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

1190 Chapter 21: Development of Multicellular Organisms
epidermal cells in fly wing sensory hair cells in mouse ear epidermal cells in fly wing sensory hair cells in mouse ear
25 µm
(A) (B) (C) (D)
10 µm
5 µm
WILD TYPE
Flamingo MUTANT
Figure 21–51 Planar cell polarity. (A) Wing hairs on the wing of a fly. Each cell in the wing epithelium forms a small, spiky
protrusion or “hair” at its apex, and all the hairs point the same way, toward the tip of the wing. This reflects a planar polarity in
the structure of each cell. (B) Sensory hair cells in the inner ear of a mouse similarly have a well-defined planar polarity, manifest
in the oriented pattern of stereocilia (actin-filled protrusions) on their surface. The detection of sound depends on the correct,
coordinated orientation of the hair cells. (C) A mutation in the gene Flamingo in the fly, coding for a nonclassical cadherin,
disrupts the pattern of planar cell polarity MBoC6 in m19.32/22.49
the wing. (D) A mutation in a homologous Flamingo gene in the mouse randomizes
the orientation of the planar cell polarity vector of the hair cells in the ear. The mutant mice are deaf. (A and C, from J. Chae et
al., Development 126:5421–5429, 1999. With permission from The Company of Biologists; B and D, from J.A. Curtin et al.,
Curr. Biol. 13:1129–1133, 2003. With permission from Elsevier.)
components of the Wnt signaling pathway, others code for specialized members
of the cadherin superfamily, while the functions of others are uncertain. These
components of planar-cell-polarity signaling are assembled at cell–cell junctions
in the epithelium in such a way as to exert a polarizing influence that can propagate
from cell to cell. Essentially the same system of proteins controls planar cell
polarity in vertebrates; mice deficient in homologs of the Drosophila planar polarity
genes have a variety of defects, including incorrectly oriented hair cells in the
inner ear, making them deaf (Figure 21–51).
Interactions Between an Epithelium and Mesenchyme Generate
Branching Tubular Structures
Animals require specialized types of epithelial surfaces for many functions,
including excretion, absorption of nutrients, and gas exchange. Where large surfaces
are required, they are often organized as branching tubular structures. The
lung is an example. It originates from epithelial buds that grow out from the floor
of the foregut and invade neighboring mesenchyme to form the bronchial tree, a
system of tubes that branch repeatedly as they extend. Endothelial cells that form
the lining of blood vessels invade the same mesenchyme, thereby creating a system
of closely apposed airways and blood vessels, as required for gas exchange in
the lung (Figure 21–52). This whole process of branching morphogenesis depends
on signals that pass in both directions between the growing epithelial buds and
the mesenchyme. Genetic studies in mice indicate that FGF proteins and their
receptor tyrosine kinases play a central part in these signaling processes. FGF signaling
has various roles in development, but it is especially important in the many
interactions that occur between a developing epithelium and mesenchyme.
In the case of lung development, FGF10 is expressed in clusters of mesenchyme
cells that lie near the tips of the growing epithelial tubes, and its receptor is
expressed in the invading epithelial cells. In FGF10-deficient mutant mice, a primary
bud of lung epithelium is formed but fails to grow out into the mesenchyme
to create a branching bronchial tree. Conversely, a microscopic bead soaked in
Figure 21–52 The airways of the lung,
shown in a cast of the adult human
bronchial tree. Resins of different colors
have been injected into different branches
of the tree of airways. (From R. Warwick
and P.L. Williams, Gray’s Anatomy, 35th ed.
Edinburgh: Longman, 1973.)
MBoC6 m22.92B/21.51