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Epithelial Flow and Force-Balance Geometry Transformation

Core Concepts
Epithelial convergent extension results from the controlled transformation of internal force balance geometry.
Shape changes in epithelia during animal development are driven by mechanical activity. Questions remain about cell-scale mechanics and coordination mechanisms. A model-based analysis framework was developed to relate cell geometry to local tension. T1 rearrangements were decomposed into internally driven active and externally driven passive contributions. Germ band extension is found to be driven by active T1 processes through positive feedback acting on tensions. The study provides insights into the mechanics of epithelial tissue during morphogenesis.
During Drosophila gastrulation, the embryonic blastoderm undergoes dramatic deformation that changes tissue topology. Epithelial tissues are under internally generated tension, revealed by recoil in response to laser ablation.
"Epithelial convergent extension results from the controlled transformation of internal force balance geometry." "Germ band extension is found to be driven by active T1 processes through positive feedback acting on tensions."

Deeper Inquiries

How does myosin dynamics control cellular behavior on a molecular level?

Myosin dynamics play a crucial role in controlling cellular behavior at the molecular level. Myosin motors exert contractile forces on actin fibers in the cell cortex, leading to the generation of cortical tension. This tension is essential for maintaining the structural integrity of epithelial tissues and plays a key role in processes such as cell shape changes, cell rearrangements, and tissue morphogenesis. The recruitment of myosin is regulated by various developmental genes and signaling pathways, which influence its activity levels within cells. Additionally, myosin dynamics are subject to positive and negative mechanical feedback mechanisms that depend on factors like stress levels and rates of strain. For example, increased stress or rapid deformation can trigger higher levels of myosin activity through positive feedback loops. This dynamic regulation allows cells to respond effectively to mechanical cues from their environment and coordinate their behaviors during processes like convergent extension. In summary, myosin dynamics control cellular behavior by regulating cortical tension, driving cell movements and rearrangements, responding to mechanical signals in the tissue microenvironment, and playing a central role in coordinating morphogenetic processes at the molecular level.

How can we reconcile solid-like capacity with fluid-like behavior in epithelial monolayers?

Epithelial monolayers exhibit both solid-like capacity to support tension (as seen in recoil responses after laser ablation) and fluid-like behavior characterized by shape changes and internal rearrangements similar to those observed in fluids. This apparent contradiction can be reconciled by understanding the unique properties of epithelial tissues: Internal Tension: Epithelial tissues have an interconnected network structure maintained by intercellular adhesions that link neighboring cells' cytoskeletons. This network structure allows them to support internal tensions generated by myosin motors at cell interfaces. Adiabatic Deformation: Epithelial tissues operate under approximate mechanical equilibrium where tissue flow occurs through adiabatic remodeling based on force balance principles rather than viscous flow typical of fluids. Force Balance Geometry: Force balance constraints relate tensions at vertices with angles formed between interfaces; this geometric relationship provides insights into how tensions drive tissue deformations while maintaining stability. Tension–Isogonal Decomposition: By decomposing tissue deformations into contributions from changing cortical tensions (active) versus isogonal modes (passive), we can understand how both solid-like tension maintenance coexists with fluid-like shape changes driven by external forces or internal stresses. Therefore, reconciling these seemingly contradictory behaviors involves recognizing that epithelial monolayers possess unique structural characteristics that allow them to exhibit both solid- and fluid-like properties depending on context-specific requirements during morphogenetic processes.

What role do intercalations play in mechanically driving tissue deformations?

Intercalations are fundamental cellular behaviors involved in mechanically driving tissue deformations during processes like convergent extension. These intercalations involve coordinated movements where neighboring cells exchange positions through specific types of transitions known as T1 transitions. The key roles played by intercalations include: Local Rearrangement: Intercalation events lead to local rearrangement of cells within an epithelial sheet without significant change in overall cell number or size. 2Mechanical Coordination: Interactions between adjacent cells during intercalation help maintain mechanical coordination across tissues allowing for coherent global morphogenetic flows. 3Tissue Remodeling: By facilitating controlled movement patterns among neighboring cells via T1 transitions, 4Active vs Passive Forces: Intercalation events provide insights into distinguishing between active (internally driven) vs passive (externally driven) contributions towards tissue deformation based on tension analysis around individual interfaces Overall,intercalation plays a critical mechanistic role in shaping embryonic development through precise spatial coordination of cellular movements and contributing to the mechanical behavior of epithelia during tissue deformations such as convergent extension or gastrulation processes