CHAPTER 14-1, PARAXIAL MESODERM: THE SOMITES AND THEIR DERIVATIVES ZOO3603C The major lineages of the amniote mesodermFour regions of the trunk mesoderm at neurulation 1. Chordamesoderm: Notochord inducing neurulation and establishing the anterior-posterior body axis. 2. Paraxial mesoderm (or somitic mesoderm): Somites producing bone, muscle, cartilage and dermis in the back 3. Intermediate mesoderm: Urogenital system consisting kidneys, gonads and their associated ducts. 4. Lateral plate mesoderm: Circulatory system consisting heart, blood vessel and blood cells.Differential gene expression in mesoderm BMP4 BMP4 Foxf1 Foxf1 Foxc1 Foxc2 Foxc1 Foxc2 Medial mesodermal compartments in the trunk 12-somite check embryo Blue, chordin in notochord Green, paraxis in somite Red, Pax2 in the intermediate mesoderm Foxc1 and Foxc2 in the intermediate mesoderm initiate Pax2 expression. Gastrulation and neurulation in the chick embryo, focusing on the mesodermal component Notochord extends beneath the neural tube. Either side of the neural tube is paraxial mesoderm Paraxial mesoderm = segmental plate (chick) =un-segmented mesoderm (other vertebrates) Gastrulation Neurulation Gastrulation and neurulation in the chick embryo, focusing on the mesodermal component Neurulation, neural tube formed Paraxial mesoderm separate into the block of cells called somite. Noggin antagonize BMP signal in paraxial mesoderm. Specification of somites In situ Pax3, marker for somite Placing Noggin-secreting cells into a prospective region of chick lateral plate mesoderm. Noggin re-specified mesoderm into somite-forming paraxial mesoderm. Gastrulation and neurulation in the chick embryo, focusing on the mesodermal component Somite Sclerotome Myotome Dermatome Syndotome The major lineages of somite Somite Sclerotome Myotome Dermatome Syndotome Vertebrae and rib cartilage Musculature of the back, ribs and limbs Dermis of the back Tendons and vascular cells • The periodicity of somite formation • Where somites form: The Notch pathway • The separation of somites from the un-segmented mesoderm • Epithelialization of the somites • Somite specification along the anterior-posterior axis • Derivatives of the somites • Determination of the sclerotome and dermatome • Determination of the myotome The formation of Somites The periodicity of somite formation Somite formation, somitogenesis “Clock and wave” mechanism  “Clock”, oscillating signal by Notch and Wnt pathway  “Wave”, moving of FGF 1. Periodicity 2. Fissure formation 3. Epithelialization 4. Specification 5. Differentiation Neural tube and somites seen by scanning electron microscopy A view after the surface ectoderm was peeled away. Paraxial mesoderm forming somites. Paraxial mesoderm Somite Where somites form: The notch pathway Notch signaling and somite formation A prospective somite boundary region created a new boundary. Notch signaling and somite formation  Non-boundary region + Notch induced a new boundary. The somite boundaries were formed at the interface at the interface between the Notch-expressing and Notch-non-expressing areas. Mechanism of Notch activity Figure 6.26 Notch protein is a juxacrine factors. 1. Delta activates Notch. 2. Notch changes its conformation. 3. Presenilin-1 cuts cytoplasmic domain of Notch. 4. Cleaved protein enters the nucleus, 5. and activates Transcription. Notch signaling and somite formation Effects of a mutation in Delta-like3 (ligand of Notch) gene on mouse ossification. Wild type Wild type The mutation had several ossification centers. Notch signaling is important to form the segmentation in somites Wavelike expression of the hairy1 gene in the chick Somite formation correlates with the wavelike expression of the hairy1 gene, which is regulated by Notch, in the chick. Every 90 minutes waving as well as the segmentation of somites Hairy1, the segmentation gene, in the posterior half of the somite Coordinating waves and clocks in somite formation Negative feedback loop 1. Wint3a stimulated Axin2 production. 2. Axin2 inhibited Wnt signaling. 3. Axin2, an unstable protein, decays. 4. With the decay of Axin2, Wnt3a can signal again. Possible scheme for the regulation of the clock through which an Fgf8 gradient regulates a Wnt oscillating clock, which in turn controls a Notch clock. Lfringe, Lunatic fringe  What keeps the segmentation clocks on both sides of the body synchronized? Answer is Retinoic Acid. The separation of somites from the un-segmented mesoderm Ephrin and its receptor constitute a possible cut site for somite formation 1. Eph is the receptor for Ephrin. 2. They can cause cell-cell repulsion. 3. Eph signaling mediates cell shape changes. 4. The change causes the separation at the border of Ephrin-B2/EphA4.Ephrin and its receptor constitute a possible cut site for somite formation In situ hybridization of Eph4A Epithelialization of the somites Epithelialization and de-epithelialization in somites of a chick embryo  A formed somite is surrounded by epithelial cells.  Epithelialization occurs first at the posterior edge of the somite.  F-actin accumulation in somites after the separation. Molecular mechanism of epithelialization in somite 1. Notch regulate Mesp2 expression. 2. Paraxis is regulated by processes involving the cytoskelton. 3. Paraxis and Mesp2 induce the synthesis of the extracellular matrix in the outer cells of somite. 4. Fibronectin and N-cadherin toward the epithelialization. 5. GDPase, Rac1 and Cdc42!can organize the actin cytoskeleton. Somite specification along the anterior-posterior axis Transplantation of the segmental plate mesoderm before somiteogenesis Segmental plate mesoderm transplanted into a different region in a younger embryo differentiates according to its original position Older Younger Just before somitegenesis Transplant the lower portion on the caudal Axial skeletons of mice in gene knockout experiments Fig. 11.43 Lumber converted into thoracic Sacral converted into sacral Hox genes expression specified the somite. Derivatives of the somites Diagram of a transverse section through the trunk of a chick embryo on days 2–4Diagram of a transverse section through the trunk of a chick embryo on days 2–4Determination of the sclerotome and dermatome Model of major postulated