Skeletal Muscle Morphogenesis Summary

Skeletal Muscle Morphogenesis

Muscle Structure

• Skeletal muscle is composed of various protein filaments, including:

  • Thick filaments: Myosin

  • Thin filaments: Actin, tropomyosin, nebulin, and troponins

  • Other structural proteins: a-actinin, desmin, titin

Connective Tissue Layers

• Epimysium: A layer of connective tissue that surrounds the entire muscle.

• Perimysium: Surrounds fascicles (bundles of muscle fibers).

• Endomysium: Surrounds individual muscle fibers.

Muscle Attachment

• Muscles attach to bones via tendons.

Learning Objectives

• Understand the embryology of muscle formation.

• Know the embryological history of muscle precursors, including their origin and migration paths.

• Understand the key genetic pathways that regulate embryonic myogenesis: specification, commitment, and differentiation.

• Understand the key similarities and differences between the programs controlling trunk and head muscle formation.

• Understand how different muscles obtain their unique form.

Somite Formation

• All trunk and limb muscles are derived from embryonic structures called somites.

• Somites are blocks of mesoderm that form the vertebral column and associated musculature.

• They are a characteristic feature of vertebrate embryos.

• Somites form in pairs on either side of the notochord and neural tube within the paraxial mesoderm.

Somite Derivatives

• Each somite generates:

  • Sclerotome: Forms cartilage of the vertebrae.

  • Myotome: Forms muscle.

  • Dermatome: Forms dermis.

  • Syndotome: Forms tendons.

Somitogenesis

• Somitogenesis involves:

  • Subdivision of somite blocks

  • Periodicity

  • Epithelialization

  • Specification

  • Differentiation

• Somitogenesis exemplifies 'progressive refinement' in development.

Clock and Wavefront Model

• Cells of the presomitic mesoderm (PSM) possess an intrinsic oscillator (the clock), which controls their somite-forming ability.

• Neighboring cells of the PSM oscillate in phase between a permissive and a non-permissive state for somite formation.

• A second component, the wavefront, corresponds to a maturation front moving posteriorly in concert with the AP differentiation gradient of the embryo.

• PSM cells form a somite when they are hit by the wavefront while in the permissive phase of the clock.

Somite Specification

• Initially, somite cells are not specified to a particular fate.

• Specification involves dorsal-ventral patterning, subdividing multipotential progenitors into distinct fates/lineages.

Somite Progenitors

• Somites contain progenitors for many tissues, including:

  • Skeletal muscle

  • Satellite cells

  • Dermis

  • Sclerotome

  • Cartilage

  • Vascular smooth muscle cells (VSMC)

  • Endothelial cells

  • Meninges

  • Joints

  • Tendon

Extrinsic Signals

• Extrinsic signals, such as those from the notochord and neural tube, specify somitic cell fates.

• These signals include Wnts, BMPs, Fgf5, and Shh.

  • Shh (short range) promotes chondrogenesis.

  • Shh (long range) + Wnt promote myogenesis.

Dermomyotome Subdivision

• The dermomyotome is subdivided into epaxial and hypaxial domains.

  • Epaxial domain: Forms back muscles.

  • Hypaxial domain: Forms body wall, limb, diaphragm, and tongue muscles.

Head vs. Trunk Muscle Formation

• Trunk muscle: Arises from segmented paraxial mesoderm.

• Head muscles: Arise from unsegmented cranial mesoderm; common precursors for branchiomeric muscles and the second heart field.

• Different transcription factors regulate specification in head vs. trunk, but the same transcription factors regulate commitment to differentiation.

Muscle Differentiation

• Muscle differentiation involves proliferation, differentiation, and fusion of myoblasts.

• Myogenic regulatory factors (MRFs) drive precursor cells to undergo terminal differentiation.

Myogenic Regulatory Factors (MRFs)

• MRFs are Class II basic Helix-loop-helix (bHLH) proteins with a basic DNA-binding domain.

• Four members in mammals: Myf5, MyoD, Myogenin, and MRF4.

Hierarchy of Transcription Factors

• A hierarchy of transcription factors is required for muscle determination and differentiation.

• This hierarchy involves genes such as:

  • Pax3

  • Pax7

  • Six1/4

  • Myf5

  • MyoD

  • MyoG

  • MRF4

Pax Genes

• Pax genes (Pax3, Pax7) are 'paired-box' homeodomain transcription factors.

• They are conserved and important in development.

• Pax3 is expressed in the presomitic mesoderm and is required for myogenic gene expression.

  • Pax3 null mutants lack diaphragm and limb muscles.

  • Pax3 -/- Myf5-/- mutants lack skeletal muscle in trunk and limbs, but not head.

• Pax7 is expressed in somitic mesoderm and plays a primary role in fetal muscle and satellite cell specification, as well as muscle repair/regeneration.

Myf5 and MyoD

• Myf5 is the first MRF expressed.

• MyoD mutants exhibit delayed hypaxial development.

• Myf5 mutants exhibit delayed epaxial development.

• MyoD + Myf5 mutants still make skeletal muscle, suggesting overlapping and partially redundant roles.

• Mrf4 can compensate in Myf5-/-; MyoD-/- double-mutant mice.

Upstream Regulators

• Pax3 and Myf5 function upstream of MyoD in somite-derived muscle development.

• Six1 and Six4 are genetically upstream of Pax3 and MRFs.

Limb Muscle Development

• Myoblasts forming limb muscles arise from hypaxial mesoderm.

