A Morphogenetic Wave that Generates Mesenchymal-to-Epithelial Transition in the Lateral Plate Mesoderm

Most mesodermal cells undergo multiple cycles of transition between an epithelial and mesenchymal state during embryonic development. While many studies have addressed the process of epithelial-to-mesenchymal transition (EMT), comparatively less is known regarding the complementary process, mesenchymal-to-epithelial transition (MET), which is essential for organogenesis and has also been proposed to be important for cancer metastasis. The current study investigated MET using the lateral plate mesoderm (LPM) of the chick embryo as a model system. We find that MET in the LPM proceeds as a wave, which divides the LPM into distinct mesenchymal, transition, and epithelial zones. In the multilayered mesenchymal zone, many apical epithelial markers, including N-Cadherin (N-Cad), Par-3 and Zo-1, but not atypical protein kinase C (aPKC), are detected as dispersed, partially co-localizing aggregates associated with cell-cell contacts. The transition zone is characterized by the appearance of aPKC and the formation of rosette-like structures characterized by wedge-shaped cells that are apical-basal polarized, with strong co-localization of apical polarity markers, but not yet arranged into distinct epithelial sheets. The transition zone is also enriched in mitotic cells. Subsequently, the rosettes resolve into two well-defined epithelial sheets that constitute the coelomic epithelium, the lining of the internal body cavity. Prior to any overt signs of apical-basal polarity, fibronectin (FN) begins to accumulate at the future basal side of the incipient epithelium. Interference with Extracellular Matrix (ECM)-integrin signaling through disruption of focal adhesion kinase (FAK) or Talin function hindered the normal progression of the epithelialization process. Cells with disrupted FAK or Talin function retained mesenchymal-like characteristics with respect to cellular morphology and apical-basal marker distribution. We propose a two-stage process for MET in the LPM. Initially, in the polarization phase, ECM-integrin-dependent signaling imparts apical-basal polarity, culminating in the activation of aPKC, to drive cell intercalation and rosette formation. Subsequently in the resolution phase, polarized rosette cells, perhaps facilitated by the weakening of cell-cell interactions that occurs during mitosis, expand their apical surface, and spread out to form new connections laterally to their fully epithelial neighbors. This sequence of events is propagated as a wave through the LPM, thus generating an integrated coelomic epithelium.