DEVELOPMENT OF THE SKELETAL SYSTEM

FATIMA TRUNKWALA GROUP 216

Embryological source of skeletal system

◦Mesoderm and ectoderm

◦Mesoderm◦ paraxial and lateral (somatic) plate mesoderm

◦Ectoderm

◦ Neural crest

Ossification• Intramembranous ossification

Bone formation in which the mesenchyme differentiated directly into the bone e.g. flat bones of the skull

• Endochondral ossification

The process of bone formation in which the mesenchymal cells give rise to cartilaginous models first which in turn become ossified and form bone e.g. long bones of the limb

Development of the skull

◦The Neurocranium; a protective case for

the brain

◦The Viscerocranium; the skeleton of the

face

Neurorocranium

◦ Membranous neurocranium

Formed by intramembranous ossification

Mesenchymal cells are derived from neural

crest and paraxial mesoderm

Cells then encircle the brain and form most of the flat

bones of the skull

Neurocranium

◦ The cartilaginous neurocranium (chondrocranium) Formed by a combination of mesodermal sclerotome and neural crest

cells

Cartilage are form around the brain beginning at the notochord

Parachordal cartilage and the occipital sclerotomes fused to form the

base of occipital bone

While the sphenoid and ethmoidal bones are formed from the

hypophysial cartilage and the trabeculae cranii

All these pieces of bones fuse with each other to form a strong base of

the skull, expect for the openings via which the cranial nerves leaves the skull

Viscerocranium

◦ Membranous Viscerocranium

◦ Dorsal portion

Undergoes intramembranous ossification and gives rise to the maxilla,

the zygomatic bone, the squamous temporal bones, the vomer and the palatine bone

◦ Ventral portion

Contains the Meckel’s cartilage

This region become surrounded by mesenchymal cells that

condenses and ossifies by membranous ossification to form the

mandible

Viscerocranium

◦ Chondral Viscerocranium

◦ Dorsal portion

Forms the malleus and incus (Meckel’s cartilage)

Forms the stapes and the styloid process (Reichert’s cartilage)

◦ Ventral portion

Ossifies and forms the lesser cornu and the upper body of the

hyoid bone

Forms the greater cornu and lower body of the hyoid bone

vertebral column◦ originate from the the sclerotomal cells

◦ During the fourth week of development these sclerotomal cells from the somites

surround-:

Ventrmedial aspect of the notochord to form the centrum and the intervertebral disc

Dorsal portion of the neural tube to form the neural arch, and

Ventrolateral aspect of the body wall to form the costal processes.

oChondrification begins in week six. Ossification begins before birth and end during the

25th year

oAt birth, three primary ossification centers are present in the centrum and in each half

of the vertebral (neural) arch

Ribs

◦ Ribs are derived from the sclerotome portion of the paraxial mesoderm which

form the costal process of the vertebrae

◦ Costal process derived mainly from the thoracic vertebrae

◦ Primary ossification centers appear in the body of the ribs and mostly become

cartilaginous during weeks 13-14 of development.

◦ Secondary ossification centers appear for the head and tubercle of the rib at

puberty

Sternum

◦ Develops from the somatic mesoderm in the ventral body wall

◦ Two sternal bars are formed on either side of the midline and these later fuses to form

the cartilaginous model of the manubrium, sternabrae (body) and the xiphoid process

◦ Ossification appear cephalo-caudally before birth except in the xiphoid process which

appears during childhood

◦ In neonate, the manubrium contains usually one main ossification center. Ossification

at the lowest segment begins shortly after birth and that of the xiphoid process during

the 3rd year of life

Appendicular Skeleton◦ Limbs are derived from the somatic layer of lateral mesoderm

◦ Mesenchymal cells of this region become activated and the limb buds become visible

as an outpocketing

◦ Mesenchyme destined for the limbs is covered by a layer of ectoderm

◦ Ectoderm thickens and forms the epical ectodermal ridge (AER) which exerts an

inductive influence and initiates growth

Appendicular Skeleton

◦ Distal end of the limb buds become flattened to form the handplates and footplates

◦ Fingers and toes are formed when the mesenchyme of the handplates and the

footplates condensed to form digital rays by apoptosis

◦ Similarly, as the shape of the limbs is being formed, mesenchyme in the buds

condenses and differentiates into chondrocytes

◦ Entire limb skeleton is cartilaginous by the end of the sixth week of development

◦ Joints are formed when chondrogenesis is arrested and a joint interzone is induced

Appendicular Skeleton◦ Development of the upper and lower limbs is similar, except that, the upper limb

appeared approximately 1 or 2 days ahead of the lower limb.

◦ Upper limb buds develop opposite the cervical segments

◦ Lower limb buds form opposite the lumbar and upper sacral segments

◦ End of the embryonic period, primary ossification begins in the diaphysis of the long

bones

◦ Endochondoral ossification gradually progresses from diaphysis of the bone toward the

end of the cartilaginous model

Appendicular Skeleton

◦ The (shaft) diaphysis of the long bone is fully ossified at birth

◦ The epiphysis is still cartilaginous and secondary ossification centers appear in the

epiphyses of these bones

◦ Persistence of the growth plates provide for interstitial growth in the length of the long

bone

◦ Periostuem provides for appositional growth in the girth of these bones

◦ Endochondral ossification advances on both sides of the plate and finally the plate

disappear and the epiphysis unite with the shaft of the bone when bone has acquired

its full length.

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