Somitogenesis Flashcards
(34 cards)
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
Summarise what happens to the myotome cells
What else do the myotome cells form?
Myotome is divided into 2 parts: a dorsal and ventral portion called the epimere and hypomere
Epimere will form the extensors of vertebral column
Hypomere will form the flexors
- Thorax - myoblasts form 3 intercostals (external, internal, innermost)
- Abdomen - myoblasts form external, internal, transversus abdominis
What is the epimere and hypomere innervated by?
Epimere - dorsal ramus of spinal nerve
Hypomere - ventral ramus of spinal nerve
Describe the migration of the myoblast precursors into the limb
Limb buds emerge at the flank
Signals are released from the limbs that attract muscle cell precursors cells from the hypomere into the limb
Myoblasts vells divide into a dorsal and ventral muscle mass
Dorsal = extensors
Ventral = flexors
Myoblasts precursors are attracted to the limb bud by scatter factors (HGF) by mesoderm in limb bud. Hypomere attract express C-met (RTK receptor)
Nerves then grow form the neural tube into muscles
What does the dermatomes form?
Dorsal dermis - migrates over the surface of the embryo
What is the dermis innervated by?
• The stripes of dermis are then innervated by a single spinal ganglion. This is maintained along the anterior-cranial caudal axis
On a separate note, how is differentiation of the somite controlled?
- The ventral portions of the neural tube and the notochord will secrete Shh which acts upon the somite to cause the expression of a transcription factor called Pax 1 which marks sclertomal cells in the ventral and medial parts of the somites
- There are signals that are released from the dorsal neural tube called Wnt. This causes the adjacent dermomyotome to differentiate towards an epimere fate and will express the transcription factor Myf5
- The lateral plate of the mesoderm will secrete BMP4 along with Wnt signals from the dorsal part of the body wall. This will induce the expression of a transcription factor in the hypomere called MyoD.
- NT-3 will induce the expression of Pax 3 which marks the dermatome compartment.
Describe the segmentation of the sclerotome
Sclerotome has a cranial and caudal portion
Segmental arteries lie between the somites
Caudal part of the first somite will fuse with the cranial part of adjacent somite to form the centrum
Allows muscles to span over adjacent vertebrae to permit movement between vertebrae
Spinal nerves pass through the somites to run through intervertebral foramen
Where does the syndetome lie?
What does it express?
Where is it found
Syndetome lies at the cranial end of the somite
Scx TF
At the block of each myotome territory
Permits the attachment of the muscles to the transverse process of each vertebrae
What gives each somite a unique axial identity?
Expression of genes within PSM
Genes are expressed in the PSM that specify the axial position of the somite
