Embryonic Development of the Orbit

The bones of the orbit develop through endochondral ossification of mesenchyme. In this process a cartilaginous precursor is eventually replaced by bone. Neural crest cells play an important role in forming the mesenchyme. Apart from forming the orbital bones, neural crest cells are involved in the formation of nerves, orbital fat, smooth muscle and other cells. Cranial neural crest cells migrate to the optic vesicle under guidance of the optic vesicle itself. Further development from week 8 sees a large orbital muscle of Müller occupying the orbital floor, levator palpebrae superioris moving towards a position above rectus superior, development of the lacrimal gland out of 5 to 6 epithelial buds and a lateral-to-frontal reorientation of the orbits thereby relatively reducing the intercanthal distance. From week 13 fat develops out of primitive blood vessels and mesenchymal condensations and from week 24 the insertions of the extraocular muscles recede from the limbus. In the last phase of fetal development the dimensions of the orbit and the amount of orbital fat increase markedly.