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

Abstract

In leech, the central annulus of each midbody segment possesses seven pairs of sensilla, which are mixed clusters of primary peripheral sensory neurons that extend their axons into the CNS where they segregate into distinct fascicles. Pathway selection by individual afferent growth cones of sensillar neurons was examined by double labeling using intracellular dye-filling with antibody labeling in early Hirudo medicinalis embryos. The monoclonal antibody Lan3-2 was used because sensillar neuronal tracts are specifically labeled by this antibody. Examining 68 individually filled neurons we found that sensillar neuron growth cones bifurcate within the CNS, that they project long filopodia capable of sampling the local environment, and that all of them appeared to choose a single particular CNS fascicle without apparent retraction or realignment of growth cones. Furthermore, each side of the bifurcating afferent growth cones always chose the same fascicle, implying a specific choice of a distinct labeled pathway. By dye-filling individual central neurons (P-cells), we show that there are centrally projecting axons present at the time sensillar afferents enter the ganglionic primordia and select a particular fascicle, and we confirm that at least the dorsal peripheral nerve is likely to be pioneered by central neurons, not by the peripheral afferents. In the sensillum studied here, we found examples of sensory neurons extending axons into one of all the available fascicles. Thus, an individual embryonic sensillum possesses a heterogeneous population of afferents with respect to the central fascicle chosen. This is consistent with the idea that segregation into distinct axon fascicles may be based upon functional differences between individual afferent neurons. Our findings argue strongly in favor of specific pathway selection by afferents in this system and are consistent with previous suggestions that there exists a hierarchy of cues, including surface glycoconjugates that mediate navigation of the sensillar growth cones and the fasciculation of their axons.

Initial formation and secondary condensation of nerve pathways in the medicinal leech

“…The dorsal P cell (PD) projects ipsilaterally within the DP nerve, whereas the ventral P cell (Pv) projects two peripheral axons, one in the anterior nerve root. Although we had previously shown that PD established the DP pathway, intracellular dye filling of Pv (which does send one axon in the anterior nerves) was more ambiguous, and its anterior peripherally directed axon was always coextensive with the ingrowing Lan3-2-positive S3 axons at the stages examined (Jellies et al, 1994b). Our present study confirmed that, although outgrowth in the MA nerve can occur early, it is preceded by ingrowth of S3 peripheral growth cones.…”

Cell interactions that affect axonogenesis in the leechTheromyzon rude

“…7A,B), where it is thought to pioneer this nerve tract (Kuwada, 1985;Jellies et al, 1994). We therefore speculated that the failure of this migration could lead to the MA/PP nerve coalescence (Fig.…”

Detection and selective avoidance of near ultraviolet radiation by an aquatic annelid: the medicinal leech

“…Leech photoreceptors are phaosomal primary receptor neurons that depolarize in response to light and produce action potentials that are conveyed into the central nervous system (CNS) (Fioravanti and Fuortes, 1972;Lasansky and Fuortes, 1969;Peterson, 1984a). Axons from the sensilla have also been shown to segregate into discrete and highly stereotyped fascicles in the CNS that might correspond to different sensory modalities (Jellies et al, 1994). The cephalic eyes in the head appear to replace several of the segmental sensilla, some of which persist (Mann, 1962;Sawyer, 1986) and may contribute to the activity recorded from nerves.…”