Morphogen transport: theoretical and experimental controversies

Abstract According to morphogen gradient theory, extracellular ligands produced from a localized source convey positional information to receiving cells by signaling in a concentration‐dependent manner. How do morphogens create concentration gradients to establish positional information in developing tissues? Surprisingly, the answer to this central question remains largely unknown. During development, a relatively small number of morphogens are reiteratively deployed to ensure normal embryogenesis and organogenesis. Thus, the intracellular processing and extracellular transport of morphogens are tightly regulated in a tissue‐specific manner. Over the past few decades, diverse experimental and theoretical approaches have led to numerous conflicting models for gradient formation. In this review, we summarize the experimental evidence for each model and discuss potential future directions for studies of morphogen gradients. WIREs Dev Biol 2015, 4:99–112. doi: 10.1002/wdev.167 This article is categorized under: Establishment of Spatial and Temporal Patterns > Gradients Signaling Pathways > Cell Fate Signaling Early Embryonic Development > Development to the Basic Body Plan