Biology 4361 Paraxial and Intermediate Mesoderm December 7, 2006
Major Mesoderm Lineages Mesodermal subdivisions are specified along a mediolateral axis by increasing amounts of BMPs more lateral mesoderm expresses higher BMP4 than midline areas differential BMP concentrations cause differential expression of the Forkhead (Fox) family of transcription factors
Major Mesoderm Lineages
Major Mesoderm Lineages
Mesoderm Formation Chick
Neural Tube and Somites Somitogenesis periodicity fissure formation epithelialization specification differentiation Anterior Posterior somitomere Somites blocks of paraxial mesoderm cells three major components: 1. sclerotome vertebrae and rib cartilage 2. myotome musculature of the back, ribs, and limbs 3. dermatome forms dermis of the back two minor components: 4. syndetome tendons 5. cells that generate vascular cells in the dorsal aorta
Chick embryo whole mounts Stage 23 26 h 26 h 26 29 h 29 33 h Prof. Judy Cebra Thomas, Swarthmore College
Chick embryo whole mounts Stage 33 38 h 40 45 h 45 49 h 48 52 h Prof. Judy Cebra Thomas, Swarthmore College
Chick embryo whole mounts Stage 50 53 h 55+ h 56+ h 52 64 h Prof. Judy Cebra Thomas, Swarthmore College
Chick embryo whole mounts Stage 64+ h 72 h 3 3.5 d 3.5 d Prof. Judy Cebra Thomas, Swarthmore College
Control of Somite Formation Somite formation depends on a clock and wave mechanism an oscillating signal (the clock ); provided by the Notch and Wnt pathways a rostral to caudal gradient provides a moving wave of an FGF, which sets somite boundaries The Notch signaling pathway is crucial Notch gene is highly expressed in the posterior region of the forming somite; just anterior to the cut somite borders are formed at the interface between the Notch expressing and non expressing cells Notch genes are expressed cyclically; providing an autonomous clock Notch controls a cascade of gene expression that separate the somites; e.g. hairy1
Regulation of Somite Formation periodic expression of Eph receptor tyrosine kinases periodic expression of ephrins (Eph ligands) Eph/ephrins involved in cell cell repulsion Eph targets include Notch signaling pathway Notch directs the placement of somite borders Notch also regulates the periodic expression of Hairy1 ~ 90 min periodicity (chick)
Epithelialization Anterior Posterior Mesenchymal mesoderm transforms into hollow epithelial ball cells polarize: sub apical surface (inward); basal membrane (outside) tight junctions form between basal lamina synthesize extracellular matrix proteins; e.g. fibronectin, N cadherin (see above) Epithelialization promoted by: fibronectin (ECM organizing protein) N cadherin (adhesion protein)
Somite Specification Somites look identical, but will form different structures; e.g. cervical, thoracic, lumbar vertebrae Each somite forms a specific type of vertebrae; not interchangeable specification occurs early segmental plate identity is established prior to somitogenesis somites specified according to Hox gene expression once specified, somites retain their patterns of Hox gene expression; even if transplanted dolphin hindlimb Hox gene product
Somite Development Sclerotome cartilage of vertebrae and part of rib Dermamyotome remaining portion of the somite contains precursors for: Dermatome dermis (mesenchymal connective tissue of the skin) Myotome muscle
Somite Development Primaxial (epaxial) myotome intercostal muscles of the ribs; deep muscles of the back Abaxial (hypaxial) myotome body wall, limbs, tongue
Determination of the Somites Sclerotome Shh (high) from the notochord and floor plate sclerotome cells secrete Pax1 (transcription factor) cartilage/vertebrae formation Dermatome neurotrophin 3, Wnt1 from roof plate (Shh antagonist) Myotome: primaxial Shh (low), Wnt1, Wnt3 abaxial Wnt (epidermis), BMP4, Fgf5 (lateral plate mesoderm) Notochord degenerates through apoptosis (mostly)
Muscle MyoD definitive muscle marker alignment mediated by membrane glycoproteins; cadherins fusion mediated by metalloproteinases; e.g. meltrins stem cells can regenerate muscle after injury
Bone The skeleton is generated from: somites vertebrae lateral plate mesoderm limbs neural crest cells brachial arch, craniofacial bones Ossification types intramembranous direct conversion mesenchyme to bone endochondral mesenchyme to cartilage to bone Mesenchymal cells commit to become cartilage Committed mesenchyme cells condense into nodules Chondrocytes proliferate; form cartilage model; secrete cartilage specific ECM Chondrocytes stop dividing; increase volume; alter the ECM to enhance mineralization; secrete angiogenesis factor VEGF
Bone, continued Blood vessels invade the cartilage model; hypertrophic chondrocytes die; replaced by osteoblasts ECM mineralizes New bone material added peripherally from the internal surface of the periosteum Internal region hollowed by osteoclasts forms the bone marrow cavity
Vertebrae Formation sclerotomes split into rostral and caudal segments re segmentation enables muscles to coordinate movement
Tendon syndetome is induced by myotome Fgf8 above inducing scleraxis scleraxis transcription is blocked by sclerotome cartilage precursors
Tendon
Major Mesoderm Lineages
Progression of Kidney Types Pronephros Mesonephros Metanephros metanephrogenic mesenchyme
Primordial germ cells PGCs Amphibians germ plasm at vegetal pole maneuvered during cleavage to endoderm at floor of blastocoel migrate along gut to genital ridges Mammals migrate into endoderm from posterior primitive streak migrate along hindgut & mesentery to the gonadal rudiment human 6 wk Reptiles, birds originate in epiblast migrate to hypoblast (germinal crescent) distributed through blood stream enter gonadal rudiment