Question 40

Skull bones are formed by the process of intramembranous ossification.

Intramembranous ossification primarily forms the flat bones of the skull, which develop directly from mesenchymal tissue without a cartilage intermediate.

A) Ribs

Ribs are not formed by intramembranous ossification; instead, they develop through endochondral ossification, where a cartilage model is formed before being replaced by bone.

B) Skull bones

Skull bones are indeed formed by intramembranous ossification. This process allows mesenchymal cells to differentiate directly into osteoblasts, leading to the formation of bone tissue that constitutes the flat bones of the skull.

C) Femur

The femur is not formed through intramembranous ossification. It is a long bone that develops via endochondral ossification, where cartilage is progressively replaced by bone as the fetus grows.

D) Humerus

Like the femur, the humerus is also a long bone that undergoes endochondral ossification. This process involves the replacement of a cartilage model with bone tissue, which is not the case for intramembranous ossification.

Conclusion

Skull bones are the only bones listed that are formed by intramembranous ossification, while the ribs, femur, and humerus develop through endochondral ossification. This distinction is crucial in understanding the different processes of bone formation and highlights why option B is the correct choice.