Chapter 10: The Axial Skeleton

The text delineates the differentiation of somites into the sclerotome, which forms vertebrae and ribs, and the dermomyotome, while also explaining the two distinct processes of bone formation: intramembranous ossification in flat bones and endochondral ossification in cartilaginous models. A significant portion of the chapter focuses on the skull, dividing it into the neurocranium, which protects the brain, and the viscerocranium, which forms the face. It describes how the membranous neurocranium forms the cranial vault with sutures and fontanelles, such as the anterior fontanelle, to allow for molding during birth, whereas the cartilaginous neurocranium forms the skull base. The summary highlights the critical role of molecular signaling pathways, specifically involving fibroblast growth factor receptors (FGFRs) and transcription factors like MSX2 and TWIST1, in regulating skeletal growth. Considerable attention is given to craniofacial abnormalities, including craniosynostosis types like scaphocephaly and brachycephaly, and skeletal dysplasias such as achondroplasia and thanatophoric dysplasia, which are often linked to genetic mutations. The development of the vertebral column is explained through the concept of resegmentation, where the caudal half of one sclerotome fuses with the cranial half of the subjacent one, creating definitive vertebrae and allowing spinal nerves to exit through intervertebral foramina. The chapter also details the formation of the intervertebral disc, noting that the notochord persists as the nucleus pulposus. Furthermore, clinical correlates regarding vertebral defects are explored, ranging from scoliosis to varying degrees of spina bifida, emphasizing the preventative role of folic acid. Finally, the text covers the formation of the ribs from vertebral costal processes and the development of the sternum from somatic mesoderm bands, addressing chest wall defects like pectus excavatum and rare sternal clefts.