Axon-glial relationships in the anterior medullary velum of the adult rat

Abstract

The anterior medullary velum is a thin sheet of CNS tissue which roofs the rostral part of the IVth ventricle and contains fascicles of myelinated fibres which, in part, arise from the nucleus of the IVth cranial nerve. This study used histochemical, immunohistochemical, and intracellular dye-injection techniques to describe cellular interrelationships in the velum in whole-mounts and in sections. Rip antibody-stained whole mounts provided a unique description of both oligodendrocyte units (defined as an oligodendrocyte and the complement of myelinated internodal segments it forms), and consecutive myelin sheaths along the same axon. A broad range of unit morphologies was categorised into four arbitrary groups, according to classical criteria, which comprised small cells supporting the short, thin myelin sheaths of 15-30 small diameter axons (Type I), through intermediate types (II & III), to the largest cells forming the long, thick myelin sheaths of 1-3 large diameter axons. Rip antibody and ferric ion-ferrocyanide staining, together with intracellular dye injection, revealed oligodendrocyte process branching patterns and their mode of engagement of myelin sheaths, nodes of Ranvier, and the spatial disposition of the outer cytoplasmic rims of myelin sheaths. The latter formed a conspicuous spiral ridge on the exterior surface of myelin sheaths which connected with the paranodal loops at each heminode. Large bundles of axons decussated through the velum, the bulk of which were IVth nerve fibres which constituted the IVth nerve rootlet. The PNS/CNS transitional zone of the IVth nerve was located 0.25-0.50 mm along the root, where astrocytic end-feet defined an abrupt margin, convex towards the periphery, where the heminodes of central and peripheral myelin were apposed, and where the basal lamina tubes of the Schwann cell units were discontinued. The basal processes of ependymal cells lining the ventricular wall of the velum, passed between axon bundles before abutting on the basal lamina of the pia. Many of these processes branched and ran along the axonal bundles. A monolayer of microglia occupied a subependymal stratum in which the non-overlapping dendritic territories of each cell formed a regular mosaic throughout the velum without any obvious interaction with either axons or other glial cells. Astrocytes were also uniformly distributed; their fine processes made up a dense lattice amongst axons, often running parallel and within the fibre bundles; stouter ones had terminal end-feet which undercoated the basal lamina of both the glia limitans externa and the blood vessels in the velum.

Neurones in the adult rat anterior medullary velum

“…Neuronal populations may have previously been overlooked because of these studies concentrating mainly on the IVth tract region (Sliney et al, 1981), where in this present study, very few neuronal somata were in fact observed. Also, the smaller size of the neurones described here (all neuronal diameters between 6 and 15 m) compared with the larger unipolar cells both in this study (45 m) and those of Berry et al (1995) may have been another factor. Studies are now under way to assess possible cell-cell interactions and circuitry in the adult rat AMV.…”

“…Brains were placed in fixative at 4°C for 1 hour and then placed in PBS for 1 hour at room temperature. Brains were then transferred to a Petri dish containing PBS, and under a binocular-scope, the AMV were dissected free from the inferior colliculi and cerebellum (see Berry et al, 1995).…”

“…It is situated between the inferior colliculi and cerebellum, caudal to the aqueduct of Sylvius, and extends from the posterior opening of the mid-brain aqueduct to the superior recess of the IVth ventricle (Garcia et al, 1983;McConnell et al, 1984;Derouiche et al, 1994;Berry et al, 1995).…”

“…In adult AMV, both myelinated and unmyelinated fibres have been demonstrated (McConnell et al, 1984;Hutchinson and McConnell, 1990;Derouiche et al, 1994. Glial aspects identified within the AMV include ependymal cells lining the IVth ventricle (Raine and Dolich, 1986;Berry et al, 1995), radial glia in the AMV of adult monkeys (Reichenbach, 1990), and more recently astrocytes and classically described subtypes (I-IV) of oligodendrocytes , 1997. However, neuronal populations were initially thought to be absent from the rat AMV (Sliney et al, 1981) or have occasionally been identified as large unipolar cells by using the neurofilament antibody RT97, with a suggestion that they may represent displaced trigeminal sensory neurones from the mesencephalic nuclei .…”

Thrombomodulin as a New Marker of Lesion-Induced Astrogliosis: Involvement of Thrombin through the G-Protein-Coupled Protease-Activated Receptor-1

“…Lesions were placed in the anterior medullary velum (AM V) (Berry et al, 1995) of Avertin-anesthetized, adult Wistar rats using a retractable, curved needle, guarded by a trochar, devised specifically for this purpose. With the needle withdrawn into the trochar, the instrument was introduced through the posterior atlanto-occipital membrane into the IVth ventricle until the tip of the instrument lay in the most rostral portion of the ventricle.…”