Figure 21–11 Pattern generation by a reaction-diffusion system. From (a) a uniform field of cells, (B) local positive feedback and (c) long-range inhibition can (D) generate patterns within the initially uniform field. the patterns can be complex, resembling the spots of a leopard (as shown) or the stripes of a zebra; or they can be simple, with creation of a single cluster of specialized cells that can, for example, go on to serve as the source of a morphogen gradient.
(Figure 21–11). Such short-range activation combined with long-range inhibition can account for the formation of clusters of cells within an initially homogeneous tissue that become specialized as localized signaling centers.
Cell diversification does not always depend on extracellular signals: in some cases, daughter cells are born different as a result of an asymmetric cell division, in which some important molecule or molecules are distributed unequally between the two daughters. This asymmetric inheritance ensures that the two daughter cells develop differently (Figure 21–12). Asymmetric division is a common feature of early development, where the fertilized egg already has an internal pattern and cleavage of this large cell segregates different determinants into separate blastomeres. We shall see that asymmetric division also plays a part in some later developmental processes.
Initial patterns are established in Small Fields of cells and refined by Sequential Induction as the embryo Grows
The signals that organize the spatial pattern of cells in an embryo generally act over short distances and govern relatively simple choices. A morphogen, for example, typically acts over a distance of less than 1 mm—an effective range for diffusion —and directs choices between several developmental options for the cells on which it acts. Yet the organs that eventually develop are much larger and more complex than this.
The cell proliferation that follows the initial specification accounts for the size increase, while the refinement of the initial pattern is explained by a series of local inductions plus other interactions that add successive levels of detail on an initially simple sketch. For example, as soon as two types of cells are present in a developing tissue, one of them can produce a signal that induces a subset of the neighboring cells to specialize in a third way. The third cell type can in turn signal 1. asymmetric division: sister cells born different uniform feld of cells long-range inhibitor (red) blocks production of activator by other cells in the neighborhood specialized clusters of cells 2. symmetric division: sister cells become different as a result of
Figure 21–12 Two ways of making sister cells different.
D and E are induced by signal C is induced on A and B, B acting on A back to the other two cell types nearby, generating a fourth and a fifth cell type, and so on (Figure 21–13).
This strategy for generating a progressively more complicated pattern is called sequential induction. It is chiefly through sequential inductions that the body plan of a developing animal, after being first roughed out in miniature, becomes elaborated with finer and finer details as development proceeds.
The rapid progress in understanding animal development has been one of the great success stories in biology over the last few decades, and it has important practical implications. Some 2 to 5% of all human babies are born with anatomical abnormalities, such as heart malformations, truncated limbs, cleft palate, or spina bifida. Advances in developmental biology help us understand how these defects arise, even if we cannot yet prevent or cure most of them.
Less obvious, but even more important from a practical point of view, is that developmental biology provides insights into the workings of cells and tissues in the adult body. Developmental processes do not halt at birth; they continue throughout life, as tissues are maintained and repaired. The fundamental mechanisms of cell growth and division, cell–cell signaling, cell memory, cell adhesion, and cell movement are involved in adult tissue maintenance and repair—just as they are in embryo development.