Evolutionary Biology 8: Comparative development 3 Flashcards
How is body axis patterning established?
Early differentiating cells collectively called ‘Organiser’ region
Organiser cells generate signals that instruct the development of neighbouring cells
Triggers differentiation of correct cell types -> correct tissue types in correct spatial locations
What cell changes are important for gastrulation-related rearrangements? What are they all controlled by?
Cells can:
- change shape
- change adhesion between one another
- divide or stop dividing
- migrate
- undergo apoptosis
All controlled by changes in gene expression
What is ‘epithelium’ in embryos?
Epithelium:
Sheets or tubes of tightly connected cells
(connected via tight junctions, gap junctions, desmosomes) that are attached to a basement membrane
- No extracellular matrix between cells
What is ‘Mesenchyme’ in embryos?
Mesenchyme:
Cells that are physically separated from one another but are loosely connected by large amounts of extracellular matrix (which keeps cells physically separated)
Can cells transition between epithelial and mesenchymal states? How?
Yes - Through changes in cell adhesion and cell shape
Differential cell affinity is important why? How is the affinity mediated?
Differential cell affinity important for developmental process
-> changes affinity for extracellular interactions, allows changes in behaviour e.g. epithelial to mesenchyme
- Mediated by adhesion proteins; expression of different adhesions proteins can change drastically which governs interactions
How does cell migration arise? Differences in mesenchymal and epithelial migration?
Arises from combination of cell motility and guidance (along the write path)
Epithelial: Orchestrated by cells at leading edge, cells behind pulled passively
Mesenchymal: Cells migrate as individuals
Four steps of cell migration?
1) Cell polarises
- external signal causes shape change via cytoskeletal rearrangements. Leading + trailing edge is well defined
2) Cell extend leading edge
- filopodia or lamellopodia form from actin polymerisation
3) Leading edge forms adhesions with extracellular substrate (via Integrins)
- Contraction of actin filaments generates force to pull cell forward
4) As leading edge extends, stretch activated calcium channels open on trailing edge -> activates proteases which degrade adhesion points at trailing end -> cell move forward
What are integrins and what are their function?
Cell adhesion proteins used during migration
- act as receptors for extracellular matrix (ECM) proteins, weakly associate and connect to cytoskeleton
6 basic processes of gastrulation rearrangement
Ingression Invagination Involution Delamination Epiboly Convergent extension
Summarise ingression + steps
Cells migrate as mesenchyme into embryo interior:
- Epithelial to mesenchyme transition
- Cells lose adhesion to epithelial sheet + polarise
- Squeeze out of epithelium + migrate as individuals to interior
Summarise invagination
Epithelial sheet of cells folds inwards to form a pocket that points to interior of embryo
- often starting point for migration of cells (such as mesoderm) into embryo
Summarise involution
Multi-layered cell sheets begins to roll into interior of embryo
-> previously external cells fold in and become internal
Summarise delamination + steps
Similar to ingression: epithelial to mesenchymal transition
- cells arrange into new sheet once migrated
- Single continuous sheet splits into multiple sheets
- New sheet delaminates into interior of embryo
Marks gastrulation in humans + birds
Summarise Epiboly
Movement of epithelial sheets - spread as a unit to enclose the inner layers of the embryo
- Ectoderm often undergoes epiboly to cover whole surface of the embryo
