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Intramembranous ossification is a process of bone formation that produces flat bones such as the parietal bones. From what I gather, it forms inside the mesenchymal tissue and uses mesenchymal cells as stem cells to produce osteoblasts.

The part I don't fully understand is, what is mesenchymal tissue? I know it's in embryos, but where is it on the embryo and what shape does it have? Is it in the shape of a parietal bone already and then the bone just develops out of it?

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During a process called gastrulation, an embryo forms 3 basic tissue layers: ectoderm, mesoderm, and endoderm. In general terms, the ectoderm forms the skin and nervous system, the mesoderm forms the connective tissue, while the endoderm forms many visceral organs such as the liver and gastrointestinal tract.

One particular group of stem cells derived from the mesoderm are the mesenchymal stem cells, which are characterized by "matrix that contains a loose aggregate of reticular fibrils". These cells can migrate and "fill" the space between other organs by differentiating into more mature connective tissue such as

  • endothelial lining of blood vessels
  • adipocytes in fat
  • myoblast in muscles
  • chondroblasts in cartilage
  • osteoblasts in bones

In terms of intramembranous ossification, the osteoblasts will proliferate into the shape of the intended bone and form mature bone directly without a cartilage intermediate.

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