Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

MORPHOGENESIS
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(A)
lamellipodia attempt
to crawl on surfaces
of neighboring cells,
pulling them inward
in direction of arrows
(B) (C) (D)
Figure 21–50 Convergent extension
and its cellular basis. (A) Schematic
diagram of cell behaviors that underlie
convergent extension. The cells form
lamellipodia, with which they attempt to
crawl over one another. Alignment of the
lamellipodial movements along a common
axis leads to convergent extension. The
process depends on the Wnt–Frizzled/
planar-cell-polarity signaling pathway and is
cooperative, presumably because cells that
are already aligned exert forces that tend to
align their neighbors in the same way.
(B–G) The pattern of convergent
extension of dorsal mesoderm during
zebrafish gastrulation at 8.8 (B, E), 9.3 (C,
F), and 11.3 (D, G) hours after fertilization.
Cells that will give rise to the notochord
are labeled in green, and cells that will give
rise to somites and muscle are labeled in
blue. The notochord and somite domains
are spatially separate from the start of the
recording (B, E), but their boundaries are
at first barely visible and only a little later
become obvious. Convergence narrows
the notochord domain to a width of about
two cells at the last time point (D, G).
(A, after J. Shih and R. Keller, Development
116:901–914, 1992; B–G, after
N.S. Glickman et al., Development
130:873–887, 2003. With permission from
The Company of Biologists.)
notochord
somite
(E)
domain
(F)
domains
(G)
appropriate region of the embryo, isolated in culture, will spontaneously narrow
and elongate, just as they would in the embryo (Figure 21–50). The alignment
of the cell movements depends on the same signaling pathway that is involved
in generating planar cell polarity within developing epithelia, as we discuss next.
MBoC6 m22.76/22.50
Planar Cell Polarity Helps Orient Cell Structure and Movement in
Developing Epithelia
Cells within an epithelium always have an apical–basal polarity (discussed in
Chapter 19), but the cells of many epithelia show an additional polarity at right
angles to this axis: the cells are all arranged as if they had an arrow written on
them, pointing in a specific direction in the plane of the epithelium. This type of
polarity is called planar cell polarity. In the wing of a fly, for example, each epithelial
cell has a tiny asymmetrical projection, called a wing hair, on its surface,
and the hairs all point toward the tip of the wing. Similarly, in the inner ear of a
vertebrate, each mechanosensory hair cell has a precisely oriented asymmetric
bundle of actin-filled, rodlike protrusions called stereocilia sticking up from its
apical plasma membrane as a detector of sound and of forces such as gravity. Tilting
the bundle in one direction causes ion channels in the membrane to open,
electrically activating the cell; tilting in the opposite direction has the opposite
effect. For the ear to function properly, the hair cells must be oriented correctly.
Planar cell polarity is also important in the respiratory tract, where every ciliated
cell must orient the beating of its cilia so as to sweep mucus upward, away from
the lungs.
Screens for mutants with misoriented wing hairs in Drosophila have identified
a set of genes that is critical for planar cell polarity. Some of these genes code for