Skeletal System Development Overview

Sep 4, 2024

Development of the Skeletal System

Introduction

  • Focus on the development of the skeletal system.
  • Goals: Develop the axial skeleton and upper/lower limbs.

Axial Skeleton Development

Skull Development

  • Neurocranium

    • Encases the brain.
    • Develops from mesoderm and neural crest cells.
    • Mesenchymal cells differentiate to chondrocytes to osteoblasts via Endochondral ossification.
    • Bones involved: Occipital, Petrous part of the temporal, Sphenoid, and Ethmoid bones.
    • Base of skull is known as Chondrocranium.

  • Membranous Neurocranium

    • Top of the skull.
    • Develops via Intramembranous ossification.
    • Bones involved: Frontal, Parietal (2), and part of the Occipital bone.
    • Important sutures: Coronal, Sagittal, and Lambdoid.
    • Fontanelles: Anterior and Posterior fontanelles allow skull flexibility and growth.

Viscerocranium

  • Facial bones develop from pharyngeal arches, primarily the first arch.
  • Bones: Mandible, Maxilla, Zygomatic, Squamous part of the temporal bone.
  • Intramembranous ossification for facial bones.
  • Includes ear bones: Malleus, Incus (from first arch), and Stapes (second arch).

Vertebrae, Ribs, and Sternum Development

Vertebrae and Ribs

  • Develop from the sclerotome of somites (part of paraxial mesoderm).
  • Somites undergo re-segmentation to allow spinal nerves passage.
  • Process: Endochondral ossification.

Sternum

  • Develops from lateral plate mesoderm.
  • Parts: Manubrium, Body, Xiphoid process.
  • Process: Endochondral ossification.

Limb Development

Upper and Lower Limbs

  • Develop from lateral plate mesoderm and somatic layer.
  • Positional genes: Hox, TBX4, and TBX5.
  • Growth factor: Fibroblast Growth Factor 10.
  • Proximal to distal growth managed by apical ectodermal ridge (AER).

Proximal to Distal Development

  • AER stimulates mesodermal proliferation via FGF4 and FGF8.
  • Differentiation from cartilage to bone: Endochondral ossification.

Anterior to Posterior Development

  • Zone of Polarizing Activity (ZPA) controls thumb to pinky development.

Dorsal to Ventral Development

  • Controlled by ectodermal genes (Wnt7a for dorsal, Engrailed-1 for ventral).

Genetic Influence on Bone and Cartilage Development

  • CbfA1 and RunX2: Key genes for osteoblast development.
  • Sox9 gene: Key gene for chondrocyte differentiation.

Conclusion

  • Understanding the genetic and developmental processes of the skeletal system leads to better insight into its formation and potential abnormalities.