Biomedical Engineering Reference
In-Depth Information
(
Green, Inoue, & Sternberg, 2008
). Even for seam cells, as described above,
Wnts are required for proper polarity orientation of seam cells rather than for
their polarization itself. To solve these contradictory observations, it has
been proposed that functions of Wnts in polarity orientation may be deter-
mined by Wnt-expressing cells (
Yamamoto et al., 2011
). In this model,
when CWN-1 is expressed anteriorly in the pharynx, it may function like
CWN-2 rather than normal CWN-1 by the activities of Wnt-modifying
enzymes or Wnt cofactors that are specifically expressed in the anterior
but not the posterior Wnt-expressing cells. This model or other possibilities
should be further explored to understand how the polarity orientation is
coordinated in a number of cells during development.
9. POLARITY REVERSAL IN NORMAL DEVELOPMENT
Although most dividing cells show the same polarity orientation
(high/low POP-1 asymmetry), two cell divisions show reversed (low/high)
polarity. Both of them are involved in the production of mirror-symmetric
structures of organs along the anterior-posterior axis.
The mirror-symmetric structure of the vulva is associated with mirror-
symmetric lineages of its precursor cells, P5.p and P7.p (
Sternberg, 2005
).
While daughters of P5.p show high/low POP-1 asymmetry as in other
daughter pairs, P7.p daughters show reversed low/high POP-1 asymmetry
(
Deshpande, Inoue, Priess, & Hill, 2005
). The polarity reversal is caused by
functions of MOM-2/Wnt and LIN-44/Wnt that is expressed in the
gonadal anchor cell anteriorly to P7.p but posteriorly to the P5.p cell
(
Green et al., 2008
). These Wnts antagonize the instructive functions of
EGL-20/Wnt that promotes high/low POP-1 asymmetry. The P7.p cell
appears to discriminate EGL-20 from the posterior region and LIN-44
and MOM-2 from the anterior cell using distinct Wnt receptors, CAM-
1/Ror for EGL-20 and LIN-17/Fzd
þ
LIN-18/Derailed for LIN-
44
þ
MOM-2. This is in good contrast with seam cells in which functions
of three receptors (LIN-17/Fzd, MOM-5/Fzd, and CAM-1/Ror) appear
to be redundant.
Another mirror-symmetric organ is gonad that consists of germ cells and
somatic gonad. Similar to the vulva, lineage of SGPs Z1 and Z4 are mirror-
symmetric with reversal of POP-1 asymmetry in Z1 (low/high) compared
with most other cells including Z4 (high/low) (
Siegfried, Kidd, Chesney, &
Kimble, 2004
). The polarity of Z1 and Z4 does not appear to require
Wnts, as it is not affected in quintuple Wnt mutants or mutants of
