The notion of an organizer is somewhat imprecise, but in a general sense there are organizers from head (see Chapter 15) to tail (Agathon et al. 2003) and many places in between, and in subsequent chapters we will see how some of them work.
The notion of an organizer, like others in biology that use a term borrowed from everyday life, should be understood with circumspection. The term is used to identify signals in a practical sense and does not mean that these cells somehow come from the outside, as the word "organizer" often implies in other uses. Developmental organizers result from prior cells in a continuum of a developmental process.
Pentameral animals such as echinoderms (e.g., starfish) have an additional kind of axis, comprising the five "arms" arrayed around the central body location; they have a central radial axis and a single, repeated PD axis going out along each arm. The genetic and developmental homologies with linearly patterned animals are not yet well understood (Popodi and Raff 2001).
Cells in different regions of the blastula become committed to three basic lineages that are the precursors of different sets of tissues in the final organism. Broadly, ectodermal cells will generate the outer covering and nervous system, cells that make up the mesoderm will become muscle and connective tissue, and endodermal cells will become the gut and its associated organs. Organisms formed from these three basic tissues are referred to as triploblastic. These first steps in the hierarchy of histological differentiation are highly varied in detail, even if the result is somewhat similar. Figure 8-3 shows a selection of developmental tissue "fates," in different classes of animals, that form in a hierarchy of commitment (e.g., see Carlson 1999; Gilbert 2003;Wake 1979). By "fate" is meant that descendants of the tissue in question differentiate into a unique hierarchy of cell or tissue types. This is a manifestation of the hierarchical, histological sequestration that is the major outcome of development, which is why the pattern can be drawn cladistically (in a treelike diagram of descent and differentiation). Commitments are made at different stages of development; these are not yet well understood, but some late-developing organ systems may already be preprogrammed in cells at very early developmental stages (the cells await a subsequent proper environment to begin elaborating their program).
The formation of these layers involves a process known as gastrulation by which the committed cells take their relative positions. Gastrulation in most animals involves the movement of cells from the outer cell layer to the inner, a process dependent on cell division and migration. Basically, through inpouching, the formation of holes in the outer layers or simply the expansion of dividing cells into the space inside the embryo, a layer forms that will develop into an inner pouch or tube that will become the gut—essentially a tube running through the animal and open to the outside world. Mesodermal cells are caught between the outer ectodermal covering and this inner tube. The higher animals are classified as protostomes (basically, mollusks, worms, insects), and deuterostomes (chordates), depending on how the gut is formed.
Among the most important invertebrate model species is the nematode Caenorhabditis elegans, which develops from a single cell into an adult with a stable 959 cells; lineages of all 959 have been entirely traced through a stereotypical developmental journey. Each organ or structure develops from a specific, very small number of cells, and gastrulation and other basic processes are correspondingly simple.
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