Biomechanics and the thermotolerance of development

Successful completion of development requires coordination of patterning events with morphogenetic movements. Environmental variability challenges this coordination. For example, developing organisms encounter varying environmental temperatures that can strongly influence developmental rates. We hypothesized that the mechanics of morphogenesis would have to be finely adjusted to allow for normal morphogenesis across a wide range of developmental rates. We formulated our hypothesis as a simple model incorporating time-dependent application of force to a viscoelastic tissue. This model suggested that the capacity to maintain normal morphogenesis across a range of temperatures would depend on how both tissue viscoelasticity and the forces that drive deformation vary with temperature. To test this model we investigated how the mechanical behavior of embryonic tissue (Xenopus laevis) changed with temperature; we used a combination of micropipette aspiration to measure viscoelasticity, electrically induced contractions to measure cellular force generation, and confocal microscopy to measure endogenous contractility. Contrary to expectations, the viscoelasticity of the tissues and peak contractile tension proved invariant with temperature even as rates of force generation and gastrulation movements varied three-fold. Furthermore, the relative rates of different gastrulation movements varied with temperature: the speed of blastopore closure increased more slowly with temperature than the speed of the dorsal-to-ventral progression of involution. The changes in the relative rates of different tissue movements can be explained by the viscoelastic deformation model given observed viscoelastic properties, but only if morphogenetic forces increase slowly rather than all at once. © 2014 von Dassow et al

Surprisingly Simple Mechanical Behavior of a Complex Embryonic Tissue

Background: Previous studies suggest that mechanical feedback could coordinate morphogenetic events in embryos. Furthermore, embryonic tissues have complex structure and composition and undergo large deformations during morphogenesis. Hence we expect highly non-linear and loading-rate dependent tissue mechanical properties in embryos. Methodology/Principal Findings: We used micro-aspiration to test whether a simple linear viscoelastic model was sufficient to describe the mechanical behavior of gastrula stage Xenopus laevis embryonic tissue in vivo. We tested whether these embryonic tissues change their mechanical properties in response to mechanical stimuli but found no evidence of changes in the viscoelastic properties of the tissue in response to stress or stress application rate. We used this model to test hypotheses about the pattern of force generation during electrically induced tissue contractions. The dependence of contractions on suction pressure was most consistent with apical tension, and was inconsistent with isotropic contraction. Finally, stiffer clutches generated stronger contractions, suggesting that force generation and stiffness may be coupled in the embryo. Conclusions/Significance: The mechanical behavior of a complex, active embryonic tissue can be surprisingly well described by a simple linear viscoelastic model with power law creep compliance, even at high deformations. We found no evidence of mechanical feedback in this system. Together these results show that very simple mechanical models can be useful in describing embryo mechanics. © 2010 von Dassow et al

Effects of Ambient Flow and Injury on the Morphology of a Fluid Transport System in a Bryozoan

Volume: 208Start Page: 47End Page: 5

Feeding the Masses: Colonial Transport in the Marine Bryozoan Membranipora membranacea

The transport system of cheilostome bryozoans is unusual among long-distance transport systems. In these colonial animals, a network of strands (the funicular system) carries nutrients to non-feeding individuals and to the growing edge of the colony. However a complex of cells appears to plug the pores that connect individuals. Focusing on the cheilostome, Membranipora membranacea, we used time lapse movies to test whether there were contractions/dilations of funicular strands, as expected if muscular pumping were to move material through the strands, and to test whether cells or large vesicles moved directionally along the strands, potentially carrying nutrients. Neither contractions/dilations of the funicular strands, nor persistent movement of particles or other features along the strands, were visible in time lapse videos (10 to 120 min at 4 to 10 sec per frame). The only visible movements were rare back-and-forth movements along the strands, or shaking of the strands. We injected materials that differed in molecular/particle size to investigate the specificity of transport at pore plates. Both fluorescein (367 Da; as sodium salt) and fluorescein-dextran (70,000 Da) moved between individuals; however 2.0μm fluorescent polystyrene beads did not. The fact that both fluorescein and fluorescein-dextran were transported suggests that transmembrane channel or transporter proteins are not required for transport; however there may be an upper size limit (<2μm) caused by something other than the pore itself. Our results are consistent with some transport mechanisms (e.g. paracellular diffusion or transcytosis at the pore plate) but inconsistent with others (muscularly-pumped flow along funicular strands, cell crawling, or transmembrane transport via transporter or channel proteins)

Feeding the Masses: Colonial Transport in the Marine Bryozoan Membranipora membranacea

