Muscle assembly in simple systems.

Muscle development has been the subject of intense scrutiny at cellular, biochemical and molecular levels, yet little is known about the factors that generate anatomically distinct muscles during embryogenesis. We now know that at least some muscles are initially organized by interactions with particular cells that appear early in development, the muscle organizers. These organizers have been described in both arthropods and annelids, and serve similar functions in both groups: they provide cellular scaffolding during the early and relatively simple anatomical stages of embryogenesis, which is then used to pattern the assembly of large numbers of pre-myocytes. Thus, single cells provide an early framework that is retained even as the embryo becomes vastly more complex. Furthermore, studies have shown that the muscle organizer is necessary for motor neuron growth cones to locate (or recognize) the appropriate target region. In other words, the motor neurons extend toward muscles during muscle histogenesis and can use the muscle organizer for guidance, rather than the definitive muscle which has not yet emerged. The discovery of these identifiable tissue organizers has opened several intriguing avenues by which the roles of cell-cell interactions in development can be further addressed. Additionally, the discovery of these cells implies that in order to understand the ways in which motor neurons are initially matched to particular muscle targets we should also study the organizers that may provide positional introductions between future synaptic partners.