even the spinal cord, may be

implicated. They may be subjected to abnormal pressure.

III.Backache.

The Ribs

• 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 butalso in the cervical, lumbar and sacral regions.

• They lie ventral to the mesenchymal basis of the transverse processes.

• In the thoracic region the entire extension (primitive costal arch) undergoes chondrification, and subsequent ossification, to form the ribs.

• 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

• 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 ossified, separately.

• 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 dergeeof non-fusion may result in a bifid xiphoid process or midline foramena.

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• The skull is developed from the mesenchyme

surrounding the brain.

• The following structures contribute to the development of the skull:

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 process.

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 conchaare 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.

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 cartilage bone.

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 situations.

Bones that are Partly Formed in Cartilage and Partly in Membrane

Anomalies of the Skull (skip)1) Anencephaly .2) Cleidocranial dystosis3) Scaphocephaly.4) Acrocephaly.5) Plagiocephaly.6) Microcephaly.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 secondary).

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 limb buds.

• With the Exception the clavicle (which is a membrane bone), they are all formed by endochondral ossification.

• The limb buds arise at he beginning of 2nd month of intrauterine life.

• 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.

• 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 segments.

• L2, L3, L4, L5, S1 and S2.

Formation of the Limbs (continue)

Joints • The tissues of the joints are derived from mesenchyme

intervening between developing bone ends.

• This mesenchyme may differentiate into fibrous tissue,

forming a:

• 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.

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 occurs8th 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

Timetable of some events mentioned in this lecture

Note ///????!!!!!!/ teratogens!!!

• The extremities are most susceptible to

teratogens during the 4th to 7th weeks;

and slightly less susceptible in the 8th week.

• ================================.

Limb formation5th wk

6th wk

7th wk 8th wk

Ectrodactyly

previously called "lobster-claw malformation

Talipes Equinovarus

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