Concept explainers
To review:
Considering the fact that actin-myosin contractile forces play crucial role in cell-to-cell adhesion molecules and “differential interfacial tension hypothesis” proposed that cell cortex contractility governs cell sorting more than cell-to-cell adhesion. For future research perspective, a better understanding of the role biophysical properties plays in mechanisms of morphogenesis.
Introduction:
Cadherins are calcium-dependent adhesion molecules, these are transmembrane proteins, which interact with other cadherin molecules to maintain intercellular connections, and they are important for the spatial segregation of cell types and to the organization of animal form. It assembles actin cytoskeleton providing mechanical forces for sheet and tube formation. A theory named “Differential Interfacial Tension Hypothesis” states for the self-rearrangement of embryonic cells and tissues, which includes sorting, mixing, and formation of patterns in heterotypic aggregates of embryonic cells, and total or partial engulfment, separation, and dissociation of tissues. This theory explains action of all currently known cytoskeletal components and cell adhesion mechanisms.
Explanation of Solution
Scientist Steinberg proposed the differential adhesion hypothesis (DAH), postulating that each type of embryonic tissue has a unique “tissue surface tension” (TST) that decides how tissues sort, just as fluid surface tension predicts how immiscible liquids demix, and (ii) differences in TST arise from differences in cell-cell adhesion. Because interactions between cells are often difficult to probe inside an embryo, investigations of the DAH have largely been carried out in vitro. By measuring forces applied to cellular aggregates in vitro, one can extract a surface tension like quantity and demonstrate that “effective surface tension” accurately predicts sorting behavior.
Other postulate is mechanical polarization in cell doublets. The cells present at tissue boundaries are different from those of interior suggesting that boundary cells possess mechanical properties. In the beginning, two cells come into contact and adhere through the external domains of cadherin molecules. The internal domains of cadherins interact with actomyosin, which leads to the reorganization of actin cortical network in a mature cell-cell contact. This further leads to up-regulation of cortical tension beside the external boundary and/or down-regulation along internal interfaces. It is postulated that an upstream extracellular biochemical signals, trigger this cascade.
Signaling pathways upstream of this reorganization may provide the tight regulation essential for proper embryonic development.
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Chapter 4 Solutions
Developmental Biology
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