Intramembranous ossification: flat bones without a cartilage model
Intramembranous ossification (bone formation within a sheet of membrane-like tissue) makes bone directly from mesenchyme (the embryo's unspecialised connective tissue), with no cartilage model first.
Remember
Bone that forms directly from mesenchyme with no cartilage stage is intramembranous; bone that replaces a cartilage model is endochondral (within cartilage).
In intramembranous ossification, mesenchymal cells (the embryo's unspecialised connective-tissue cells) gather together and turn directly into osteoblasts, the bone-building cells. No cartilage model is laid down first.
The osteoblasts lay down osteoid, the soft, unmineralised organic part of bone, built largely from collagen. It then mineralises (hardens with calcium salts). The first bone appears as bony spicules (tiny slivers of bone). Osteoblasts trapped in the hardening bone become osteocytes, the mature bone cells.
The spicules are then remodelled (reshaped) into trabeculae, the struts that make up spongy bone. Spongy bone remains within the bone, while compact bone develops at the surface.
It forms the clavicles (collarbones), the flat bones of the face and most cranial bones. The parietal and frontal bones are flat bones of the skull vault (the dome that encloses the brain), and they form directly from mesenchyme. Intramembranous ossification begins before birth, and the sutures (the joints between skull bones) are not yet closed at birth, so the head can deform slightly during delivery.
The alternative is endochondral ossification, in which a hyaline cartilage model (a template of smooth, glassy cartilage) is gradually replaced by bone. It forms the long bones of the limbs, such as the femur and humerus, as well as the ribs, vertebrae and the base of the skull. So the word skull alone is not enough: it is the flat bones of the vault that form directly. A rib, which can look flat, still forms from a cartilage model.
Key facts
- Route
- mesenchymal cells turn directly into osteoblasts, with no cartilage stage.
- Cell sequence
- mesenchymal cell, then osteoblast, then osteocyte once trapped in the hardening bone.
- Bones formed
- the clavicles, the flat bones of the face and most cranial bones, including the parietal and frontal bones.
- Bone laid down
- osteoid mineralises into bony spicules, which are remodelled into trabeculae (the struts of spongy bone, which remains within the bone) while compact bone develops at the surface.
- Cartilage route
- endochondral ossification replaces a hyaline cartilage model and forms the long bones of the limbs, ribs, vertebrae and the base of the skull.
- Shape alone
- a rib can look flat but forms from a cartilage model, so shape does not show how a bone formed.
Exam traps
- The shaft of the femur, a long bone, forms on a cartilage model by endochondral ossification, so it does not form directly from membrane.
- A bone that looks flat, such as a rib, can still form from a cartilage model, as vertebrae do, so its shape alone does not show how it formed.
- Vertebrae form from a cartilage template by endochondral ossification, unlike the flat bones of the skull vault, so they do not form directly in mesenchyme.
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