Summary of "Embryology: Somitogenesis: Turning Paraxial Mesoderm into Dermatome, Myotome, and Sclerotome"
Summary of Embryology: Somitogenesis: Turning Paraxial Mesoderm into Dermatome, Myotome, and Sclerotome
Main Ideas and Concepts
1. Mesoderm Differentiation and BMP Signaling
- The mesoderm differentiates into three main parts:
- Paraxial mesoderm (closest to midline)
- Intermediate mesoderm
- Lateral plate mesoderm (farthest lateral)
- BMP (Bone Morphogenetic Protein) gradients influence this differentiation:
- Paraxial mesoderm exposed to low BMP levels
- Intermediate mesoderm exposed to medium BMP levels
- Lateral plate mesoderm exposed to high BMP levels without inhibition (e.g., by noggin)
2. Lateral Plate Mesoderm Development
- Splits into two layers forming a cavity called the intraembryonic coelom:
- Somatic (parietal) lateral plate mesoderm: forms limb buds, lateral and ventral body wall, and part of the heart sac.
- Splanchnic (visceral) lateral plate mesoderm: forms the gut tube mesentery and contributes to the gut and respiratory tract.
- The cavity formed eventually becomes the pericardial, pleural, and peritoneal cavities (the “three P’s“).
- Communication occurs between intraembryonic and extraembryonic coeloms during embryonic folding.
3. Cardiogenic Mesoderm and Heart Formation
- A third wave of epiblast cells migrates lateral to the primitive streak to form the cardiogenic mesoderm (cardiac crescent).
- Cells passing through different parts of the primitive streak form different heart components:
- Cranial streak: great vessels (aorta, pulmonary trunk, vena cava)
- Middle streak: ventricles
- Caudal streak: atria
- A fourth wave forms the septum transversum, precursor to the diaphragm and liver.
4. Somitogenesis (Formation of Somites)
- Paraxial mesoderm segments into epithelial blocks called somites, starting around day 19 in the cervical region.
- Initial clumps in cranial regions are called somitomeres (less segmented).
- Somites form sequentially from cranial to caudal, about 3 pairs per day.
- The wave front model explains somite formation via gradients of retinoic acid (RA), FGF, and Wnt signaling:
- RA turns off FGF and Wnt in the presomitic mesoderm, allowing somite formation.
5. Somite Differentiation into Three Key Components
-
Sclerotome (ventromedial part):
- Induced by Sonic Hedgehog (Shh) and noggin signaling.
- Expresses Pax1 and Pax9 genes.
- Cells become mesenchymal again (“secondary mesenchymal tissue”).
- Gives rise to vertebrae, ribs, dura mater, and meninges.
- Sclerotome cells migrate around the neural tube and notochord.
- Vertebrae form from two adjacent sclerotomes.
-
Dermatome and Myotome (dorsolateral part):
- Wnt signaling activates Pax3, Pax7, and Paraxis genes at the ends of the dorsolateral somite.
- Mesenchymal cells migrate beneath epithelial cells forming:
- Dermatome: forms dorsal dermis of the trunk.
- Myotome: forms skeletal muscles of the axial skeleton and limbs.
- Dermatome remains epithelial; myotome becomes mesenchymal.
-
Somitocoel cells (mesenchymal cells in the somite center):
- Also called “somitocoele cells” or “arthrotome” cells.
- Contribute to the annulus fibrosis of intervertebral discs and articular surfaces of zygapophyseal (facet) joints.
- Some contribute to the nucleus pulposus along with notochord cells.
6. Somite Contributions to Adult Structures
- Somites give rise to:
- Axial skeleton (vertebrae, ribs, part of the skull)
- Muscles of the axial skeleton (intrinsic back muscles, limb muscles)
- Most of the dermis of the back (dorsal dermis)
- Meninges (dura, arachnoid, pia mater)
- Blood vessels (via angiogenesis)
- Other cell types including adipocytes, fibroblasts, myocytes, endothelial cells, and nervous system cells
7. Additional Notes
- The lateral plate mesoderm forms the limb buds and lateral/ventral body wall dermis.
- The septum transversum forms the diaphragm and contributes to liver development.
- Embryonic folding is critical for cavity formation and organ positioning.
- There is inconsistency in terminology across embryology texts (e.g., somatic vs. parietal mesoderm).
Methodology / Key Processes
Mesoderm Differentiation
- Exposure to BMP gradient leads to specification into paraxial, intermediate, and lateral plate mesoderm.
- Lateral plate mesoderm splits into somatic and splanchnic layers, forming the intraembryonic coelom.
Somite Formation (Somitogenesis)
- Around day 19, paraxial mesoderm in the cervical region undergoes mesenchymal-to-epithelial transition.
- Somitomeres form in cranial regions (less segmented).
- Somites form caudally from cranial to caudal direction at a rate of approximately 3 pairs per day.
- Retinoic acid produced by formed somites inhibits FGF and Wnt in presomitic mesoderm, allowing new somite formation (Wave front model).
Somite Differentiation
- Central somite cells become mesenchymal somitocoel cells, contributing to annulus fibrosis and joint structures.
- Ventromedial somite cells induced by Shh and noggin express Pax1/Pax9 and form the sclerotome, which gives rise to vertebrae, ribs, and meninges.
- Dorsolateral somite cells activated by Wnt express Pax3, Pax7, and Paraxis, forming the dermomyotome, which differentiates into dermatome (dorsal dermis) and myotome (skeletal muscles).
Sclerotome Migration and Differentiation
- Cells migrate around the neural tube and notochord.
- Produce proteoglycans (chondroitin sulfate) to support chondrocytes and osteoblasts.
- Two adjacent sclerotomes contribute to one vertebral body.
Myotome Differentiation
- Medial myotome forms intrinsic back muscles (extensors).
- Lateral myotome forms ventrolateral body wall muscles and limb flexors.
Dermatome Contribution
- Forms dorsal dermis of the trunk.
- Lateral plate mesoderm forms lateral/ventral dermis and limb dermis.
Important Terminology and Models
- BMP (Bone Morphogenetic Protein): Key morphogen in mesoderm patterning.
- Somitogenesis / Segmentation / Epithelialization: Formation of somites from paraxial mesoderm.
- Wave Front Model: Explains somite formation via gradients of retinoic acid and FGF/Wnt.
- Sclerotome, Dermatome, Myotome: Three somite derivatives.
- Somitocoel Cells: Mesenchymal cells in somite center contributing to annulus fibrosis and joints.
- Cardiogenic Mesoderm / Cardiac Crescent: Third wave of epiblast cells forming heart structures.
- Septum Transversum: Fourth wave forming diaphragm and liver precursor.
Speakers / Sources Featured
- Primary Speaker: Unnamed embryology lecturer (likely a professor or instructor providing a detailed embryology lecture for a chiropractic or medical course).
- Textbook References:
- Langman’s Medical Embryology (referred to as “Langman”)
- Larson’s Embryology (referred to as “Larson”)
- Carlson’s Embryology (referenced for detailed somite research)
- Other Mentions:
- Dr. Gillard (referenced for embryonic folding video)
- Various embryology authors (not named individually) with noted inconsistencies in terminology
Overall, this lecture provides an in-depth explanation of how the paraxial mesoderm forms somites, and how these somites differentiate into the sclerotome, dermatome, and myotome, ultimately giving rise to key musculoskeletal and connective tissues. It also covers lateral plate mesoderm differentiation, heart development, and the importance of signaling molecules like BMP, Shh, noggin, retinoic acid, FGF, and Wnt in embryonic patterning.
Category
Educational
