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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from the properties of individual cells. In this section, we consider how the rearrangements
of cells during animal development give shape to the embryo and
to all the individual organs and appendages of the body.
A small number of cell processes are basic to morphogenesis. Individual cells
can migrate through the embryo along defined tracks. They can crawl over one
another in a coordinated way to elongate, constrict, or thicken a tissue. They can
segregate from their neighbors and form physically separate groups. They can
change their shape so as to deform an epithelial sheet into a tube or a vesicle.
By stretching out while holding on to their companions, specialized sets of cells
can form growing tubular networks such as the system of blood or lymph vessels.
Mass migrations, as occur in gastrulation, can transform the entire topology of
the embryo. Underlying all these processes are changes in cell shape and changes
in cell contacts—either with other cells or with extracellular matrix. We begin by
considering the migration of individual cells.
Cell Migration Is Guided by Cues in the Cell’s Environment
The birthplace of cells is often far from their ultimate location in the body. Our
skeletal muscles, for example, derive from muscle cell precursors, or myoblasts, in
somites, from which they migrate into the limbs and other regions. The routes that
the migrant cells follow and the selection of sites that they colonize determine the
eventual pattern of muscles in the body. The embryonic connective tissues form
the framework through which the myoblasts travel, and these tissues provide the
cues that guide myoblast distribution. No matter which somite they come from,
the myoblasts that migrate into a forelimb bud will form the pattern of muscles
appropriate to a forelimb, and those that migrate into a hindlimb bud will form
the pattern appropriate to a hindlimb. It is the connective tissue that provides the
patterning information.
As a migrant cell travels through the embryonic tissues, it repeatedly extends
surface projections that probe its immediate surroundings, testing for cues to
which it is particularly sensitive by virtue of its specific assortment of cell-surface
receptor proteins. Inside the cell, these receptors are connected to the cortical
actin and myosin cytoskeleton, which moves the cell along. Some extracellular
matrix molecules, such as the protein fibronectin, provide adhesive sites that help
the cell advance; others, such as chondroitin sulfate proteoglycan, inhibit locomotion
and repel immigration. The nonmigrant cells along the migration pathway
may likewise have inviting or repellent macromolecules on their surface; some
may even extend filopodia to make their presence known.
Among the many guiding influences, a few stand out as especially important.
In particular, many types of migrating cells are guided by chemotaxis that
depends on a G-protein-coupled receptor (called CXCR4), which is activated by
an extracellular ligand called CXCL12. Cells expressing this receptor can snuffle
their way along tracks marked out by CXCL12 (Figure 21–45). Chemotaxis toward
sources of CXCL12 plays a major part in guiding the migrations of lymphocytes
germ cells
(A) 4-somite stage
somites
CXCL12
notochord
(B) 15-somite stage
Figure 21–45 CXCL12 guides migrating
germ cells. Zebrafish germ cells migrate to
domains that express CXCL12. As the sites
of CXCL12 expression change, cells follow
the CXCL12 track and are guided to the
region where the gonad develops at a later
developmental stage. (A) At the 4-somite
stage, germ cells move from a position
that is close to the midline to more lateral
regions where CXCL12 is expressed. (B) As
the CXCL12 expression retracts, germ cells
are guided to more posterior positions.