The skeletal system develops from mesoderm and ectoderm tissues. Bones form through two processes - intramembranous ossification which forms flat bones, and endochondral ossification where cartilage is replaced by bone. The skull develops from both membranous and cartilaginous structures which fuse together. Vertebrae develop from somites and the ribs and sternum form from mesoderm in the body wall. Limbs bud from lateral plate mesoderm and develop cartilage models which are later replaced by bone through endochondral ossification.

1. DEVELOPMENT OF
THE SKELETAL SYSTEM
FATIMA TRUNKWALA
GROUP 216

2. Embryological source of skeletal
system
◦Mesoderm and ectoderm
◦Mesoderm
◦ paraxial and lateral (somatic) plate mesoderm
◦ Ectoderm
◦ Neural crest

3. 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

4. Development of the skull
◦The Neurocranium; a protective case for
the brain
◦The Viscerocranium; the skeleton of the
face

5. 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

6. 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

7. 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

8. 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

9. 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.
o Chondrification begins in week six. Ossification begins before birth and end during the
25th year
o At birth, three primary ossification centers are present in the centrum and in each half
of the vertebral (neural) arch

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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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