The a
ncestors of moder
n Metazoa were co
nstructed i
n large part by the foldi
ngs a
nd distortio
ns of two-dime
nsio
nal sheets of epithelial cells. This cha
nged
![](http://www.scie<font color=)
ncedirect.com/scidirimg/e
ntities/223c.gif" alt="
not, vert, similar" title="
not, vert, similar" border="0">600 millio
n years ago with the evolutio
n of mese
nchymal cells. These cells arise as the result of epithelial cell delami
natio
n through a reprogrammi
ng process called a
n epithelial to mese
nchymal tra
nsitio
n (EMT) [Shook D,
Keller R. Mecha
nisms, mecha
nics a
nd fu
nctio
n of epithelial-mese
nchymal tra
nsitio
ns i
n early developme
nt. Mech Dev 2003;120:1351–83; Thiery JP, Sleema
n JP. Complex
networks orchestrate epithelial-mese
nchymal tra
nsitio
ns. Nat Rev Mol Cell Biol 2006;7:131–42]. Because mese
nchymal cells are free to migrate through the body cavity, the evolutio
n of the mese
nchyme ope
ned up
new ave
nues for morphological plasticity, as cells evolved the ability to take up
new positio
ns withi
n the embryo a
nd to participate i
n novel cell–cell i
nteractio
ns; formi
ng
new types of i
nter
nal tissues a
nd orga
ns such as muscle a
nd bo
ne [Thiery JP, Sleema
n, JP. Complex
networks orchestrate epithelial-mese
nchymal tra
nsitio
ns. Nat Rev Mol Cell Biol 2006;7:131–42; Hay ED, Zuk A. Tra
nsformatio
ns betwee
n epithelium a
nd mese
nchyme:
normal, pathological, a
nd experime
ntally i
nduced. Am J Kid
ney Dis 1995;26:678–90]. After migrati
ng to a suitable site, mese
nchymal cells coalesce a
nd re-polarize to form seco
ndary epithelia, i
n a so-called mese
nchymal–epithelial tra
nsitio
n (MET). Such switches betwee
n mese
nchymal a
nd epithelial states are a freque
nt feature of Metazoa
n gastrulatio
n [Hay ED, Zuk A. Tra
nsformatio
ns betwee
n epithelium a
nd mese
nchyme:
normal, pathological, a
nd experime
ntally i
nduced. Am J Kid
ney Dis 1995;26:678–90] a
nd the
neural crest li
neage [Duba
nd JL, Mo
nier F, Dela
nnet M, Newgree
n D. Epitheliu-mmese
nchyme tra
nsitio
n duri
ng
neural crest developme
nt. Acta A
nat 1995;154:63–78]. Sig
nifica
ntly, however, whe
n hijacked duri
ng the developme
nt of ca
ncer, the ability of cells to u
ndergo EMT, to leave the primary tumor a
nd to u
ndergo MET at seco
ndary sites ca
n have devastati
ng co
nseque
nces o
n the orga
nism, allowi
ng tumor cells derived from epithelia to i
nvade surrou
ndi
ng tissues a
nd spread through the host [Thiery JP, Sleema
n JP. Complex
networks orchestrate epithelial-mese
nchymal tra
nsitio
ns. Nat Rev Mol Cell Biol 2006;7:131–42; Hay ED, Zuk A. Tra
nsformatio
ns betwee
n epithelium a
nd mese
nchyme:
normal, pathological, a
nd experime
ntally i
nduced. Am J Kid
ney Dis 1995;26:678–90]. Thus, the molecular a
nd cellular mecha
nisms u
nderpi
nni
ng EMT are both a
n esse
ntial feature of Metazoa
n developme
nt a
nd a
n importa
nt area of biomedical research. I
n this review, we discuss the commo
n molecular a
nd cellular mecha
nisms i
nvolved i
n EMT i
n both cases.