Development of Skeletal System &

Limbs

Dr. Sama ul HaqueDr. Rania Gabr

Objectives

Development of skull. Development of limbs. Limb innervation. Anomalies of limbs Development of vertebrae. Anomalies of vertebrae. Development of ribs & sternum.

Based on the mode of development, there are two types of bones in the body:

Cartilage bones, that develop via:

1- Intracartilagenous (endochondral ossification). A cartilage model first forms and is eventually replaced with bone e.g. formation of the bones of the axial & appendicular skeletons and the cranial base.

4

• Membrane bones, that develop via

• 2- Intramembranous ossification.

• Bone forms directly from mesenchymal cells without the prior formation of cartilage e.g. majority of bones of the face and skull

The skull bones develop from the mesoderm around the mesoderm around the developing braindeveloping brain..

The skull consists of:1.1. NeuroNeurocranium:

protective case for brain

2.2. VisceroViscerocranium: skeleton of face

Development of skullBones of skull that

ossify byIntramembranous

ossification:1.Frontal2.Parietal3.Zygomatic4.Squamous temporal5.Maxilla6.MandibleBase of skull develops by develops by

intracartilagenous intracartilagenous ossificationossification

Skull of a Newborn

Cartilaginous Neurocranium (Chondrocranium)

AnencephalyAnencephaly, also known as “open skull,” is a birth defect in which the major parts of the brain, scalp, and skull of the fetus do not form completely.

Anencephaly occurs when the neural tube, does not close properly between the third and fourth weeks of pregnancy. An infant who is born with anencephaly has some or most of the brain missing. These infants are unconscious, cannot feel, and are usually blind and Infants with anencephaly are stillborn in about 75% of cases. Newborns who survive die within several hours, days, or weeks

Craniosynotosis

Craniosynostosis consists of premature fusion of 1 or more cranial sutures, often resulting in an abnormal head shape. It may result from a primary defect of ossification (primary craniosynostosis) or, more commonly, from a failure of brain growth (secondary craniosynostosis). Simple craniosynostosis is a term used when only 1 suture fuses prematurely. Complex or compound craniosynostosis is used to describe premature fusion of multiple sutures.

Achondroplasia Achondroplasia is caused by a gene mutation that impairs the growth of bone in the limbs and causes abnormal growth in the spine and skull.Approximately 20-50% of all children with achondroplasia will experience a neurological impairment. This is caused by compression created as they literally grow faster than their bones. The stunted bone growth at the base of the skull and the spine can cause the spinal cord and brain stem to become compressed. This can lead to key nervous system structures — like the brain stem, spinal cord, spinal nerve roots and cerebrospinal fluid (CSF) spaces — to also compress. 

One of the causes of dwarfism

Development of Limb Buds:

Limb buds appear at the end of 4th week.

Forelimb appears 1st then Hindlimb.

Core of Mesoderm covered by Ectoderm.

Limb Buds Develop (Days 31-32)

15

Limb buds appear at the end of 4th week as outpocketing from ventro-lateral aspect of body.

Each bud is composed of a core of mesoderm which is covered by ectoderm.

Each limb bud is at first extended at a right angle to the trunk. Each bud has 2 surfaces: ventral and dorsal, and 2 borders :cranial and caudal.

Limb Buds Develop (Days 31-32)

Each limb bud shows 2 constrictions or annular creases which divide the bud into 3 segments.

The upper limb is divided into: arm, forearm and hand plate.

The lower limb is divided into: thigh, leg and foot plate.

The distal segment of the UL bud becomes flattened to form the hand plate ,that of the LL flattens to form the footplate.

Fingers and toes are formed as a result of disintegration of the thinned tissue between the developing fingers and toes. At 7th week, upper limb rotates 90 degree laterally while lower limb rotates 90 degree medially.

Development of Limbs

Hand Foot

Digits develop as a result of Apoptosis (Programmed Cell Death)

Limb development5th Week: Limb bud

appears as mesoderm covered with ectoderm.

6th Week: Constriction produces hand or foot plate and skeleton now is totally cartilaginous .

7th Week: Endochondral ossification begins.

8th Week: Upper & lower limbs appropriately named.

5W 6W

7W 8W

Mesenchyme from lateral mesoderm

Induces growth of mesenchyme & its transformation into cartilage

Cartilage ossifies by:Endochondral ossificationEndochondral ossification

Myoblasts migrate from myotomes to form:Muscles of limbsMuscles of limbs

Limb Innervation (side view)

As the bud elongates it carries with it the dermatomes from the corresponding spinal segments and drags with them the ventral rami of the

corresponding spinal nerves.

The UL carries dermatomes from C4 to T2.

The LL from T12 to S4.

24

Anomalies of the limbs

Anomalies of the limbs

Mesoderm:-Paraxial (Somites)

a. Myotomes:-Limb & Axial muscles

b. Sclerotomes:-Axial skeleton (vertebrae)

c. Dermatomes:-Dermis of skin

Stages in the development of a somite

Development of Vertebral Column

Vertebrae are developed after 4 stages:1- First stage: Formation of

notochord.2- Second stage:( Blastemal

stage)=Mesodermal condensations: Formation of mesenchymal vertebral column.

Sclerotomes surround notochord and neural tube.

3- Third stage: (Chondrification) of mesenchymal vertebral column.

4- Fourth stage:( ossification) of the vertebrae.

Other Anomalies of Vertebrae

Lumbarization of 1st sacral vertebra.

Sacralization of 5th lumbar vertebra.

Increase(Supernumerary vertebrae) or

Decrease(Missed vertebrae) in number of vertebrae. Due to an error in sclerotomic segmentation.

Abnormal curvatures of vertebral column as kyphosis and scoliosis.

Development of Sternum

Anomalies of Sternum

Cleft sternum: failure of fusion between lower parts of sternal plates.

Perforated sternum: failure of fusion of central part of sternum.