Development of Neural Tube and Somites (PDF)

04/11/23 Development of neural tube and somites and embryo folding: part 2: Learning objective: de ne and describe the process of somite formation and differentiation. Learning objective: appreciate the derivatives if the mesoderm ( in general ). Differentiation of mesoderm: During Gastrulation, spe...

04/11/23 Development of neural tube and somites and embryo folding: part 2: Learning objective: de ne and describe the process of somite formation and differentiation. Learning objective: appreciate the derivatives if the mesoderm ( in general ). Differentiation of mesoderm: During Gastrulation, speci c regions of the epiblast migrate through different parts of the node and primitive streak to form mesoderm. The path of migrating cells has been mapped and their ultimate fates determined: cells migrating at the central and most cranial part of the node will form the notochord. Epiblast cells migrating laterally form paraxial, intermediate and lateral plate mesoderm. Development of somites: By the beginning of the third week, paraxial mesoderm starts to be organised into segments, somitomeres ( precursor populations of cells ). Towards the end of the third week, somitomeres further organise into somites, each side of the neural tube. The new somites appear in cranio-caudal sequence at a rate of approximately 3 pairs/day until at the end of the fourth week 42-44 pairs have formed. Somite differentiation: As the somite matures, various regions become committed to forming only certain cell types: Scelerotome: this is the ventro-medial region that contains cells that will give rise to cartilage cells ( chondrocytes ) of the vertebrae and ribs, forming bone component of the axial skeleton. Dermamyotome is a double layered structure constituting of: Dermatome: which is the central region of the dorsal layer of dermamyotome that will generate the dorsal of the back skin. Myotome: the lower layer of the dermamyotome that contains cells that will divide to produce myoblasts that will give rise to axial and appendicular( limb ) skeletal muscle. Somite innervation: somites determine the migration pattern of neural crest cells and spinal ner ve axons. As somites appear, neural crest cells simultaneously migrate from the dorsal neural tube into somites and differentiate into melanocytes and also contribute to the development of dorsal root ganglia. Each dermatome and Myotome retains its inner vation from its segment of origin. Somites are temporary but an essential part of the developing body plan of the vertebrates. They are the building blocks of the vertebrae body plan, essential for segmentation, bone and musculature development as well as creating a template for the nervous system. Although all somites looks identical, they will form different structures at different positions along the anterior-posterior axis. Video on somite development: https://www.youtube.com/watch?v=9N-q7z2Zz4s