Part VI. Embryonic Origins

Part VI

Embryonic Origins The stem and progenitor cells discussed in Parts IV and V that produce the vertebrate skeletons there is more than one skeleton have two distinct embryological origins in two of the four germ layers: mesoderm (Chapter 16) and neural crest (Chapter 17). Chapter 16 deals with the somitic origin of the vertebrae, ribs, pectoral and pelvic girdles, development of the muscles of the limbs from somitic cells and the developmental and evolutionary implications of more than one cell lineage generating the elements of the pectoral girdle. Origins of vertebrae from the perinotochordal sheath in fish and from sclerotomal mesenchyme in tetrapods are discussed. Chapter 17 deals with the neural crest origin of the craniofacial and visceral arch skeletons, with whether trunk neural crest is skeletogenic or odontogenic and with cartilage and bone that form in the heart and great vessels, much of which is of neural crest cell origin. Chapter 18 deals with initiation of skeletal elements by epithelial cells interacting with a population of mesenchymal cells to form a condensation of chondro- or osteogenic cells. These interactions localise skeletogenic mesenchyme within embryos and allow condensation and differentiation of cartilage or bone (or dentine or enamel) under the control of Endothelin1 (Edn1) and members of the Dlx gene family. Interactions between neuromasts of the lateral line system and initiation of dermal bone formation also are discussed.