Lecture 16 Somitogenesis and neural crest cell migration March 21, 2011.doc

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Lecture 16 March 21, 2011
Eph-Ephrin signaling
(1) Somitogenesis and intersomitic boundary formation
(2) Neural crest cell migration
Notes verbatim from cited literature with minor modifications
Somitogenesis and intersomitic boundary formation
Watanabe Takahashi (2010) Tissue morphogenesis coupled with cell shape changes
Current Opinion in Genetics & Development 20:443–447
When organogenesis proceeds in embryos, a simple organ primordium progressively produces
complex subtypes of tissues. Such process is often enabled by a morphological segregation of newly forming
tissues. Tissue segregation is seen, for instance, during early brain formation where mid-brain and hind-brain
regions become distinct with a boundary between them. Another striking example of tissue segregation is
provided by the complete separation of newly formed somite from the unsegmented presomitic mesoderm
(PSM) (Figure 1, see slide). This process is the main topic of this review. In general, tissue separation is often
accompanied by robust changes in cell shape, such as transitions between epithelial and mesenchymal states.
Tissue separation and its concomitant changes in cell shape need to be precisely coordinated, because if such
coordination were to fail, the newly formed tissue would be distorted with incorrect morphological integrity.
Although there is much information about how transcriptional regulation determines organ development (mainly
with mutant model animals), and how molecular signals regulate cell shape changes
(mainly with cultured cells in vitro), it remains relatively unknown how cell shapes and tissue shapes
coordinately change during morphogenesis. Recently, using somitic boundary formation in vertebrate embryos
several groups have revealed novel mechanisms that enable the coordination between somitic boundary
formation and concomitant cell epithelialization. A central player acting during this coordination is repulsive
Ephrin-Eph signals. Interestingly, Ephrin-Eph signal is also important for tumor suppression, where tumor
segregation is coupled with cell epithelialization.
Somitogenesis: tissue separation is coupled with mesenchymal-to-epithelial transition
Somites are transient embryonic structures that are antero-posteriorly aligned on both sides of the midline. The
somites are repeatedly and regularly segmented with a clear structural separation between each entity (Figure 1).
As segmentation proceeds in an anterior-to-posterior direction, a small portion of the anterior end of the PSM
becomes separated by a morphological boundary (also called gap or cleft). Gap formation in PSM, a
mesenchymal tissue, is soon followed by an epithelial conversion of the gap-facing cells by the process known
as mesenchymal-to-epithelial transition (MET). Consequently, individually separated (segmented) somites have
a spherical shape outlined by epithelial cells. Some cells remain mesenchymal even after segmentation. They are
located at the center of the somite, enclosed by the epithelial cells. Thus, the structure of a newly formed somite
is very simple, consisting of two types of cells, epithelial outside and mesenchymal inside, with little or no cell
proliferation and apoptosis.
Three advantages make somitic segmentation an excellent model to study the mechanisms
underlying tissue separation and MET. First, PSM can be manipulated experimentally with transgenes using the
electroporation technique, particularly effective in chick embryos. Second, since segmentation reiterates
regularly in time and space, the next-forming boundary in PSM can unambiguously be identified and
manipulated. Third, the MET, occurring during segmentation, allows in vivo assessment of the roles of
candidate molecules that were implicated in cell epithelialization by in vitro studies. Below, we first review
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