Lecture 15 - Musculoskeletal Development II Flashcards
All skeletal muscle originates in either:
somites or somitomeres
Stages in the differentiation of skeletal muscle tissue:
Myogenic cells: •Originate in somites •Resemble mesenchymal cells •Restricted to muscle-forming line •Mitotic cells Myoblasts: •Derived from myogenic cells •Postmitotic
Stages in the differentiation of skeletal muscle tissue:
Myotubes:
•Formed when myoblasts line up and adhere to one another
•Requires calcium-dependent CAMs
•Involved in mRNA and protein synthesis
•Characterized by appearance of actin, myosin, troponin and tropomyosin
•Formation of myofibrils with sarcomeric arrangement
•Nuclei move to periphery
Muscle fibers
Satellite Cells
- Attach to myotubes before basal lamina is laid down Located between sarcolemma and basal lamina of myofiber
- Mitotic and myogenic
- Able to fuse with muscle fiber and provide for growth
- Note that satellite cells do not form new muscle fibers.
- Generally quiescent
- Function as stem cells
- Can become mitotic in times of stress
- Give rise to myogenic precursor cells: (Replace damaged muscle by proliferating, fusing, and differentiating into skeletal muscle fibers.)
Muscle Histogenesis
Primary myotubes:
- Formed by fusion of earliest (embryonic) myoblasts
- May already be distinguished as fast or slow in some species
- Differentiation occurs before innervation
Muscle Histogenesis
Secondary myotubes:
- Smaller than primary
- Formed alongside primary from late (fetal) myoblasts
- Presence of motor axons may be necessary to form secondary myotubes.
- Contained within same basal lamina and are electrically coupled.
Muscle Histogenesis
Secondary myotubes:
- Smaller than primary
- Formed alongside primary from late (fetal) myoblasts
- Presence of motor axons may be necessary to form secondary myotubes.
- Contained within same basal lamina and are electrically coupled.
Mitotic capability of myogenic cells:
FGF and TGF-βmaintain myogenic cells in labile state
Loss of mitotic capability:
Due to p21: •Produced by myogenic cells in response to myogenic regulatory factors.
Myogenic regulatory factors:
MyoD family:
•Able to convert non-muscle cells to cells capable of expressing muscle proteins.
•Helix-loop-helix transcription factor
•Forms dimer and binds to E box in enhancer region of myogenic genes
•Binding is enhanced when E12(transcription factor) replaces one member of the dimer to form a heterodimer.
•Transcriptional inhibitor idcan replace one member of the MyoD homodimer to form a heterodimer, resulting in poor binding ability.
•Pax-3 and Myf-5 separately can activate MyoD and cause myogenic cells to become myoblasts.
•Increasing levels of MyoD along with Myf-5 result in expression of myoblast genes by myoblasts and expression of myogenin.
•Myogenin results in expression of myotube genes and Myf-6. •Myf-6 leads to expression of myofiber genes.
morphogenesis of muscles is dependent on
associated connective tissue framework
Muscles of trunk origin
- Epaxial muscles arise from dorsal lip of myotome.
- Epaxial tendons arise from syndetomelayer within somites.
- Hypaxial muscles arise from ventral buds of myotome.
- Tendons of hypaxial muscles arise from lateral plate mesoderm.
Muscles of trunk origin
- Epaxial muscles arise from dorsal lip of myotome.
- Epaxial tendons arise from syndetomelayer within somites.
- Hypaxial muscles arise from ventral buds of myotome.
- Tendons of hypaxial muscles arise from lateral plate mesoderm.
Muscles of limbs origin
- Limb muscles arise from ventrolateral dermomyotome.
* Tendons of limb muscles arise from lateral plate mesoderm.
Muscles of head and neck origin
- Mostly derived from paraxial somitomeres
- Extraocular muscles arise from prechordal plate.
- Most of the cranial musculature is derived from the unsegmented paraxial mesoderm.
- Some cranial musculature (lower jaw) is derived from splanchnic mesoderm.
