University:
Amherst College
Course:
BIOL 220 | Developmental Biologyl
Academic year:

2022
Views:

259

Pages:

17
Author:

PramCralttx5

Mesoderm I Some Mesodermal Derivatives Chordamesoderm notochord skeletal muscle kidney facial muscle blood cells Anatomy of Trunk in Frog and Chick Frog Chick Spatial Relationships of Major Lineages of the Mesoderm Early Subspecializations of Trunk Mesoderm • Somites segment into blocks • Lateral plate and somites split (form coelom) Transverse sections Paraxial Mesoderm • Separates into blocks of cells: “somites” – Transient structures – Organize the segmental pattern of vertebrate embryos – Determine migration path of neural crest cells and spinal nerve axons – Form vertebrae, ribs, and the skeletal muscles of the back, body wall, and limbs Somitic Regions • Dermatome --> dermis of back (connective tissue) – Other dermis comes from other mesenchymal cells (non-somite) • Myotome --> trunk and limb muscles – Somatic (striated) muscle • Sclerotome --> cartilage (replaces notochord) – bone of vertebrae and ribs Differentiation of Skeletal Muscle Cells • Muscles – Skeletal muscle • from myotome of somites – Involuntary muscles (smooth muscle and cardiac muscle) • from mesodermal lateral plate • Muscle fibers are made of multinucleated muscle cells – 0.05mm diameter in humans – Myofibril = contractile unit • actin and myosin How is the Myotome Specified? • Paracrine factors signal the myotome precursors to express MyoD, Myf5 • Muscle-specific genes are activated • Myoblasts = committed muscle cell precursors from spinal chord, notochord, lateral plate, epidermis MyoD • MyoD and other related transcription factors control determination – Only expressed in muscle lineage • Transfect in other cell types turns them to muscle in culture • Knockout both myoD and myf5 – kills muscle precursor cells but either can give muscle determination (redundant) Models for Myotube Formation • How could you make multinucleate cells? – Nuclear division without cell division – Fusion of multiple cells Myoblasts into mytotubes Models for Myotube Formation • Division Model Fusion Model How Can We Choose? • Division Model Fusion Model • To distinguish: start with embryos derived from fusion of embryonic cells from 2 different mice strains (genotypes) encoding different enzymes • Detectable marker = isocitrate dehydrogenase (2 subunits) – Genotype AA --> protein A --> enzyme AA – Genotype BB --> protein B --> enzyme BB How to Detect Different Isotypes? • Division Model Fusion Model How To Do This in a Living Organism: The Tetraparental Mouse Result • Division Model • All tissues (including heart muscle) Fusion Model skeletal muscle Where Does Skeletal Tissue Arise? • Axial skeleton (from somites) • Limb skeleton (from lateral plate mesoderm) • Branchial arches and cranofacial bones (from neural crest cells)

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