Steady-state epithelial apical flatness is characterized by MLCK morphodynamics and asynchronous Ca ²⁺ oscillations, but not underlying ECM geometry

The canonical simple epithelium is a flat sheet-like tissue of horizontally packed cells. While the basal surface is delineated by the basement membrane of extracellular matrix (ECM), little is known about how a flat apical surface is maintained, or if apical/basal dynamics are coordinated. The current study tests the role of the apical domain, to define mechanisms involved in maintaining a flat apical geometry in an epithelium. When the basal geometry is modulated, Madin-Darby Canine Kidney (MDCK) cells adjust their morphology to maintain an overall apical flatness of the confluent layer. Pharmacological and transgenic disruption of non-muscle myosin ATPase, and MLCK activity results in an uneven apical structure, and overall loss of the flat geometry typical of a confluent epithelium. Surprisingly, transgenic experimentation showed that forces maintaining individual MDCK cell flatness are cell-autonomous. Finally, Ca ²⁺ imaging reveals an asynchronous calcium flux across a confluent epithelium, suggesting a myosin II-mediated mechanism for maintaining a flat apical architecture. Our results highlight that apical/basal cellular surfaces may not be tightly coordinated, but rather independently regulated. This study provides a new paradigm for how apical flatness is regulated at steady state.