The transport system of cheilostome bryozoans is unusual among long-distance transport systems. In these colonial animals, a network of strands (the funicular system) carries nutrients to non-feeding individuals and to the growing edge of the colony. However a complex of cells appears to plug the pores that connect individuals. Focusing on the cheilostome, Membranipora membranacea, we used time lapse movies to test whether there were contractions/dilations of funicular strands, as expected if muscular pumping were to move material through the strands, and to test whether cells or large vesicles moved directionally along the strands, potentially carrying nutrients. Neither contractions/dilations of the funicular strands, nor persistent movement of particles or other features along the strands, were visible in time lapse videos (10 to 120 min at 4 to 10 sec per frame). The only visible movements were rare back-and-forth movements along the strands, or shaking of the strands. We injected materials that differed in molecular/particle size to investigate the specificity of transport at pore plates. Both fluorescein (367 Da; as sodium salt) and fluorescein-dextran (70,000 Da) moved between individuals; however 2.0μm fluorescent polystyrene beads did not. The fact that both fluorescein and fluorescein-dextran were transported suggests that transmembrane channel or transporter proteins are not required for transport; however there may be an upper size limit (<2μm) caused by something other than the pore itself. Our results are consistent with some transport mechanisms (e.g. paracellular diffusion or transcytosis at the pore plate) but inconsistent with others (muscularly-pumped flow along funicular strands, cell crawling, or transmembrane transport via transporter or channel proteins)

L (2010) Surprisingly simple mechanical behavior of a complex embryonic tissue

Abstract Background: Previous studies suggest that mechanical feedback could coordinate morphogenetic events in embryos. Furthermore, embryonic tissues have complex structure and composition and undergo large deformations during morphogenesis. Hence we expect highly non-linear and loading-rate dependent tissue mechanical properties in embryos

Duration of actomyosin contractions depends on temperature.

<p>(A) Sequential frames from a representative time-lapse sequence collected from the basal cortex of an animal cap ectoderm explant cultured on fibronectin-coated glass substrate. F-actin dynamics are revealed in cells expressing the actin-binding domain from moesin coupled to EGFP (moe-GFP) (left column). This sample collected at 16°C. (A′) Schematic of frames matching those in (A) highlighting the cell outline (dotted line) and hexagonal regions of the cell cortex identified as “F-actin contractions.” Regions are categorized as contractions when their integrated intensities are 50% greater than the mean intensity of the basal cell cortex. (B) Duration of individual F-actin contractions across the three temperature regimes. (C) Frequency distribution of the duration F-actin contractions at three temperatures. Note abundant short duration contractions at the low temperature regime.</p

Temperature dependence of compliance and strength of induced contraction.

<p>(A) Representative kymographs of microaspiration with electrically induced contractions at 900 seconds at 16°C (upper) and 26°C (lower). (B) Fit of power law viscoelastic model to the aspirated length from 600 to 900 s for the 26°C case. (C) Flow chart for analysis of contractions. (D) Contraction analysis. ‘X's indicate half-max, max, and return to half-max for each curve. Panels B and D show data from the lower embryo in A. Arrowheads in A and B indicate electrical stimuli. (E) β, (F) maximum displacement during induced contraction, (G) duration (half-maximum until return to half-maximum displacement) of contraction, (H) compliance at 1 s (triangles) and at 300 s (circles), (I) maximum apical tension during induced contraction, (J) duration of apical tension. Triangles and circles: individual embryos; X's: means.</p

Comparison of viscoelastic models of morphogenesis for ramped versus stepped forces.

<p>(A) Diagram of model. Summed contractions (wavy lines) average out to stepped or ramped stresses (σ) depending on when cells begin contracting. When applied to the viscoelastic material with compliance J[t], the deformations (strains, ε) follow the time course of ramped forces more closely than stepped force. This can be visualized as adding up strains due to a series of stepped forces applied over time (dotted lines on right). (B) Predictions for R_CP, the ratio of the time for morphogenesis (blastopore closure) to the time for patterning (D-V progression of involution), as a function of the time for patterning at temperature T, normalized to the time for patterning at 16°C, for ramped <i>vs.</i> stepped models for different values of β. Yellow dots: grand mean of experimentally observed values. The curves automatically converge to the right hand dot (at 16°C) where T₂ = T₁ since t_C at T₁ is used to calculate R_CP at T₂. (C) Comparison of the observed R_CP at 26°C to the predictions for models with ramped or stepped forces, and with temperature invariant or varying mechanical properties (inset: prediction for stepped force model with temperature dependent mechanical properties on a log scale.) Error bars indicate confidence intervals. (D) Histogram of bootstrap resampling estimates of R_CP at 26°C for each model (10,000 resamples total).</p

Blastopore closure at high and low temperatures.

<p>(A) Upper: vegetal view of an embryo showing the blastopore soon after the start of dorsal superficial involution. Lower left: kymograph of blastopore closure at 26°C, taken along the yellow line from the dorsal side to the ventral side, showing the points when dorsal (DI) and ventral (VI) superficial involution begin, and when the blastopore closes (BC). Right: kymograph taken along a line from the dorsal to the ventral side at 16°C. (B) The ratio (R_CP) of the time for blastopore closure to the time for dorsal-to-ventral progression of involution versus the time (t_P) for dorsal-to-ventral progression of involution. Dots indicate individual embryos. X's indicate medians for clutches (4 to 8 embryos each).</p