Tissue Patterning: Translating Design Principles from In Vivo to In Vitro. 
Recapitulating the architecture of native tissue remains a significant challenge, impeding the progress of engineering tissues.  Imposing appropriate organization is especially challenging in tissues that contain multiple cellular components in complex structural units.  One solution is to mimic developmental processes in embryos.  In an embryo, cells are organized by tissue patterning, whereby induction of fate-determining genes is spatially controlled to generate patterns of cell differentiation and maturation.  Following patterning, the imposed cell organization is further reinforced by implementation of compartment boundaries, which prevent intermingling of cells from distinct phenotypic domains, thereby ensuring preservation of proper cell organization in growing and reorganizing tissues.  Both morphogenic processes utilize a conserved set of fundamental principles, the implementation of which leads to highly regulated cell organization.  In this article, we review these patterning principles in vivo and reflect on the progress made by tissue engineers in mimicking tissue patterning ex vivo.