Intermingling of central and peripheral nervous tissues in rat dorsolateral vagal rootlet transitional zones

Evolutionary and developmental understanding of the spinal accessory nerve

The vertebrate spinal accessory nerve (SAN) innervates the cucullaris muscle, the major muscle of the neck, and is recognized as a synapomorphy that defines living jawed vertebrates. Morphologically, the cucullaris muscle exists between the branchiomeric series of muscles innervated by special visceral efferent neurons and the rostral somitic muscles innervated by general somatic efferent neurons. The category to which the SAN belongs to both developmentally and evolutionarily has long been controversial. To clarify this, we assessed the innervation and cytoarchitecture of the spinal nerve plexus in the lamprey and reviewed studies of SAN in various species of vertebrates and their embryos. We then reconstructed an evolutionary sequence in which phylogenetic changes in developmental neuronal patterning led towards the gnathostome-specific SAN. We hypothesize that the SAN arose as part of a lamprey-like spinal nerve plexus that innervates the cyclostome-type infraoptic muscle, a candidate cucullaris precursor.

Initial motor axon outgrowth from the developing central nervous system

Rat and chick studies show that the earliest motor rootlet axon bundles emerge from all levels of the neural tube between radial glial end feet which comprise the presumptive glia limitans. The loose arrangement of the end feet at the time of emergence facilitates this passage. The points of emergence are regularly spaced in relation to the long axis of the neural tube and are not defined by any cell contact with its surface. Each rootlet carries a covering of basal lamina from the neural tube surface, which forms a sleeve around it. It is only after bundles of ventral rootlet axons have emerged that cells associate with them, forming clusters on the rootlet surface at a distance peripheral to the CNS surface of both species. A tight collar of glial end feet develops around the axon bundle at the neural tube surface shortly after initial emergence. These arrangements are in sharp contrast to those seen in the sensory rootlets, where clusters of boundary cap cells prefigure the sensory entry zones at the attachments of the prospective dorsal spinal and cranial sensory rootlets. Boundary cap cells resemble cluster cells and a neural crest origin seems the most likely for them. The study clearly demonstrates that no features resembling boundary caps are found in relation to the developing motor exit points.

Comparative postnatal development of spinal, trigeminal and vagal sensory root entry zones

Somatic and visceral sensory information enters the central nervous system (CNS) via root entry zones where sensory axons span an environment consisting of Schwann cells in the peripheral nervous system (PNS) and astrocytes and oligodendrocytes in the CNS. While the embryonic extension of these sensory axons into the CNS has been well-characterized, little is known about the subsequent, largely postnatal development of the glial elements of the root entry zones. Here we sought to establish a comparative developmental timecourse of the glial elements in the postnatal (P0, P3, P7, P14) and adult rat of three root entry zones: the spinal nerve dorsal root entry zone, the trigeminal root entry zone, and the vagal dorsal root entry zone. We compared entry zone development based on the expression of antigens known to be expressed in astrocytes, oligodendrocytes, oligodendrocyte precursor cells, Schwann cells, radial glial fibres and the PNS extracellular matrix. These studies revealed an unexpected distribution among glial cells of several antigens. In particular, antibodies used to label mature oligodendrocytes (RIP) transiently labelled immature Schwann cell cytoplasm, and a radial glial antigen (recognized by the 3CB2 antibody) initially decreased, and then increased in postnatal astrocytes. While all three root entry zones had reached morphological and antigenic maturity by P14, the glial elements comprising the PNS-CNS interface of cranial root entry zones (the trigeminal root entry zone and the vagal dorsal root entry zone) matured earlier than those of the spinal nerve dorsal root entry zone.

Peripherally-derived olfactory ensheathing cells do not promote primary afferent regeneration following dorsal root injury

Olfactory ensheathing cells (OECs) may support axonal regrowth, and thus might be a viable treatment for spinal cord injury (SCI); however, peripherally-derived OECs remain untested in most animal models of SCI. We have transplanted OECs from the lamina propria (LP) of mice expressing green fluorescent protein (GFP) in all cell types into immunosuppressed rats with cervical or lumbar dorsal root injuries. LP-OECs were deposited into either the dorsal root ganglion (DRG), intact or injured dorsal roots, or the dorsal columns via the dorsal root entry zone (DREZ). LP-OECs injected into the DRG or dorsal root migrated centripetally, and migration was more extensive in the injured root than in the intact root. These peripherally deposited OECs migrated within the PNS but did not cross the DREZ; similarly, large- or small-caliber primary afferents were not seen to regenerate across the DREZ. LP-OEC deposition into the dorsal columns via the DREZ resulted in a laminin-rich injection track: due to the pipette trajectory, this track pierced the glia limitans at the DREZ. OECs migrated centrifugally through this track, but did not traverse the DREZ; axons entered the spinal cord via this track, but were not seen to reenter CNS tissue. We found a preferential association between CGRP-positive small- to medium-diameter afferents and OEC deposits in injured dorsal roots as well as within the spinal cord. In the cord, OEC deposition resulted in increased angiogenesis and altered astrocyte alignment. These data are the first to demonstrate interactions between sensory axons and peripherally-derived OECs following dorsal root injury.

