Diversity in the molecular and cellular strategies of epithelium-to-mesenchyme transitions: Insights from the neural crest

Abstract

Although epithelial to mesenchymal transitions (EMT) are often viewed as a unique event, they are characterized by a great diversity of cellular processes resulting in strikingly different outcomes. They may be complete or partial, massive or progressive, and lead to the complete disruption of the epithelium or leave it intact. Although the molecular and cellular mechanisms of EMT are being elucidated owing chiefly from studies on transformed epithelial cell lines cultured in vitro or from cancer cells, the basis of the diversity of EMT processes remains poorly understood. Clues can be collected from EMT occuring during embryonic development and which affect equally tissues of ectodermal, endodermal or mesodermal origins. Here, based on our current knowledge of the diversity of processes underlying EMT of neural crest cells in the vertebrate embryo, we propose that the time course and extent of EMT do not depend merely on the identity of the EMT transcriptional regulators and their cellular effectors but rather on the combination of molecular players recruited and on the possible coordination of EMT with other cellular processes.

Figure 1

Different modes of EMT. (A) complete EMT. All epithelial cells undergo EMT coincidently, causing the complete dislocation of the epithelial structure and resulting in the formation of a single mesenchyme. (B) Partial EMT. A small number of epithelial cells undergo EMT individually and separately over time such that the epithelial structure is maintained intact during the whole process. (C) en masse EMT. Groups of cells in a defined portion of the epithelium undergo simultaneously coordinated movements of ingression so that they are displaced out of the epithelium which heals to fill the gap. Both partial and en masse EMT result in the formation of two distinct tissues: the epithelium which persists after EMT and the newly-formed mesenchyme. Comparison of the molecular and cellular events accompanying the different modes of EMT suggest that they do not differ in the molecular players that are involved but in the underlying regulatory processes. Epithelial cells are depicted in beige, cells undergoing EMT in green, and mesenchymal cells are in blue. Green solid bars: basement membrane; purple lines: fibrillar extracellular matrix.

Figure 2

Molecular players involved in EMT. Studies on epithelial cell lines established in vitro and on cancer cells showed that transition between epithelial to mesenchymal cells is promoted by a great variety of growth factors and morphogens, e.g., TGFβ, BMPs, HGF/SF, FGF, wnt and Notch which all impinge via a few families of transcription factors (Snail, Zeb, Twist and Fox) on the expression of a variety of cellular components involved in the maintenance of the epithelial structure: cell adhesion molecules of the junctional complexes, such as E-cadherin, desmoplakins, occludins and claudins, cell polarity molecules, such as Par and Crumb, cytoskeletal components, such as cytokeratins, and basement membrane components, e.g., laminins. Conversely, mesenchymal cells express a new repertoire of adhesion, cytoskeletal and matrix components that enables them to populate their environment. Epithelial cells are depicted in beige, cells undergoing EMT in green, and mesenchymal cells are in blue. Green solid bars, basement membrane; purple lines, fibrillar extracellular matrix.

Figure 3

Different strategies of neurulation in vertebrates. (A) Primary neurulation in the chick at the midbrain level. This process involves columnarization of a preexisting epithelium which then rolls, folds and bends dorsally into a tube. Note that during primary neurulation, the notochord generally preexists to the neurulation event. In amphibians, the whole neuraxis forms by primary neurulation whereas in amniotes, only the rostral neural tube in the head and upper trunk are concerned. (B) Secondary neurulation in the chick. It involves condensation of a mesenchymal blastema followed by epithelialization, first dorsally then progressively more ventrally. A lumen then appears by cavitation in the midline to form a neural tube. During secondary neurulation, the notochord forms coincident with the neurulation event. This type of neurulation occurs in the lumbosacral region of amniotes. (C) Neurulation in zebrafish. In this species, the neural tube does not form by rolling up and elevation of folds but by movements of convergence extension in a multilayered neural plate. This results in the formation first of a keel, then a rod. Cavitation also occurs in the midline but most likely involves different cellular events than during secondary neurulation. The neural tube is depicted in orange; the notochord in red, the ectoderm in light purple and the mesoderm in white.

Figure 4

Kinetics of NC cell specification, delamination and migration during primary neurulation at cranial and truncal levels of the chick embryo. (A) In the midbrain, NC undergo massively EMT soon after neural tube closure, resulting in large streams of migrating cells. The total duration of delamination does not exeed 12 h. (B) in the anterior trunk, at the brachial level, NC cells undergo EMT about 8–12 h after neural tube closure. Delamination is progressive and concerns a limited number of cells at the same time. Thus, the duration of the delamination process lasts about 30–40 h. The neural tube is depicted in beige; the notochord in red, the ectoderm in light purple, the mesoderm in white. NC cells in the process of specification are in yellow, delamination in green, and migration in blue.

Figure 5

Molecular players of EMT during NC cell delamination at cranial and truncal levels in the chick embryo. The massive delamination of NC cells at cranial levels is correlated with expression of a greater number of transcriptional regulators than at truncal levels, suggesting cooperation between transcription factors to allow coordination of EMT events in cells. + moderate expression; ++ strong expression; +/− expression in a limited number of cells (e.g., for cad7 at cranial levels) or gradually decreasing during migration (e.g., for RhoB in migrating cells). Expression of Sox-9 and Sox-10, two major genes involved in NC specification and migration, is indicated for reference.