Paraxial and Intermediate MesodermBiology 4361July 22, 2009Paraxial and Intermediate Mesoderm OverviewDevelopment of major mesodermal lineagesSomites:- formation- specification and differentiationMuscleBonePrimordial germ cellsVertebraeTendonKidneyMesodermal structures:Chick – Early Development19-22 h23-26 hChick – Early Development33-38 h40-45 hChick – Early DevelopmentMajor Mesoderm LineagesMesodermal subdivisions are specified along a mediolateral axis by increasing amounts of BMPs- lateral mesoderm expresses higher BMP4 than midline areas- different BMP concentrations cause differential expression of the Forkhead (Fox) family of transcription factors – results in different mesodermal fatesMajor Mesoderm LineagesMajor Mesoderm LineagesParaxial and Intermediate Mesoderm OverviewDevelopment of major mesodermal lineagesSomites:- formation- specification and differentiationMuscleBonePrimordial germ cellsVertebraeTendonKidneyMesodermal structures:Somite DerivativesMajor somite components: 1. Sclerotome – vertebrae, ribsand rib cartilage2. Myotome – musculature of the back, ribs, and limbs3. Dermatome – forms dermis of the backMinor components: 4. Syndetome – tendons5. cells that generate vascular cells in the dorsal aortaSomitogenesisSomitogenesis- periodicity- fissure formation- epithelialization- specification- differentiationSomitomereAnteriorPosteriorParaxial mesoderm(presomitic mesoderm)Regulation of Somite FormationSomites are formed by a “clock and wave” mechanism:- FGF expression follows Hensen’s node regression from rostral-to-caudal, which triggers…- an oscillating signal (the “clock”): Notch, which stimulates gene expression, (e.g. hairy1), that appears in repeating “waves”- with each wave of gene expression, another somite is formedHow to construct an “expression clock”Trigger ClockClockInhibitor(unstable)EffectorFgf8(low?)Hairy1Wnt3aAxin(unstable)NotchLunaticfringe(unstable)SegmentationHypothetical model for somite productionRegulation of Somite Formation- regulates the periodic expression of hairy1(etc.)~ 90 min periodicity (chick)Notch directs the placement of somite bordershairy1expressionFGFRAa, Alizarin staining of a corn snake showing 296 vertebrae, including 3 cervical, 219 thoracic, 4 cloacal (distinguishable by their forked lymphapophyses) and 70 caudal. b, Time course of corn snake development after egg laying (118-somite embryo on the far left) until the end of somitogenesis ( 315 somites). Céline Gomez, Ertu rul M. Özbudak, Joshua Wunderlich, Diana Baumann, Julian Lewis & Olivier Pourquié Nature 454, 335-339(17 July 2008) Regulation of Somite FormationEpithelializationEpithelialization promoted by:- fibronectin (ECM organizing protein)- N-cadherin (adhesion protein)Mesenchymal mesoderm transforms into hollow epithelial ball- synthesize extracellular matrix proteins; e.g. fibronectin, N-cadherin (see above)- tight junctions form between basal lamina- cells polarize: sub-apical surface (inward);basal membrane (outside)Anterior PosteriorSpecification and DifferentiationSomite identity specified according to Hox gene expression- once identity is specified, somites retain their patterns of Hox gene expression pattern, even if transplantedSomites look identical, but will form different structures; e.g.- cervical, thoracic, lumbar vertebrae- tissue specification occurs early – prior to somitogenesisEach somite forms a specific type of vertebrae; not interchangeable- somite tissue specified by:notochord (Shh)neural tube floor plate (Shh)neural tube roof plate (Wnts)epidermal ectoderm (BMPs)lateral plate mesoderm (Fgfs)Somite DevelopmentSclerotome - cartilage of vertebrae and part of ribDermamyotome - remaining portion of the somite contains precursors for:Dermatome – dermis (mesenchymal connective tissue of the skin) Myotome – muscleSomite DevelopmentPrimaxial (epaxial) myotome – intercostal muscles of the ribs; deep muscles of the backAbaxial (hypaxial) myotome – body wall, limbs, tongueDetermination of the SomitesSclerotome – Shh (high) from the notochord and floor plate- sclerotome cells secrete Pax1 (transcription factor) - cartilage/vertebrae formationDermatome – neurotrophin-3, Wnt1 from roof plate (Shh antagonist)Determination of the SomitesMyotome:- primaxial – Shh (low), Wnt1, Wnt3- abaxial – Wnt (epidermis), BMP4, Fgf5 (lateral plate mesoderm)Notochord – degenerates through apoptosis (mostly; remnants remain as nucleus pulposus)Paraxial and Intermediate Mesoderm OverviewDevelopment of major mesodermal lineagesSomites:- formation- specification and differentiationMuscleBonePrimordial germ cellsVertebraeTendonKidneyMesodermal structures:MuscleMyoD – definitive muscle markerAlignment mediated by membrane glycoproteins,cadherinsFusion mediated bymetalloproteinases; e.g. meltrinNote - stem cells can regeneratemuscle after injury- satellite, mesenchymal SCsBoneThe skeleton is generated from: - somites → vertebrae- lateral plate mesoderm → limb bones- neural crest cells → pharyngeal arch, craniofacial bonesOssification typesMesenchymal cells commit to become cartilageCommitted mesenchyme cells condense into nodulesChondrocytes proliferate; - form cartilage model; - secrete cartilage-specific extracellular matrix (ECM)Chondrocytes stop dividing; increase volume (hypertrophy)ECM shifts: mineralizationsecrete angiogenesis factor - VEGFEndochondral – mesenchyme to cartilage to boneIntramembranous – direct conversion – mesenchyme to boneShh → Pax1BMPs → N-caherin, N-CAMSox9VEGFEndochondral Bone FormationBone/Cartilage Differentiation(bone formation)Endochondral Bone, continuedBlood vessels invade the cartilage model; hypertrophic chondrocytes die; replaced by osteoblasts(sclerotome)ECM mineralizes(CaPO4)New bone material added peripherally from the internal surface of the periosteumOsteoclasts (lateralplate mesoderm)hollow internal region- bone marrow cavityperiosteumVertebrae FormationSclerotomes split into rostral and caudal segmentsRe-segmentation enables muscles to coordinate movementNotochord induces mesenchyme to attract sclerotome cells- differentiates into cartilage (endochondral bone formation)TendonSyndetome is induced by myotome cells (above) Fgf8 inducingscleraxisin first row of sclerotome cells (below)Scleraxistranscription is blocked by sclerotome cartilage precursors(Pax1) (Scleraxis)TendonNote – ant, post.Major Mesoderm LineagesBMP Lim1Pax2Pax8Development of the