the mesoderm gives rise to the connective tissues—at first, to the loose mesh of cells in the embryo known as mesenchyme, and ultimately to cartilage, bone, and fibrous tissue, including the dermis (the inner layer of the skin). the mesoderm also forms the muscles, the entire vascular system—including the heart, blood vessels, and blood cells—and the tubules, ducts, and supporting tissues of the kidneys and gonads. the notochord forms from the mesoderm and serves as the core of the future backbone and the source of signals that coordinate the development of surrounding tissues.
the ectoderm will form the epidermis (the outer, epithelial layer of the skin) and epidermal appendages such as hair, sweat glands, and mammary glands. It will also give rise to the whole of the nervous system, central and peripheral, including not only neurons and glia but also the sensory cells of the nose, the ear, the eye, and other sense organs. (B, after t. Mohun et al., Cell 22:9–15, 1980. With permission from elsevier.)
The ectoderm, mesoderm, and endoderm formed during gastrulation constitute the three germ layers of the early embryo. Many later developmental transformations will produce the elaborately structured organs. But the basic body plan and axes set up in miniature during gastrulation are preserved into adult life, when the organism may be billions of times larger (Movie 21.2).
the Developmental potential of cells Becomes progressively restricted
Concomitant with the refinement of the body plan, the individual cells become more and more restricted in their developmental potential. During the blastula stages, cells are often totipotent or pluripotent—they have the potential to give rise to all or almost all of the cell types of the adult body. The pluripotency is lost as gastrulation proceeds: a cell located in the endodermal germ layer, for example, can give rise to the cell types that will line the gut or form gut-derived organs such as the liver or pancreas, but it no longer has the potential to form mesoderm-derived structures such as skeleton, heart, or kidney. Such a cell is said to be determined for an endodermal fate. Thus, cell determination starts early and progressively narrows the options as the cell steps through a programmed series of intermediate states—guided at each step by its genome, its history, and its interactions with neighbors. The process reaches its limit when a cell undergoes terminal differentiation to form one of the highly specialized cell types of the adult body (Figure 21–4). Although there are cell types in the adult that retain some degree of pluripotency, their range of options is generally narrow (discussed in Chapter 22).
Underlying the richness and astonishingly complex outcomes of development is cell memory (see p. 404). Both the genes a cell expresses and the way it behaves depend on the cell’s past, as well as on its present circumstances. The cells of our body—the muscle cells, the neurons, the skin cells, the gut cells, and so on—maintain their specialized characters largely because they retain a record of the extracellular signals their ancestors received during development, rather than because they continually receive such instructions from their surroundings. Despite their radically different phenotypes, they retain the same complete genome that was present in the zygote; their differences arise instead from differential gene expression. We have discussed the molecular mechanisms of gene regulation, cell memory, cell division, cell signaling, and cell movement in previous chapters. In this chapter, we shall see how these basic processes are collectively deployed to create an animal.
The anatomical features that animals share have undergone many extreme modifications in the course of evolution. As a result, the differences between species are usually more striking to our human eye than the similarities. But at the level of the underlying molecular mechanisms and the macromolecules that mediate them, the reverse is true: the similarities among all animals are profound and extensive. Through more than half a billion years of evolutionary divergence, all animals have retained unmistakably similar sets of genes and proteins that are responsible for generating their body plans and for forming their specialized cells and organs.