Spontaneous functional viscerosensory regeneration into the adult brainstem

The dorsal root entry zone of the vagus nerve (vDREZ) is uniquely characterized by peripheral tissue insertions (PTIs) deep to the brainstem surface, consisting of Schwann cells and a reticulum of astrocytic processes. Because Schwann cells permit peripheral axonal regeneration, the capacity of vagal medullary PTIs to allow centripetal regeneration of visceral afferents after vagal dorsal rhizotomy in adult rats was investigated. The present work shows that vagal axons spontaneously regenerate into appropriate and ectopic brainstem nuclei. They accomplish this by first growing along PTIs but then extend along basal laminas of medullary blood vessels. Electrically stimulated regenerated vagal afferents induced Fos expression (indicating functional connectivity) within appropriate but not ectopic nuclei. The unique structure of the vDREZ can thus support spontaneous functional regeneration of visceral primary afferent axons into the adult CNS.

Axons and glial interfaces: ultrastructural studies

At most vertebrate nerve transitional zones (TZs) there is a glial barrier which is pierced by axons passing between the CNS and PNS. Myelinated axons traverse this in individual tunnels. The same is true of larger non-myelinated axons. This holds widely among the vertebrates, for example, the large motor axons of the sea-lamprey Petromyzon (which also possess TZ specializations not found in mammals). Smaller non-myelinated axons traverse the TZ glial tunnels as fascicles and so the barriers are correspondingly less comprehensive for them. Accordingly, in nerves composed of non-myelinated axons, such as the vomeronasal or the olfactory, a TZ barrier stretching across the nerve is effectivelyabsent. The chordateAmphioxus differsfrom the vertebrates in lacking a TZ barrier throughout. Invertebrates also lack glial barriers at the TZs between ganglia and interconnecting nerve trunks. The glial barrier at the dorsal spinal root TZ (DRTZ) has considerable value for analysing protocols aimed at achieving CNS regeneration, because it provides a useful model of the gliotic reaction at sites of CNS injury. Also, it is especially amenable to morphometric analysis, and so enables objective quantification of different protocols. Being adjacent to the subarachnoid space, it is accessible for experimental intervention. The DRTZ was used to investigate the value of neurotrophin 3 (NT3) in promoting axon regeneration across the TZ barrier and into the CNS following dorsal root crush. It promoted extensive regeneration and vigorous non-myelinated axonal ensheathment. On average, around 40% of regenerating axons grew across the interface, compared with virtually none in its absence. These may have traversed the interface through loci occupied by axons prior to degeneration. Many regenerating axons became myelinated, both centrally and peripherally.

Schwann cell invasion of the central nervous system of the myelin mutants

Schwann cells are excluded from the CNS during development by the glial limiting membrane, an area of astrocytic specialisation present at the nerve root transitional zone, and at blood vessels in the neuropil. This barrier, however, can be disrupted and, with the highly migratory nature of Schwann cells, can result in their invasion and myelination of the CNS in many pathological situations. In this paper we demonstrate that this occurs in a number of myelin mutants, including the myelin deficient (md) and taiep rats and the canine shaking (sh) pup. While it is still relatively uncommon in the rodent mutants, the sh pup shows extensive Schwann cell invasion along the neuraxis. This invasion involves the spinal cord, brain stem, and cerebellum and increases in amount and distribution with age. In situ hybridisation studies using a Pzero riboprobe suggest that the likely origin of these cells in the sh pup is the nerve roots, primarily the dorsal roots. Paradoxically, Schwann cell myelination of the CNS increases with time in the sh pup despite a marked, progressive gliosis involving the glia limitans and neuropil. Thus the mechanism by which these cells migrate into the CNS through the gliosed nerve root transitional zone or from vasa nervorum remains unknown. Extensive Schwann cell CNS myelination may have therapeutic significance in human myelin disease.