• Pax3 is a critical regulator of myogenesis in the limb.

Gene Regulatory Networks

• Conserved transcriptional regulatory networks regulate myogenesis.

• These networks share core elements but differ in some details in different muscles.

Satellite Cells

• Satellite cells originate in somites.

• Pax3+, Pax7+ cells in the central dermomyotome become tissue-resident stem cells involved in tissue maintenance and repair.

Head Muscles

• Most craniofacial muscles do not originate from segmented mesoderm.

• These include branchiomeric muscles.

• While the core MRFs requirement remains similar, the factors that act upstream to regulate these MRFs in the progenitors of head muscles differs for different head muscles.

Head Muscle Origins

• Extra-Ocular Muscles (EOMs) originate from cranial paraxial mesoderm.

• Jaw, facial, pharyngeal, and laryngeal muscles (branchiomeric) originate from cranial paraxial mesoderm (CPM).

• Tongue and neck muscles originate from somites.

Branchiomeric Myogenesis

• Tbx1 regulates branchiomeric myogenesis (relevant for DiGeorge Syndrome aetiology).

• Pitx2 is required for first branchial arch muscle development.

Head Muscle Satellite Cells

• Distinct origins and genetic programs of head muscle satellite cells.

Head and Cardiac Muscle Progenitors

• Head muscles share progenitors with some cardiac muscle.

Clonal Analysis

• Clonal analysis reveals common lineage relationships between head muscles and second heart field derivatives.

Regulatory Network in Branchial Mesoderm

• Lhx2 is expressed in branchial arch core mesoderm, not somites.

• Lhx2 and Tcf21 genetically interact with Tbx1.

• Each TF binds to promoter regions of the other members of the network.

• This regulatory network of transcription factors in branchial mesoderm controls head and cardiac muscle morphogenesis.

Cranial vs Trunk Muscle Specification

• In head vs. trunk, different transcription factors regulate specification to myogenesis, but the same transcription factors regulate commitment to differentiation.

Common Progenitor Cells

• Common progenitor cells give rise to both muscles of the pharyngeal arches and to a number of structures of the heart.

Head and Trunk Muscles: Summary

• Cranial muscles arise from non-segmented mesoderm anterior to the somites.

• Specification of cranial muscles is through Tbx1 and Pitx2, with differing requirements depending on the anterior-posterior origin of the progenitor cells.

• Head muscle stem cells arise from progenitor cells that do not express Pax3 or Pax7.

• Common progenitor cells give rise to both muscles of the pharyngeal arches and to a number of structures of the heart.

Muscle Patterning

• Muscles come in a broad variety of shapes and sizes that reflect their function.

Fibre Types

• Four main fibre types in adult muscle:

  • I slow oxidative

  • IIa fast oxidative

  • IIb fast glycolytic

  • IIx super-fast glycolytic

• Most skeletal muscle is a mixture of three types of fibres; all the skeletal muscle fibres of any one motor unit are all the same.

• Adult muscle fibre composition is plastic: motor nerve activity, exercise.

• During embryonic stages, mature fibre type has not been established.

Fibre Type Specification

• Sox6, Six1, and Six4 are required for the Fast program.

• Wnt signaling induces BMP4 to specify the slow phenotype in foetal myoblasts.

Muscle Pattern Determination

• Muscle pattern is determined by extrinsic signals.

• Demonstrated using Duck-Quail Chimaeras.

Limb Musculature

• Limb musculature originates from somitic myoblasts.

• Limb bones and tendons are formed from lateral plate mesoderm progenitors.

• The progenitors of the limb muscle and tendon/bone have different embryological origins.

Musculoskeletal Unit

• Muscles, tendons, and bones form the core musculoskeletal unit.

Muscle Connective Tissue (MCT)

• Formation of individual limb muscles depends on Muscle Connective Tissue (MCT).

• Disruption can lead to ectopic muscles, absent muscles (hypoplasias), or abnormal shaped/positioned muscles (dysplasia).

• Some degree of variation exists in musculature in the normal population (e.g., palmaris longus muscle).

Tbx Genes in Limb Development

• Tbx5 and Tbx4 are expressed in the forelimb and hindlimb, respectively.

• Conditional deletion of Tbx5 after limb initiation stages results in mispatterned soft tissues (muscles and tendons) throughout the forelimb.

• Tbx5 acts non-cell autonomously in the MCT on nascent muscles and tendons.

N-Cadherin and B-Catenin

• N-Cadherin and B-Catenin are downregulated in Tbx5 mutant MCT.

Muscle Translocation

• Translocation of superficialis muscles from the hand to the arm in normal development.

• Muscle translocation depends on tendon and muscle activity.

• Superficial flexor muscles differentiate in the hand and relocate as differentiated myotubes into the arm.

The ‘In-Out’ Mechanism in the Limbs

• Precursors of muscle that end up in a medial (axial) location initially migrate into the limb bud periphery and then move back out to their final location.

• Examples are cloacal/perineal muscles, the precursors of which initially migrate into the hindlimb bud.

• In the pectoral girdle, the pectoral muscles and latissimus dorsi migrate into the forelimb bud initially and then extend back to axial locations.

Summary of Myogenesis

• Myogenesis involves a complex series of steps from founder stem cells to adult satellite cells.

The above notes summarize the key aspects of skeletal muscle morphogenesis, covering muscle structure, embryological origins, regulatory factors, and patterning mechanisms.