Ultrastructure of an identified array of growth cones and possible substrates for guidance in the embryonic medicinal leech, Hirudo medicinalis

Abstract

The oblique muscle organizer (Comb- or C-cell) in the embryonic medicinal leech, Hirudo medicinalis, provides an amenable situation to examine growth cone navigation in vivo. Each of the segmentally iterated C-cells extends an array of growth cones through the body wall along oblique trajectories. C-cell growth cones undergo an early, relatively slow period of extension followed by later, protracted and rapid directed outgrowth. During such transitions in extension, guidance might be mediated by a number of factors, including intrinsic constraints on polarity, spatially and temporally regulated cell and matrix interactions, physical constraints imposed by the environment, or guidance along particular cells in advance of the growth cones. Growth cones and their environment were examined by transmission electron microscopy to define those factors that might play a significant role in migration and guidance in this system. The ultrastructural examination has made the possibility very unlikely that simple, physical constraints play a prominent role in guiding C-cell growth cones. No anatomically defined paths or obliquely aligned channels were found in advance of these growth cones, and there were no identifiable physical boundaries, which might constrain young growth cones to a particular location in the body wall before rapid extension. There were diverse associations with many matrices and basement membranes located above, below, and within the layer in which growth cones appear to extend at the light level. Additionally, a preliminary examination of myocyte assembly upon processes proximal to the growth cones further implicates a role for matrix-associated interactions in muscle histogenesis as well as process outgrowth during embryonic development.

Possible role of the receptor protein tyrosine phosphatase HmLAR2 in interbranch repulsion in a leech embryonic cell

“…1(D)], which are established much earlier in development. Other than the CC processes and these muscle layers, few cellular components can be consistently observed in this region (Kopp and Jellies, 1994).…”

“…The oblique orientations require the presence of cues capable of directing the CC lateral growth cones on an anterolateral diagonal course and the medial growth cones in a posteromedial direction. However, serial EM reconstruction of the space in which CC processes grow, between circular and longitudinal muscle layers, failed to reveal any physical substrates that might be candidates to steer the growth cones along their courses (Kopp and Jellies, 1994). Lacking a special substrate, an alternative possibility is that CC growth cones integrate orthogonal cues, either bound to the adjacent muscle layers or present as soluble anteroposterior and mediolateral gradients, to achieve their unusual oblique trajectories.…”

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

“…4). The results from 68 individually filled neurons show that sensillar neuron growth cones bifurcate within the CNS, they project long filopodia capable of sampling the local environment, and all of them choose a single particular CNS fascicle without apparent retraction or realignment of growth cones (Jellies et al, 1994b). Furthermore, each side of the bifurcating afferent growth cones always chose the same rostrally and caudally directed fascicle, implying a specific choice of a distinctly labeled pathway.…”

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

“…The leech Hirudo medicinalis with its accessible embryonic stages and uniquely identifiable neurons, has been used extensively for studies of neuron and muscle development (Jellies and Kristan, 1988a;Baptista et al, 1990;Jellies, 1990;French et al, 1992;Kristan et al, 1993;Thorey and Zipser, 1993;Kopp and Jellies, 1994) as well as for connectivity and regeneration (Van Essen and Jansen, 1977;Elliott and Muller, 1983;Camhi and Macagno, 199 1 ;Muller et al, 1992 ). Leeches possess constellations of peripheral sensory neurons that extend axons into the CNS (Laverack, 1969;Kretz et al, 1976;Gascoigne and McVean, 1993), many of which are clustered in discrete sensilla along the central annulus of each midbody segment (Derosa and Friesen, 1981;Philips and Friesen, 1982;McVean et al, 1990).…”