Because the notochord, a powerful inductive agent, only reaches up to the
cranial flexure, it encourages a cartilage template only at the base of the
skull. Above the notochord, a mesoderm-originated covering lies over the developing
brain, preventing cartilage from forming [B]. This
membrane ultimately gives rise to the roof and side bones of the skull as
well as the dermis covering them. The plate-like bones are separated during
development by connective tissue sutures [A]. Premature
fusion of these sutures results in craniosynostosis, which causes an abnormally
shaped skull and associated neural problems. This condition helps to explain
the signaling mechanisms between the forming dermal bone and the underlying
neural tissue.
The dura
mater is the hardest of three layers encasing the brain, and lies closest
to the ossifying mesenchyme. In the embryonic stage, the dura mater and the
mesenchyme express a lot of Msx-1,
Msx2, as well as Bmp-4
and FGF-9. This signaling keeps the sutures between
the plates open, allowing the skull bones to grow and accommodate the expanding
brain. After birth, sonic hedgehog is expressed
at the edges of the skull bones, causing the dura mater to stop expressing
signaling factors. This signaling dictates the mophogenesis of the bones.
See
this paper for more information
Malformations of the brain are accompanied by malformations of the skull. When the brain is very small, microcephaly occurs: the skull forms a very small vault. In the opposite situation, where the brain is swollen with excess cerebrospinal fluid, the skull is greatly enlarged in a situation called hydrocephaly[A]. The signifigance of this communication is that the communication between neural and mesenchymal tissue causes the skull to form a custom made case for the brain during its rapid proliferation in the embryonic stage.
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