In Chapter 19, we saw how cells cohere to form epithelial sheets or surround themselves with extracellular matrix to create connective tissues. We also discussed how the basic features of tissues, such as the polarity of epithelia, arise 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
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.