18. Segmentation Flashcards
(22 cards)
What is segmentation?
Partitioning of structures into repeating units
How do segments differ in simple organisms versus vertebrates?
In simple organisms, segments are identical with no specialization; in vertebrates, segments become specialized with different identities.
What are somites?
Transient developmental structures that form segments
What is the presomitic mesoderm (PSM)?
Unsegmented tissue that precedes somite formation
What do somites ultimately form?
- Vertebrae
- Ribs
- Dermis
- Body wall
- Skeletal muscle
What structures flank the neural tube during development?
Somites
What are the three mesodermal regions mentioned?
- Intermediate mesoderm (forms kidney and gonads)
- Lateral plate mesoderm (forms circulatory system, heart, body cavity)
Which signals do newly formed somites receive?
- Wnt signals from ectoderm
- Sonic hedgehog from notochord
- BMP and FGF from lateral plate
What is the function of the sclerotome?
Formed from ventral cells undergoing epithelial-to-mesenchymal transition
What is the segmentation clock?
Periodic oscillations of gene expression in presomitic mesoderm
How does the timing of the segmentation clock vary across species?
- 90 minutes in chick
- 2.5 hours in mouse
- Faster in zebrafish, slower in humans
What pathways oscillate in the segmentation clock?
- Notch pathway components
- Wnt pathway components
- FGF pathway components
What are the gradients of signaling molecules in the wavefront?
- FGF and Wnt gradients (high posterior, low anterior)
- Retinoic acid gradient (high anterior, low posterior)
Where does segmentation occur in relation to the clock and wavefront?
Where the oscillatory wave meets the wavefront
What does Notch signaling mediate in segmentation?
Cell-cell communication
What does Mesp2 activate in the posterior presomitic mesoderm?
Eph/ephrin signaling
What determines segmental identity?
Hox genes
What happens when PSM is inverted in grafting experiments?
Results in reverse order of segments
What are the clinical implications of defects in somitogenesis?
- Congenital scoliosis
- Congenital vertebral defects
- Spondylocostal dysostosis (SCDO) syndromes
What modern techniques allow visualization of oscillations?
- Transgenic reporters
- Ex vivo culture of PSM tissue
- 2D cell cultures
- Organoid models
True or False: The mechanisms of segmentation are the same across all vertebrates.
True
Fill in the blank: The process of segment formation is known as _______.
Somitogenesis
