BIO420

1. Paraxial Mesoderm

2. The Mesoderm
• The mesoderm lies between the
ectoderm and endoderm
• Its cells are divided into three parts:
• Paraxial
• Intermediate
• Lateral Plate
• The paraxial mesoderm is just
lateral to the the notochord
• It gives rise to the vertebrae,
skeletal muscles, and connective
tissue of the skin

3. Regions
• The regions of the trunk and head mesoderm are as follows:
• The central region of trunk mesoderm is the chordamesoderm (axial mesoderm). This
tissue makes the notochord and part of the vertebrate disks.
• The paraxial (somatic) mesoderm is on both sides of the notochord. This will eventually
produce muscle and connective tissue of the back (like parts of the vertebrae, muscles,
etc.)
• Part of the paraxial mesoderm also produce the head mesoderm- skeleton, muscle,
and connective tissue of the skull
• The intermediate mesoderm is lateral to the paraxial- urogenital system (kidneys, gonads)
• Lateral plate mesoderm- Most lateral. It will help form the heart, blood vessels, pelvis
and limb skeleton

4. Cell Types of the
Somite
• Somites are condensations of paraxial mesoderm. They
can be divided into dermomyotomes and sclerotomes
• Dermomyotomes are divided into:
• Dermatomes are the more superficial. They will
become the dermis of the skin of the back.
• Myotomes will from the muscles of the back,
rib cage, and ventral body wall
• Sclerotomes form the vertebrae and rib cage.

5. Establishing the Paraxial Mesoderm and Cell
Fates Along the Anterior-Posterior Axis
• Where the types of mesoderm are in relation to being medial or lateral depends on concentrations of BMP.
• Lateral areas have higher concentrations of BMP
• There is more Noggin towards the notochord. Noggin inhibits BMP
• As far as the anterior-posterior axis, there are many opposing morphogens.
• Towards the posterior is FGF8 and Wnt3a.
• On the anterior portion is Retinoic acid, which represses FGF8
• Hox genes determine what particular structures will form in’ each segment along the anterior-posterior axis.

6. Somitogenesis (somitomeres)
• Somitomeres are going to appear before somites
• The cranial paraxial mesoderm faintly clumps into block-like structures on each side of the
midline
• They appear sequentially starting from the cranial region
• The clumping process is called segmentation
• After the first forms, the next immediately forms caudally to it (until it reaches seven pairs)
• In segmentation, mesenchymal mesoderm is converted back to epithelial tissue

7. Somites
• After the first seven pairs of somitomeres form, the first pair of somites form
• Somites have a more pronounced segmentation.

8. How are Somites Built?
• Mesodermal cella at the caudal end make high levels of FGF8 and Wnt (these inhibit
somite formation)
• These genes need to be shut off in the presomitic tissue
• How to turn off FGF8 and Wnt? The most recent pair of somites will make retinoic acid
and release it into the presomatic tissue. This turns FGF8 and Wnt off

9. Clock-Wavefront Model
• Retinoic Acid is high in the segmenting cells while FGF and Wnt are high in the
proliferating cells.
• As the new somites are formed sequentially (and caudally), it is live a wave.
• Notch gene also has to get turned on in the presomitic tissue
• Notch produces two transcription factors which cause somite development:
• Lunatic fringe
• C-Hairy
• A barrier needs to form between each somite. For this they use Ephrin B

10. Sclerotome
Development
• This is the first large division of the somite
• When the somite is formed, two signaling
molecules form the notochord and neural
tube
• Shh and Noggin
• Shh and Noggin will turn on Pax1 and Pax9
• Pax 1 and Pax 9 will convert the epithelial
tissue back into mesenchymal tissue (it was
previous converted into epithelial tissue)
• They are now sclerotome cells which will go
on to make the vertebrae and ribs

11. Dermomyotome
• Sclerotomes are made with the medial ½ of the somite
• Dermomyotomes are made with the dorsolateral ½ of the somite
• The neural tube and ectoderm secrete Wnt
• Wnt turns on Pax3 and Pax 7
• Pax 3 and Pax 7 transform epithelial mesoderm to mesenchymal cells (except for the tissue in the
center)
• This new mesenchymal + epithelial center is now dermomyotome