PMC

Embryological origin of craniocervical bones. Origin of craniocervical bones according to embryology. The 4th and 5th somites are the origin of the proatlas sclerotome and then the occipital bone. The 5th and 6th somites are the origin of the C1 sclerotome. In the 4th developmental week, the occipital and cervical bones are separated by a severance line.

On each side of the neural tube the unsegmented paraxial mesoderm (presomitic) (A) forms segmented tissular blocks named somites (B) that lose their epithelial structure throw differentiation. Then, the somites differentiate into a ventro-medial portion (sclerotome) and a dorso-lateral portion (dermomyotome) (C). The sclerotome cells migrate around the neural tube and the notochord and from dermamyotome forms the myotome (D) (after Larsen’s Human Embryology)

Primary somitogenesis in a chick embryo. A At very early gastrulation, the mostly unneurulated neural plate caudal to the prochordal plate (head process) is flanked on each side by the presomitic (presegmented) mesoderm, or PSM. B The first pair of somites are formed just caudal to the otic vesicles. The PSM column elongates from addition of new cells from the caudal embryonic pole rostral to Hensen’s node. C The new somites (SN) are formed in a rostrocaudal direction so that the older more matured somites (SM) are rostral, i.e. closer to the cephalic end of the embryo. D The older matured somites (SM) have undergone dorsoventral differentiation (purple colour) into sclerotome and dermomyotome. The intermediate-aged somites (SI) (orange colour) are pre-differentiated units without dorsoventral specification. The PSM is always at the most caudal end closest to Hensen’s node. Rostrocaudal sequential somitogenesis parallels progression of primary neurulation. HN Hensen’s node, PS primitive streak, T telencephalon, R rhombencephalon, NT neural tube, NP neural plate

Chick embryo during sequential somite formation close to the end of gastrulation. The rostral most mesoderm consists of matured somites (SM) already having undergone dorsoventral differentiation into dermomyotome and sclerotome, flanking the formed neural tubes (NT); followed by the newer intermediate-aged pre-differentiated epithelial somites (SI), the new somites (SN) and the presomitic mesoderm (PSM) on each side of the unneurulated neural plate (NP). HN Hensen’s node, PS primitive streak, T telencephalon, R rhombencephalon