To identify the cellular cues that guide zebrafish neuronal growth cones to their targets, we examined interactions between identified motor growth cones and identified muscle fibers and tested whether these fibers were required for growth cone navigation. Caudal primary motoneurons (CaPs) and middle primary motoneurons (MiPs) are identified motoneurons that innervate cell-specific regions of the myotome. Growth cones of both cells initially extend along a common pathway and then pause at a set of identified muscle fibers, called muscle pioneers, before diverging along cell-specific pathways. Muscle pioneers are intermediate targets of both CaP and MiP Liu and Westerfield, 1990); both motoneurons extend their growth cones directly to the muscle pioneers on which the first functional neuromuscular contacts form, suggesting that muscle pioneers may provide guidance information to these growth cones. We tested this idea by ablating muscle pioneers and observing the resulting motor axonal trajectories. Both CaP and MiP ultimately formed normal axonal arbors after muscle pioneer ablation, showing that muscle pioneers are unnecessary for formation of correct axonal trajectories; however, although final cellular morphology was correct in the absence of muscle pioneers, MiP growth cones branched abnormally or extended ventrally beyond the common pathway. Ablation of CaP and the muscle pioneers together increased the aberrant behavior of the MiP growth cone. Our results provide evidence that an intermediate target, the muscle pioneers, affects motor axonal extension without altering target choice, suggesting that other cues also contribute to proper pathway navigation. Key words: acetylcholine receptors; neuromuscular junctions; axogenesis; zebrafish motoneurons; muscle pioneers; pathway navigationThe environment through which growth cones navigate contains cues that regulate neuronal pathfinding (Frank and Wenner, 1993;Goodman and Shatz, 1993;Keynes and Cook, 1995). En route to their synaptic targets, neurons may project axons to intermediate targets that influence subsequent pathway choice . For example, some grasshopper pioneer neurons project to guidepost cells (Bentley and Keshishian, 1982) that are required for normal pathfinding (Bentley and Caudy, 1983). Chick hindlimb motoneurons (Lance-Jones and Landmesser, 1981; Tosney and Landmesser, 1985a,b;Landmesser, 1992) and spinal commissural neurons (Bovolenta and Dodd, 1991) also project to specific intermediate targets, which appear to influence extension. These results suggest that intermediate targets serve as choice points for pathway selection.We examined the role of potential intermediate targets in the pathway choices of zebrafish primary motoneurons. We focused on the first two primary motoneurons to extend growth cones out of the spinal cord: C aP and MiP (see Fig. 1) Myers et al., 1986). The C aP growth cone pioneers a common pathway to the nascent horizontal myoseptum, where it pauses before selecting its cell-specific pathway along ventral myotome Myers e...

Substantial progress has been made in understanding the genetic basis of temperature-compensated circadian clocks. Ultradian rhythms, with a period shorter than 24 h, are at least as widespread as circadian rhythms. We have initiated genetic analysis of defecation behavior, which is controlled by an ultradian clock in Caenorhabditis elegans. The defecation motor program is activated every 45 sec, and this rhythm is temperature compensated. We describe mutations in 12 genes that either shorten or lengthen the cycle period. We find that most of these mutations also disrupt temperature compensation, suggesting that this process is an integral part of the clock These genes open the way for molecular genetic dissection of this ultradian clock

No abstract

To understand how life works, it is essential to understand physics and chemistry. Physics informs and enlightens biology in myriad dimensions, yet many biology courses proceed with little or no consideration of physical properties or principles. The intersection between physics and biology is explored in this review of online media.