Somitogenesis Flashcards
What are somites?
• Somites are transient embryonic structures that form many structures
What do somites form?
- Somites form most of the bones in the trunk of the skeleton and bones on the base of the skull are formed from somites. The appendicular skeleton is not formed from somites
- Almost all of the muscles are derived from somites, both on the trunk and appendicular regions.
What do somites form from?
Pre-somatic mesoderm (PSM)
Describe the order in which somites form?
Cranial -> caudal
What do somites form in the body?
• Axial skeleton – somites o Vertebrae, ribs (not sternum) • Axial muscles – somites o Vertebral, thoracic, abdominal • Appendicular muscles – somites o Flexors, extensors • Appendicular skeleton – limb buds – not formed from somites • Limbs, digits, girdles
What are the divisions of the paraxial mesoderm which component forms the somites?
• The paraxial mesoderm is divided along the anterior-posterior axis. At the cranial end we have the head mesoderm, at the trunk is the somatic mesoderm.
What will the sclerotome form?
What will the myotome form?
What will the dermatome form?
Bone
Muscle
Dermis
What generates the PSM which will form those somites?
• The presomitic mesoderm (PSM) is generated by the cranial (rostral) primitive streak & the tail bud, involving proliferation, specification and emigration
How many somites will form?
• The total number of somites formed is also characteristic of the species, independent of variations in embryonic size (e.g. zebrafish 33 pairs, human 44, mouse 65)
What does the PSM form before the somite is formed?
What do they differentiate into?
• Initially paired epithelial spheres each side of neural tube
• Differentiate to:
o Dermomyotome and sclerotome (vertebrae)
o Myotome: epimere, hypomere, limb muscle
o Dermatome: dorsal dermis
o Syndetome: tendons
Describe the organisation of the sclerotome and how it forms
Epithelial somites are present on each side of the neural tube at day 22
The epithelial somites will undergo epithelial to mesenchymal transition such as in gastrulation. The mesenchymal cells will migrate (they are sclerotomal) and will surround them neural tube and notochord.
The sclerotome cells will continue to proliferate and migrate. They will eventually surround the neural tube and the notochord.
The cells will go on to form the vertebrae that surround the neural tube (spinal cord)
They initially develop cartilage which is then mineralised by a process called endochondral ossification.
They will begin to form the transverse processes that will divide the epimer from the hypomere, so the dermatomyotome will form the epimer at the top and hypomere at the bottom
Describe the fate of the sclerotome
Sclerotome will form the centrum that surrounds the neural tube and notochord
The Notochord will form the intervertebral disks
Neural arches will surround the neural tube - fails you get spina abfinida
Sclerotome will also form the costal processes such as the ribs
Sternum forms from somatic mesoderm not somites
What does removal of the spinal ganglia and notochord cause?
- If you remove the spinal ganglia, then the neural arches remain unsegmented so we know that the adjacent nerves are important in subdivision of the sclerotome
- If you remove the notochord then the centrum remains unsegmented
What does the myotome cells form and how are they divided?
- The myotome will form both the flexors and extensors of the trunk including the ventral wall muscles, distal ribs, dorsal and ventral muscles masses of limbs
- We mentioned that the transverse processes will grow outwards from the sclerotome and will divide muscles into epimer (dorsal) and hypomere (ventral).
Where do myotome cells also migrate?
Into limb buds
