Somitogenesis Flashcards
What is the axial musculoskeletal system?
- includes the vertebrae and ribs, the muscles attached to them and dermis overlying them
- during development it also includes the muscles of the limb but not the bones
- all of these tissues come from the embryonic somite
what three main components do somites form?
- axial skeleton
- skeletal muscle
- dermis
what are somites and where are they located?
- somites are segments of tissue along the body axis
- located on either side of the neural tube
What is somitogenesis?
- the formation of somites
What is the name of the mesenchymal tissue that forms somites?
- pre-somatic mesoderm (PSM) forms somites
what happens at the cranial end of the PSM and the caudal end?
- at the cranial end of the PSM somites from
- at the caudal end of the pre-somatic mesoderm cells continue to divide and lengthen the region
How many somites does it take to form one singular vertebra?
- one pair of somites = 1 vertebra
How long does it take for one pair of somites to segment from the pre-somatic mesoderm (in chicks)?
- 90 mins providing the temperature conditions are optimal
Segmentation of the PSM is dependent on a molecular clock, what is the name of the molecular clock?
- time measured on the molecular clock - a gene called hairy
- e.g., ach cycle if hairy expression lats 90 mins so a somite forms every 90 mins in a chick
What happens at the cranial end of the PSM in order for somites to from?
- cells at the cranial end of the pre-somatic mesoderm undergo a mesenchymal to epithelial transition to form somites
Where are the most developed somites found?
- most developed somites found at the cranial end of the PSM
what 3 tissues can somites from?
- sclerotome
- myotome
- dermatome
What part of the body is formed from the sclerotome?
- the axial skeleton (vertebrae and ribs)
What must be formed from the somite before the myotome and dermatome can form?
- the dermatomyotome
What is formed from the myotome and dermatome?
- myotome = forms the skeletal muscle in the body and limbs
- dermatome = forms the dermis
How and where is the epithelial ball (the somite) formed?
- the PSM undergoes a mesenchymal to epithelial transition in caudal immature somites
How and where is the sclerotome formed?
- In maturing cranial somites, the ventromedial portion of the somites undergoes and epithelial to mesenchymal transition to form sclerotome
In the maturing somite, after sclerotome formation, the dorsal aspect remains epithelial and is called the dermomyotome - what do the dermomyotomal cells the go on to from?
- dermomyotomal cells go on to form the myotome
Can myotomal cells divide?
- myotomal cells can NEVER divide
- cells continually recruited from the dermomyotome
How is the dermatome formed?
- once all of the myotomal cells have ingressed from the dermomyotome, the cells remaining in the dermomyotome become mesenchymal tissue - the dermatome
Describe re-segmentation?
- Sclerotome re-segments. Fissure develops in middle of sclerotome (A-P).
- Posterior half of one half of somite fuses with anterior half of adjacent sclerotome.
- Myotome does not re-segment. Therefore, muscle binds adjacent segments.
- Sclerotome encases neural tube and sclerotome differentiates first into cartilage and then bone. Forms vertebrae.