Muscle structure and adaptation Flashcards
What is muscle formed from?
somites (paired blocks of paraxial mesoderm on either side of the notochord)
What is Somitogenesis?
formation of somites
How do somites form?
by paracrine signalling from the neural tube and notochord triggering a mesenchymal-to-epithelial transition of the paraxial mesoderm, forming a hollow ball of epithelial cells (epithelial somite)
What happens to the epithelial somite?
due to other paracrine signalling, epithelial somite further subdivided into 4 major cells which go on to form specific tissue types:
- sclerotome
- myotome
- syndetome
- dermomyotome
Sclerotome forms…
bone, ribs and cartilage
Myotome forms…
muscle precursors (which form muscle)
Syndetome forms…
tendons
*syndetome is in between myotome and sclerotome
Dermomyotome forms…
new source of muscle cells that later populate the myotome and give rise to the dorsal dermis
Describe the process of myogenesis.
1) Paracrine factors signal for the mesodermal cells to produce myogenic regulatory factors Myf5 and MyoD, which commit those cells to a myogenic fate and become myoblasts
2) Myoblasts then differentiate and proliferate in presence of growth factors until they exit the cycle after expression of another myogenic regulatory factor Myogenin
3) Myogenin causes terminal differentiation of muscle fibres and differentiates the myoblasts into myotubes, and structural proteins start being expressed (e.g. myosin + actin)
4) Myotubes align and fuse together to form multinucleated muscle fibres.
Why is muscle development biphasic?
After the initial formation of large primary muscle fibres, smaller secondary muscle fibres then form on their surfaces which make up the bulk of the muscle
What are Satellite cells?
third group of muscle cells (primary and secondary fibres, satellite cells)
-muscle stem cells which sit dormant on muscle fibres until they are activated in the case of regeneration and postnatal growth where they can start dividing and forming myotubes which then fuse to form the muscle fibres
What is the Embryonic muscle fibre number?
at the end of embryogenesis, the number of muscle fibres that you have are what you have for the rest of your life (muscle fibre number is therefore set from birth and this is genetically determined)
-HOWEVER can be manipulated during embryogenesis
What affects fibre number during embryogenesis?
Although fibre number is genetically determined, it can be affected by:
- temperature
- hormones
- nutrition
- innervation
These affect myogenic regulatory factor (MRF) expression e.g. My5, MyoD and Myogenin
Fibre number is increased by…
hyperplasia
Fibres increase in mass by…
hypertrophy (increasing muscle mass postnatally)
What is Postnatal Hypertrophy (increase in skeletal muscle mass)?
during growth, muscle fibres require more protein and therefore muscle stem cells (satellite cells) start dividing and making myotubes which fuse to make muscle fibres, producing more structural proteins
-increases cross-section and size of the muscle fibre
Why are muscle fibres multinucleated?
because muscle fibre requires a lot of mitochondria and structural proteins along the length of the muscle fibre (very long cell), therefore many nuclei are needed to produce the structural proteins for the proper functioning of the muscle fibre
What is Postnatal Hyperplasia (increase in muscle fibre number)?
Evidence it happens in animals:
Avian Stretch Model
- weight put on a quale wing and there was muscle development on its back
- quale was euthanised and muscle fibres counted, showing an increase in muscle fibre
Cat Weight Lifting Model
- cats trained to flex paw and lift weight for the release of some food
- as a result, an increase in fibre number in paw required to lift the weight
Lacking evidence it happens in humans, however 2 proposed mechanisms:
>muscle fibre splits and there is hypertrophy causing an increase in muscle mass
>proliferation of satellite cells which then form their own muscle fibres
Describe the Variability between muscles.
all sarcomere structure is the same, however there is a lot of molecular variability between muscles due to multiple isoforms of myofibrillar proteins produced by alternative splicing or promoters
Different myosin isoforms have…
different chemomechanical transduction and ATP hydrolysis, conveying a more rapid/speed of contraction
What are the Different Titin isoforms?
convey different elastic properties to the sarcomere
What are the Different Troponin and Tropomyosin isoforms?
different sensitivies to Ca2+ released from the sarcoplasmic reticulum
Which isoforms contribute to resistance to fatigue?
Myosin and Troponin isoforms
What are the different Types of Muscle Fibre?
Type I Muscle Fibres (slow twitch)
Type II Muscle Fibre (fast twitch)
