September 14, 2004 – Epithelium and mesenchyme represent two
extremes in the organization of groups of cells. Epithelial cells
array themselves into flat sheets or rolled tubes, while mesenchymal
cells appear less coordinated in their structure-forming activities
and form fewer and looser connections with other cells. These tissue-level
differences are reflected at the level of the individual cell as well.
Under the microscope, mesenchymal cells are amorphous and lack the
distinct apical-basal polarity that characterizes their epithelial
counterparts. Both types of cells contribute to the body’s function
in distinct ways, with epithelium being the essential structural and
physiological component of organs and tissues such as the kidney and
the lining of the gut, and mesenchyme forming all migratory cells,
including metastatic cancer cells, as well providing support for the
epithelium in various contexts. Despite (or perhaps because of) these
differences, our anatomy contains countless examples of interaction
between epithelial and mesenchymal cells. Indeed, nearly every one
of the body’s structures, from internal organs to limbs to teeth,
is made up of a mesenchymal and an epithelial component.
Developmental biologists are particularly interested in the ability
of each of these cell types to be converted into its counterpart,
epithelial cells become mesenchymal, while in other situations the
reverse transformation occurs. Now, Yoshiko
Takahashi (Team Leader, RIKEN CDB Laboratory for Body Patterning;
Kobe, Japan) and colleagues report a heretofore unknown mechanism
by which mesenchymal-epithelial transitions (METs) are regulated
in the embryogenesis of the chicken. The article, published in the
September 14 issue of Developmental Cell, describes how a pair of
intracellular molecules affects the ability of mesenchymal cells
to convert into epithelial cells during the formation of somites
early in embryogenesis. Somites are transitory structures that appear
in a head-down direction in strictly timed and ordered pairs as
the chicken develops, dividing the embryo into body patterning segments,
and later giving rise to the vertebrae, ribs and the muscles of
the trunk.
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Somites forming in 2-day
chicken embryo |
The presomitic mesoderm, from which somites form, consists
of a pair of strips made up of mesenchymal cells running down the
embryo’s back. After the boundaries of an individual somite
have been established, the cells in the border region undergo a
transformation, acquiring the polarized structure and properties
of epithelial cells. This results in a somite body in which an outer
layer of epithelial cells surrounds a mesenchymal core. Takahashi
et al. focused on this mesenchymal-epithelial transition to study
how mesenchymal cells known for their formlessness are able to make
the transition to become highly organized epithelial cells. They
used a method that they had previously developed for introducing
DNA into somitogenic cells by electroporation to study the role
of a group of molecules known as Rho-family small GTPases. This
family, which includes Rho, Rac and Cdc42, is known to play an important
role in the dynamic organization of the cytoskeleton, the framework
of elements that determines many aspects of a cell’s morphology
and structure.
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Forming somites undergo
dynamic changes in cell polarity,
revealed by distribution of cytoskeleton (red) |
The team found that the expression of constitutively active Cdc42
prevented mesenchymal cells from undergoing their normal epithelialization,
while interference with Cdc42 function by a specific inhibitor caused
an unusually high number of the mesenchymal cells to take on epithelial
characteristics, demonstrating the central role of this GTPase in
somitogenic MET. These findings were made possible by a novel experimental
system developed in the Takahashi lab that takes advantage of the
ability to introduce cDNA along with a fluorescent protein directly
into a living embryo’s cells by momentarily and reversibly
disrupting their outer membranes by the application of a tiny electrical
current, a process called electroporation. This technology allowed
the team to track cells that had been electroporated with the mutant
forms of Cdc42 and observe as they contributed to somite formation.
Electroporation of dominant negative and constitutively active
forms of Rac1 also indicated a role for that molecule in the mesenchymal-epithelial
transition. Rac1 appears to act in a tightly constrained dose-dependent
fashion to regulate MET, as both constitutively active and dominant
negative forms of the protein disturb somitic epithelialization.
Takahashi and colleagues further demonstrated that developing somites
in the chicken are able to activate Rac1, suggesting a function
for the protein in normal somitogenesis. When they extended their
study to investigate possible crosstalk between Rac1 and Paraxis,
the only transcription factor known to be essential for somitic
MET, they found that Paraxis does indeed require Rac1 in its function
as an epithelialization factor, strengthening the argument for its
importance in the somitogenic process.
The development of a system for introducing DNA directly into somite-forming
cells and the demonstration of the central role played by cytoskeleton-regulatory
molecules in the cell shape changes characteristic of the mesenchymal-epithelial
transition represent significant steps toward a clearer comprehension
of how cellular reorganizations are achieved during development.
And with epithelial-mesenchymal transitions featuring in fundamental
biological processes from organ development to cancer metastasis,
such knowledge may one day contribute to our ability to understand
these processes, and perhaps even to right them, when they go awry.
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