Node distribution and packing density in the rat CNS-PNS transitional zone

The density of nodes of Ranvier was examined at CNS, PNS, and transitional zone (TZ) levels of rat lumbar ventral motoneurone fibres. It was found to be significantly greater in the TZ than at the other levels: The difference was sevenfold for the ventral root and at least fourfold for central fibre levels. Node distribution and spacing was examined within the two main types of TZ found in rat ventral rootlets: the first, in which the TZ is short and is approximately on a level with the surface of the cord; and the second, in which it is much longer and extends into the proximal part of the rootlet. Node spacing was estimated as nearest neighbour distance, the true distance between adjacent node centres. This is a better estimate of node spacing than simple density since it measures the actual linear distance between nodes over which any interaction between them would be likely to take place. Despite marked differences in the dimensions of the two types of TZ, nearest neighbour distance distribution was very similar in each, suggesting that similar mechanisms may influence their spacing during development. The TZ contains especially large amounts of interstitial tissue, mainly composed of astrocyte processes, separating the fibres traversing it. The proportion of the TZ composed of interstitium was over three times that in the ventral root and nearly twice that at the CNS level studied. The large amounts of astrocytic tissue in the TZ may be related to the high packing density of nodes. It may function to regulate extracellular ionic concentrations in the TZ and to maintain a stable ionic environment for the transitional nodes.

The CNS-PNS transitional zone of the rat. Morphometric studies at cranial and spinal levels

The transitional zone is that length of rootlet containing both central and peripheral nervous tissue. The CNS-PNS interface may be defined as the basal lamina covering the intricately interwoven layer of astrocyte processes which forms the CNS surface and which is pierced by axons passing between the CNS and PNS. Study of transitional zone development defines morphologically the growth, relative movement and interaction of central and peripheral nervous tissues as they establish their mutually exclusive territories on either side of the CNS-PNS boundary, and helps to explain the wide variations in the form of the mature transitional zone. Nerve rootlets at first consist of bundles of bare axons. These become segregated by matrices of fine Schwann cell processes peripherally and of astrocyte processes centrally. The latter may prevent Schwann cell invasion of the CNS. Astrocyte processes branch profusely and come to form the principal central nervous tissue component of the transitional zone. Developmental changes in the transitional zone vary markedly between nerves, reflecting differences in its final morphology. Widespread relative movements and migration of CNS and PNS tissues take place during development, so that the central-peripheral interface changes shape and position, commonly oscillating along the proximodistal axis of the rootlet. For example, developing cervical ventral rootlets contain a transient central tissue projection, while that of lumbar ventral rootlets and to a lesser extent that of cervical dorsal rootlets alternately increase and decrease in length. In the developing cochlear nerve, a central tissue projection is present before birth, but regresses somewhat before a marked outgrowth of central nervous tissue along the nerve takes place, which reaches into the modiolus during the first week postnatum. During development, some astrocytic tissue may even break off and migrate distally into the root, giving rise to one or more glial islands within it. During the period immediately preceding birth, Schwann cells come to be present in very large numbers in that part of the rootlet immediately distal to the CNS-PNS interface, the proximal rootlet segment. Here they form prominent sleeves or clusters of closely packed cells which intertwine with and encapsulate one another on the rootlet surface. Such Schwann cell overcrowding in the proximal rootlet segment could result in part from distal overgrowth of the rapidly expanding CNS around axon bundles, which might strip the Schwann cells distally off the bundle segments so engulfed.(ABSTRACT TRUNCATED AT 400 WORDS)

Myelin-axon relationships established by rat vagal Schwann cells deep to the brainstem surface

The central-peripheral transitional zones of rat dorsolateral vagal rootlets are highly complex. Peripheral nervous tissue extends centrally for up to several hundred micrometers deep to the brainstem surface along these rootlets. In some instances this peripheral nervous tissue lacks continuity with the peripheral nervous system (PNS) and so forms an island within the central nervous system (CNS). In conformity with the resulting complexity of the CNS-PNS interface, segments of vagal axons lying deep to the brainstem surface are myelinated by one or more intercalated Schwann cells, contained in peripheral tissue insertions or islands, at either end of which they traverse an astroglial barrier. Intercalated Schwann cells are thus isolated from contact or contiguity with the Schwann cells of the PNS generally. They are short, having a mean internodal length of around 60% of that of the most proximal Schwann cells of the PNS proper, which lie immediately distal to the CNS-PNS interface and which are termed transitional Schwann cells. The thickness of the myelin sheaths produced by intercalated Schwann cells is intermediate between that of transitional Schwann cells and that of oligodendrocytes myelinating vagal axons of the same calibre distribution. This is not due to limited blood supply or to insufficient numbers of intercalated Schwann cells, the density of which is greater than that of transitional Schwann cells. These factors are unlikely to restrict expression of their myelinogenic potential. Nevertheless, the regression data show that the setting of the myelin-axon relationship differs significantly between the two categories of Schwann cell. Thus, the myelinogenic response of Schwann cells to stimuli emanating from the same axons may differ between levels along one and the same nerve bundle. Mean myelin periodicity was found to differ between sheaths produced by intercalated and by transitional Schwann cells.