Multimorphic growth cones in the embryonic medicinal leech: Relationship between shape changes and outgrowth transitions

Abstract

Comparative studies of growth cone morphology may provide insight into the mechanisms underlying motility and navigation in vivo. Here we analyzed the morphology of a unique set of growth cones in the embryonic medicinal leech, Hirudo medicinalis. The comb or C-cell is a transient cell found as a bilateral pair in each midbody segment. Early in development, from embryonic day (E)7 to E11, each C-cell adds and orients about 70 parallel growth cones that remain relatively nonmotile until E12 when rapid process outgrowth is initiated. Individual C-cells from E10 to E14 were injected with Lucifer yellow and growth cones were traced with a camera lucida. Growth cone morphology was quantified from the drawings. Lamellar regions increased in area with age and change in extension rate. Young, relatively nonmotile growth cones had numerous short filopodia in many orientations, while at highly motile stages filopodial number decreased, length increased, and orientation became more restricted in the direction of outgrowth. Thus, while filopodia were distributed symmetrically, such that the average filopodial angle was predictive of the direction of outgrowth at all stages, younger (relatively nonmotile) growth cones project more filopodia in many directions than do older more motile growth cones. These results suggest that: (1) alterations in morphology may reflect developmentally regulated changes in extension and the local environment, (2) these growth cones maintain a large area for environmental sampling as they increase extension rate, even as filopodia become more restricted in orientation, and (3) C-cell growth cones might progressively alter their affinity for local cellular cues as they initiate rapid and directed outgrowth. The C-cell of embryonic leech may provide a relatively simple system in which to test these ideas experimentally.

Multiple strategies for directed growth cone extension and navigation of peripheral neurons

“…The extrasensillar neurons still differentiate [ Fig. 7(B, small arrowheads)], but their axonal outgrowth is stunted, without apparent direction, Drawings for B are after Kopp and Jellies ( 1993).] cell (Fig.…”

“…These cells seem to use a space-filling strategy similar to that seen in the tiling of amacrine cells in the vertebrate retina (Jellies and Kristan, 1991), but with the added characteristic of highly directed projections. The growth cones from left and right C cells ignore both segment boundaries and the ventral midline, ultimately crossing over each other to weave a tapestry of obliquely oriented processes (Jellies and Kristan, 199 1 ; Kopp and Jellies, 1993). Although process outgrowth by the C cells, like the peripheral sensory cells, seems to establish a stereotyped set of projections.…”

“…Most of the processes sprout directly from the soma, but a small number are apparently added as branches of existing processes (Jellies and Kristan, 199 1 ;Kopp and Jellies, 1993). The ability to produce branches may be limited to growth cone bifurcation during the initial extension phase ofgrowth because: ( 1) such branches are not found lateral to the nephridiopore (Kopp and Jellies, 1993), (2) branches are not induced by experimentally creating extra body wall territory that the growth cones might extend into (Jellies and Kristan,199 I), and (3) there is a decreasing number of bifurcated growth cones over time, which is inversely related to the increase in branch points on processes (Kopp and Jellies, 1993). Thus, the shaft of the process may be extremely stable and, as a result, C cells might be incapable of producing collateral processes by sprouting except at a region limited to the active growth cone proximal to the soma.…”

Specific pathway selection by the early projections of individual peripheral sensory neurons in the embryonic medicinal leech

“…provide much of the motile machinery for process elongation (Bray and Hollenbeck, 1988;Heidemann and Buxbaum, 1991 ) and mediate navigation by interacting with environmental cues (Letourneau, 1992). Facing a complex and changing embryonic environment, growth cones may use several qualitatively different mechanisms for guidance and use them either simultaneously or in particular spatial and temporal sequences (Caudy and Bentley, 1987;Jellies andKristan, 1988a, 1991;Tosney, 1992;Wolszon and Macagno, 1992;Kopp and Jellies, 1993). In this view, the growth cone is truly a sensory/motor organelle capable of inte-I1 88 Jellies et al grating a variety of signals and executing appropriate responses (Kater and Mills, 1991;Letourneau et al, 1992).…”

A molecular cell biology of lithium

“…Spreading corresponds to periods of sensing and integration of guidance cues from the environment, and can precede a direction change [25]. The cycling between the contracted and spread forms, however, is independent of chemotaxis, and represents an underlying autonomous behaviour of motile growth cones.…”