Nonbinary branching of myelinated dendrites at nodal networks on afferent terminal arbors in paddlefish electroreceptors

Abstract

The branching of vertebrate neuronal processes, axons or dendrites, is predominantly binary, where a proximal neuronal process bifurcates into two distal progeny. Here, we illustrate counterexamples where neuronal processes branched into more than 2, up to 6, progeny branches, in one step. These were branch points of myelinated dendrites in the peripheral terminals of ALLn cranial nerve afferents innervating the Lorenziniantype ampullary electroreceptors on the rostrum of paddlefish, Polyodon spathula. We imaged afferent terminals fluorescently (in widefield stacks, with deconvolution) after immunolabling of afferent terminals for neuronal cytoplasmic neurofilament-H (NEFH), myelin markers (MBP, P0), or nodal ion channels (Nav1.x, Kv1.1), or after migration of DiI in dendrite membranes. Branched afferent terminals formed a laminar radial radiation beneath a receptive field, parallel to the skin surface, with two serial stages: (1) Starting at each afferent's centric first branchpoint, each of a small group of 2-4 (most often 3) afferents branched radially into 2-4 (usually 3) generations of myelinated dendrites, whose internodes were covered by sheaths immunoreactive for antigenic markers of myelin (MBP+/P0+), ending distally at~15 heminode presumed spike initiation zones. (2) From the latter, bundles of unmyelinated (MBP-/P0-) sensory neuron (NEFH+) processes projected distally to innervate electrosensory neuroepithelia of adjacent ampullary organs. The nonbinary branch points that we imaged were in stage-1, on myelinated dendrites. Branch points always coincided with composite systems of nodes, in which each progeny dendrite started at a narrowed nodal segment expressing voltage gated sodium ion channels at high density. These narrowed nodal segments of multiple progeny branched in parallel from a parent's blunt distal end. Hence the branch point nodal complexes formed multisite excitable systems, likely strongly coupled due to proximity. 1 + 2 where N varies from 1.5 to 2 corresponding to symmetrical d/D ratios of 0.63-0.71 [2].

Large-Scale Convergence of Receptor Cell Arrays Onto Afferent Terminal Arbors in the Lorenzinian Electroreceptors of Polyodon

“…The sheaths had crenulated surfaces (Figure 11B), with a surface lamina revealed by binding of WGA or peanut agglutinin lectins. Dark transverse striations ( Figures 7G, 11B) were attributed to melanin (Russell et al, 2019). Proof that dendritic branches were functionally myelinated would require EM demonstration of compact myelin wrappings, as at other afferent terminals (Quick et al, 1979).…”

“…(G) DiI tracing of distal afferent branches by transcellular retrograde diffusion of lipophilic DiI, from small crystals placed on the EN of two adjacent ampullary organs. Migrating proximally, DiI tracing revealed candidate terminal glia (tg), sites where myelination began at presumed heminodes (h), and third-or fourth-generation branch points (3 • bp, 4 • bp) of myelinated dendrites (md), which appeared banded due to external melanin (Russell et al, 2019) were due to visible pairs or fascicles of fibers. Projection bundles followed approximately direct paths to AOs (Figure 7), but often the projection bundles to adjacent AOs could form a "Y" shape ( Figure 7B), diverging at midlength but not branching there.…”

“…The branch points of myelinated dendrites always corresponded to compound nodes of Ranvier (Quick et al, 1979;Russell et al, 2019), typically with narrowed Nefh+ processes (arrow, Figure 11A). Many of the branch points of myelinated arbors were binary, that is, with two progeny branches.…”

“…Many of the branch points of myelinated arbors were binary, that is, with two progeny branches. However, non-binary branch points with three or more progeny myelinated neuronal processes were observed routinely (Russell et al, 2019). Their ≥3 progeny branches arose in one step, not as serial bifurcations.…”

“…Another useful identifier of dendritic nodes (inline or at branch points) was a local region of reduced Nefh immunoreactivity ( Figure 11A) (Russell et al, 2019). This low FIGURE 11 | Myelinated afferent arbors of one electroreceptor's large multipart receptive field on the base of the rostrum.…”