Quicktime Movie - Skeletal System 3D stage 22

From Embryology
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<Flowplayer height="588" width="560" autoplay="true">Stage22_skeletonlarge.flv‎</Flowplayer> 3 Dimensional Reconstruction

Based upon a serial reconstruction from individual embryo slice images. Carnegie Stage 22 embryo (27mm Human embryo, approximate day 56)

The mesoderm contributes the skeletal structures of the body and neural crest contributes to the head. Note the relative size and position of individual structures at this stage of development. Below the animation is a more complete description of each system.

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Skeletal System Development

The skeletal system is primarily mesodermal in origin, with additional input from the neural crest. The neural crest contributes to the craniofacial skeleton, with somites and the lateral plate mesoderm forming the axial skeleton and the limbs respectively.

The first stage in bone development is migration of preskeletal cells to the correct anatomical location. This is associated with an epithelium and basement membrane, which interact with the preskeletal cells, causing them to undergo condensation. Cell condensation is the process whereby populations of cells gather together and differentiate into a single cell type, be it cartilage, bone, muscle, tendon etc... Adhesion molecules, epithelial cell surface receptors and extracellular matrix molecules initiate condensation and set its boundaries. The next step in bone formation is differentiation of the cells to chondrocytes or osteoblasts.

2 types of skeletal ossification can occur:

  1. Intramembranous – primarily in the skull
  2. Endochondral – the majority of bones, particularly the vertebrae, limb long bones etc…

The difference between the two is that endochondrial ossification involves formation of a cartilage template which later ossifies to bone, while intramembranous ossification bypasses this step, with the ossification taking place over a membranous template.

The cartilage template formed in endochondrial ossification by chondrocytes. Unlike bone, cartilage is avascular, thus an important step in ossification is development of vasculature. This achieved by the chondrocytes which hypertrophy and begin to secrete Vascular Endothelial Growth Factor (VEGF), subsequently undergoing apoptosis. VEGF induces vasculature to form, bringing with it immature osteoblasts, which begin to form bone.