Muscle Ebryology Flashcards
What is gastrulation?
-Process of cell division and migration resulting in formation of 3 germ layers.
How does gastrulation occur?
-Cells migrate towards raised primitive streak and push downwards into epiblast layer, then pushes hypoblast out of the way to form 3 layers.
What is the mesoderm important for?
-It is the building block of all muscle
What distinct regions does the mesoderm divide into?
- Paraxial mesoderm
- Intermediate mesoderm
- Lateral plate mesoderm
- Extraembryonic mesoderm
When does the mesoderm differentiate?
-Between days 17 and 21
What is the notochord and its function?
-Area in the middle with thickened cells that is a signalling centre that controls specification of surrounding cells.
Why do the oropharyngeal and cloacal membrane have no mesoderm layer?
-They will break down to form the mouth and the anus.
What does the paraxial mesoderm form?
-Somites in the embryo
How does the paraxial mesoderm form and where does it lie?
- Forms from cells moving bilaterally and cranially from the primitive streak.
- Lies adjacent to notochord and neural tube
What does the intermediate mesoderm form?
-Genitourinary system
What 2 layers does the lateral plate mesoderm divide into?
- Somatic or parietal layer
- Splanchnic or visceral layer
What cavity splits the 2 lateral plate mesoderm layers?
-Intraembryonic coelom
What area of the mesoderm forms skeletal muscle?
-Paraxial mesoderm
What area of mesoderm forms smooth muscle (gut and derivatives)?
-Visceral layer, lateral plate mesoderm around the gut tube
What area of mesoderm will form smooth muscle (pupils, mammary and sweat glands)?
- Not mesoderm
- Ectoderm
What area of mesoderm will form cardiac muscle?
-Visceral layer, lateral plate mesoderm around heart tube.
What does the paraxial mesoderm get organised into?
-Somites
Where and when do somites form?
-Form along side the developing neural tube in a craniocaudal sequence over time from day 20
What rate do somites appear and why can this be useful?
- Appear at approx. 3 pairs a day until the end of week 5.
- Useful as can be used to accurately determine age of the embryo by counting no. of pairs.
What transition does mesenchymal cells undergo during somite formation?
-Mesenchumal to epithelial transition
What is the unsegmented mesoderm called?
-The pre somitic mesoderm
What happens to the pre somitic mesoderm before it segments?
-It gets patterned and many molecular factors are involved in this process. Including the notochord.
What are some of the molecules that are involved in somite formation?
- FGF
- Wnt
- Notch
Describe the clock and wave mechanism to regulate somite formation?
- Genes tell cells to switch between a permissive and non-permissive state in a constantly timed fashion (notch).
- A wave of factors sweeps along the length of the embryo and interacts with the cells that are permissive at the right time in the right area. (FGF 8)
- The process is helped along by Wnt.
- In summary, overlapping gradients control somite formation.
By then end of week 5 around how many somites are formed and what will these go onto form?
- By the ned of week 5, 42-44 pairs are present.
- These will go onto form the axial skeleton
What happens to somites when they have formed?
-They start to differentiate
How does somite differentiation occur?
- By start of week 4, cells in the ventral and medial wall of somites start to lose epithelial characteristics and become mesenchymal like again.
- This portion is calles the sclerotome.
- Cells at the dorsal half form the dermomyotome.
What does the sclerotome go onto form?
-Ribs and vertebrae
What happens to the dermomyotome?
- It splits again to form the: -dermatome
- myotome
What does the dermatome go onto form?
-Dermis of the back
What does the myotome go onto form?
-Muscles
What are myoblasts?
-Muscle cell precursors
What are myocytes and how would they form?
-Are mature muscle cells formed from myoblasts
How do myoblasts differentiate?
-Myoblasts undergo cell division under the influence of growth factors.
What happens to myoblasts when there is depletion of growth factors?
- they stop dividing
- Secrete formation onto ECM and bind to it via integrin crucial step
How do myoblasts differentiate into myotubes?
- Myoblasts align into chains and fuse, cell membranes disappear and mutinucleated myotubes from (primary myotubes).
- Myogenin mediates this differentiation.
What regulates the development of muscle?
-Genes and myogenic factors eg. MYOD, MYO5
What are MYOD and MYF5 and what do they do?
- Transcription factors
- Activate muscle specific genes
- Enable differentiation of myogenic precurosrs in the dermomyotome into myoblasts
- Are very powerful as they can convert non muscle cells into cells expressing all muscle proteins.
What happens at the neural tube that helps regualte muscle development?
-Wnt proteins (activate) and BMP proteins (inhibit) combine to activate MYOD in the dermomyotome; creating a group of muscle cell precursors, which express MYF5.
What happens at the notochord to regulate muscle development
-Sonic hedgehog and noggin induce sclerotome formation.
What happens at the lateral plate mesoderm that regulated muscle development?
-Wnt and BMP activate MYOD and MYF5
What does smooth muscle (apart from cilliary muscle, sphincter pupillae of eye (ectoderm)) originate from?
-Splanchnic mesoderm/visceral mesoderm
What is responsible for smooth muscle cell differentiation?
-Serum response factor (SRF)
What can enhance SRF activity?
- Upregulated by kinase phophorylation pathways
- Myocardin related transcription factors enhance SRF activity.
How is skeletal muscle formed and what controls this?
- Myoblasts fuse to form long multinucleated fibres (myotubes)
- Controlled by MYOD, MYF5, Myogenin
What are tendons derived from and what controls this?
-Derived from sclerotome and under control of TF scleraxis.
What does cardiac muscle originate from?
-Splanchnic/viseceral mesoderm surrounding the developing heart tube.
What genes are and arent involved in early cardiac muscle development?
- IS tinman
- Isnt MYOD
How do myoblasts adhere in cardiac muscle?
-Via intercalated discs
