The central-peripheral transition zone of cervical spinal nerve roots in Jimpy mutant and normal mice. Light- and electron-microscopic study

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.

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)

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

The morphology of the CNS-PNS transitional zone of adult rat dorsolateral vagus nerve rootlets is uniquely complex. A typical rootlet contains a transitional zone over 300 microns long, consisting of a central tissue projection extending distally into each rootlet and a peripheral tissue insertion extending for a longer distance deep into the brainstem. The peripheral tissue insertion is continuous with the peripheral tissue of the free rootlet through channels traversing or running parallel to the central tissue projection. Accordingly, the vagal CNS-PNS interface is topologically much more complex than that found elsewhere. In some rootlets the peripheral tissue in the brainstem constitutes an isolated island deep within the neuraxis. In others, peripheral continuity is established only through a cross connection with the peripheral tissue insertion of a neighbouring rootlet. About one fifth of all vagal myelinated axons alternate between the CNS and PNS tissue compartments. This distinguishes the vagus from all other nerves studied to date. These axons are myelinated by Schwann cells distal to the transitional zone, by oligodendrocytes in the central tissue projection and by one or more short intercalated Schwann internodes further centrally, mostly in the peripheral tissue insertion, where their perikarya commonly form closely apposed aggregates. More than four fifths of all unmyelinated axon bundles alternate between central and peripheral tissue compartments, commonly more than once. In the peripheral tissue insertion axons are enveloped by series of non-myelinating Schwann cells. Schwann processes commonly extend for over 50 microns into the central compartment at each central-peripheral transition. Around one fifth of peripherally unmyelinated axons have an oligodendrocytic sheath in the central compartment. Of these axons possessing more than one intercalated Schwann internode, over one quarter display alternation of myelinated and unmyelinated segments in the peripheral tissue insertion. Astrocytes in the transitional zone segregate PNS tissue, a role played by sheath cells further peripherally in the vagal rootlets. Astrocytes form the surface limiting membranes of the central tissue projection and the barrier between the peripheral tissue insertion and the surrounding brainstem. The barrier consists only of an attenuated layer of processes. This is deficient in places, where oligodendrocytic myelin sheaths are directly exposed to the endoneurial space of the peripheral tissue insertion and in some instances are apposed to myelinating or non-myelinating Schwann cells. Such communication between the central and peripheral compartments is unique to the vagal transitional zone. The findings are consistent with a range of possible events during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Schwann cell myelination of the myelin deficient rat spinal cord following X-irradiation

The myelin-deficient (md) rat is an X-linked myelin mutant that has an abnormality of oligodendrocytes and a severe paucity of myelin throughout the CNS. This lack of myelin makes it an ideal model in which to study the cellular interactions that occur when "foreign" myelinating cells are induced in the milieu of this nonmyelinated CNS. In this study, Schwann cells were induced in the lumbosacral spinal cord by exposing it to radiation, a technique demonstrated repeatedly in other nonmutant strains of rats. Md rats and their age-matched littermates were irradiated (3,000 to 4,000 R) at 3 days of age and perfused 16-22 days later after pulse labeling with tritiated thymidine. In the md rat, Schwann cell invasion progressed from the area of the spinal cord-nerve root junction and extended into the dorsal columns and adjacent gray matter. Autoradiographic evidence revealed that many of these cells incorporated 3H-thymidine, indicating that they were undergoing proliferation. Ultrastructural observations showed that there was an integration of these intraspinal Schwann cells with the cells normally occurring in this environment, i.e., oligodendrocytes and astrocytes. The extent of migration and division of Schwann cells, as well as their interactions with glial cells, were similar to those seen in the nonmutant irradiated littermates. These studies provide conclusive evidence that md rat axons are normal with respect to their ability to provide trophic and mitogenic signals to myelinating cells.