= Solution
\b[Polarity proteins generate tissue form by controlling where cells build surfaces, place junctions and divide.] Their morphogenetic roles can be separated experimentally from a wholesale loss of epithelial identity. Three examples illustrate different outputs: positioning a single lumen, orienting neighbour exchange, and limiting tube dimensions.
The first example is <Cdc42>-dependent <lumen formation> in three-dimensional <Caco-2> epithelial cysts. Cells in a normal cyst coordinate their <apical-basal polarity> so that apical surfaces face one central lumen. During growth, a correctly oriented <spindle apparatus> places daughter cells into the epithelial wall rather than establishing inappropriate internal cell arrangements. <Cdc42> depletion disrupts spindle orientation and produces abnormal, often multiple, luminal spaces. Importantly, the cells can still establish apical membrane identity: in this model the crucial defect is the coordination and positioning of apical surfaces during division, rather than an absolute inability to make any apical surface. This distinguishes tissue-level organization from polarity within an isolated cell.<Cdc42 controls spindle orientation to position the apical surface during epithelial morphogenesis by Aron B. Jaffe and others>[Caco-2 three-dimensional cysts, Cdc42 depletion and spindle-orientation analysis]{ref}
Mechanistically, <Cdc42> connects cortical organization to the <PAR polarity complex>, polarized trafficking and spindle-positioning machinery. However, the relative contributions depend on the model: a <Cdc42> trafficking phenotype in another epithelial line cannot simply be substituted for the <Caco-2> spindle result. A convincing test combines <live-cell imaging> of division and the first ectopic lumen with apical and basal markers, verifies target depletion and rescues with functional <Cdc42>. Compare the total number and viability of cells, because a dying cell population can also produce abnormal cavities. A perturbation that restores spindle positioning while leaving other outputs of <Cdc42> impaired would help isolate the causal route. These controls make the single-lumen phenotype an interpretable morphogenetic output.
The second example is <Bazooka>, the fly <PAR-3> protein, during <Drosophila germband extension>. This uses <planar cell polarity> within the epithelial sheet as well as apical-basal organization. <Bazooka> and junctional components become enriched at a complementary set of cell interfaces to <myosin II>. Spatially patterned gene activity organizes this planar asymmetry, and loss of <Bazooka> disrupts normal polarized cell behaviour and extension. Thus a protein associated with epithelial junction assembly is redeployed to make different junctions behave differently in the same sheet.<Patterned gene expression directs bipolar planar polarity in Drosophila by Jennifer A. Zallen and Eric Wieschaus>[Planar distributions of Bazooka and myosin and dependence on patterned gene expression]{ref}
Oriented junction shrinkage and new-junction formation permit <cell intercalation> while maintaining epithelial continuity. The important geometry is that junctions separating anterior and posterior neighbours shrink and resolve into interfaces with the complementary orientation, producing extension along the body axis. Junction names can be confusing because they may describe either the neighbours or the line itself; tracking vertices and neighbour exchange avoids this ambiguity. <Bazooka> is a spatial organizer, not itself the force-generating motor. To test its contribution, quantify its localization, myosin localization, junction lifetimes, cell rearrangements and tissue strain after an acute or mosaic perturbation that leaves enough epithelial structure for movement to occur. A general epithelial collapse would establish an essential structural role but would not isolate its role in planar coordination.
The third example is control of fly tracheal tube size by the apical determinant <Crumbs> and basolateral regulators including <Yurt> and <Coracle>. <Crumbs> promotes the apical domain, so restricting its activity helps constrain the amount of luminal cell surface and hence tube dimensions. Mutations in yurt produce overlong tracheal tubes, and reducing <Crumbs> dosage suppresses important length defects; <Crumbs> overexpression has the opposite tendency. This genetic interaction connects the balance of apical and basolateral regulators to organ geometry rather than simply to the presence of an epithelial sheet.<Epithelial polarity proteins regulate Drosophila tracheal tube size in parallel to the luminal matrix pathway by Patrick Laprise and others>[Tube measurements, Crumbs dosage suppression and <Vermiform> secretion controls]{ref}
The luminal <extracellular matrix> is a crucial control in this example. Septate-junction-associated proteins can also affect delivery of the matrix-modifying protein <Vermiform>, potentially changing tube shape indirectly. Yet yurt mutants retain <Vermiform> secretion, and reducing <Crumbs> can suppress coracle length defects without restoring the secretion defect. <Scribble> mutants also show long tubes but are not equivalently rescued by the tested <Crumbs> reduction. The result supports parallel and partly distinct routes, rather than a universal linear pathway in which every basolateral protein merely inhibits <Crumbs>. Compare cell number, apical area, tube length and diameter independently: a longer tube need not result from extra divisions. Quantitative dosage interactions and rescue are particularly informative because they can reveal a size-control function while preserving much of epithelial organization.
Together, these examples link polarity to three separable operations. <Cdc42> coordinates the location of new apical surfaces during division; <Bazooka> contributes to directional junction remodelling within a sheet; and the Crumbs-regulator balance sets the extent of a luminal surface. Conversely, changing forces and geometry can alter junctional organization, so the relationship is reciprocal. The common principle is that epithelial polarity supplies spatial constraints on growth and rearrangement. It is a dynamic component of <morphogenesis>, not merely a static property to be established before morphogenesis starts.
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