6533b82ffe1ef96bd1295998

RESEARCH PRODUCT

Spatial and Temporal Expression Patterns in Animals

subject

description

All metazoan animals comprise a body plan of different complexity. Since it is well established—especially on the basis of molecular and cell biological data—that all metazoan phyla, including the Porifera (sponges) evolved from a common ancestor, the fundamental common principles of pattern formation in Metazoa have been studied to a great extent. Common to all metazoan body plans is the formation of at least one axis that runs from the apical to the basal region; examples for this organization step are the Porifera and the Cnidaria (diploblastic animals). Animals belonging to the triploblasts, the Protostomia (model systems: insects and nematodes) and the Deuterostomia (mammals), comprise in addition to the anteroposterior axis also a dorsoventral axis that runs at a right angle to the first. Body plan formation starts from totipotent germ cells that lose their plastic differentiation propensity during their proliferation/differentiation. In the course of the developmental processes, a differential expression of cell–cell and cell–matrix adhesion molecules proceeds, which also involves the formation of cell junction molecules. After formation of tissue units, immune molecules are expressed, which allow the discrimination between self and nonself (establishment of the individuality), a process that is also controlled by apoptotic regulatory systems. Subsequently, extracellular solute factors (morphogens) and other secreted molecules in concert with transcription factors establish the axes. Organizer regions are formed which are localized in diploblasts (Porifera and Cnidaria) in the oscule or mouth region, respectively. In triploblasts, as it is well documented in the vertebrate Spemann organizer or Hensen's node, cells are formed in a distinct region, which displays the property to induce a complete body axis. In parallel, skeleton formation (exo- or endoskeleton) supports the body construction. The major difference in the genetic systems controlling body plan patterning between diploblasts and triploblasts lies in the fact that only the triploblasts (both Protostomia and Deuterostomia) comprise clusters of homeotic genes. In diploblasts, those clusters are missing and nonclustered paired-class genes and LIM/homeodomain genes control pattern formation. In metazoans, in diploblasts as well as in triploblasts, body plan formation starts by polarization of the embryo; the subsequent development is followed in diploblasts only to the stage of serial module patterning, while in triploblasts, the development to the adults includes compartmental and the subsequent segmental patterning. Keywords: Axis; Bilateria; Body Plan; Diploblasts; Metazoa; Morphogenesis; Pattern; Porifera (Sponge); Segments; Serial Modules; Somites; Triploblasts