Subpial Space Research Articles

The meningeal compartments of the median eminence and the cortex. A comparative analysis in the rat.

The intervascular segments of the leptomeninges of the rat were studied by the use of horseradish peroxidase (HRP) in short-term experiments. HRP was injected (i) intravenously, (ii) into the lateral ventricle, (iii) into the cortex, and (iv) into the meninges. The composition of the meninges covering the median eminence (ME) was analyzed in comparison to the results obtained with the parietal cortex. The meninges covering the cortex show the following pattern of layers and compartments: The intercellular compartment comprises the intercellular clefts of the neuropil, the subpial space, and the intercellular clefts of the leptomeninges. The pial space establishes a second compartment. The third compartment is the arachnoid space. The intercellular clefts of the dura form the fourth compartment. At the border of the ME, the neurothelium and the outer arachnoid layer are rolled up to form a tissue frame around a hollow pit that is covered by a diaphragm consisting of meningeal cells; the latter separate the hemal milieu of the ME from that of the dura. The hemal and the cerebrospinal fluid (CSF) milieus may communicate to a limited extent only within the subpial space adjacent to the ME. The CSF-containing compartments of the pial and arachnoid spaces terminate at the brain-facing insertion of the tissue frame. According to the present results, an anatomical basis for a short-loop feedback from and to the neurohemal region of the ME via the CSF does not exist.

Electron microscopic immunocytochemical localization of tyrosine hydroxylase in the area postrema of rat

The ultrastructural morphology and specialized neuronal, vascular, and ventricular associations of tyrosine hydroxylase-labeled neurons are examined within the area postrema of rat brain. Specific antiserum to the purified enzyme is localized throughout the rostrocaudal and dorsoventral extent of the area postrema by means of the peroxidase-antiperoxidase technique. In all regions, peroxidase immunoreactivity for tyrosine hydroxylase is distributed throughout the cytoplasm of selectively labeled neuronal perikarya and processes. The perikarya contain a large nucleus, infolded nuclear membrane, numerous cytoplasmic organelles, and form axosomatic synapses with unlabeled terminals. The majority of the labeled processes are dendrites, which contain ribosomes, microtubules, mitochondria, and scattered vesicles. These dendrites are postsynaptic to unlabeled axon terminals and show membrane specializations with other labeled dendrites and perikarya. In contrast to dendrites, peroxidase-labeled profiles clearly distinguished as axons or axon terminals are sparse and never show membrane specializations with other neuronal or nonneuronal structures within the area postrema. Numerous large processes which could be either axons or dendrites are associated with blood vessels and the ventricular surface of the area postrema. With respect to blood vessels, processes are located either in direct apposition to the external glial membrane, or less frequently, within the perivascular space. The ventricular processes are either associated with blood vessels in the subpial space or distributed among the cilia and villi at the anterior margins of the area postrema. The neuronal and nonneuronal associations of the tyrosine hydroxylase-labeled processes are consistent with a receptor or chemosensor function for catecholamines in this circumventricular organ.

The distribution of orthogonal arrays and their relationship to intercellular junctions in neuroglia of the freeze-fractured hypothalamo-neurohypophysial system.

Using freeze-fracture techniques, we have investigated membrane specializations of the glia associated with the hypothalamo-neurohypophysial system of the rat. In the paraventricular (PVN) and supraoptic (SON) nuclei, astrocytes in areas of high neuronal density (i.e., magnocellular regions) display orthogonal arrays of 6--7 nm particles solely near gap junctions, while astrocytes in areas of lower neuronal density (i.e., parvocellular regions), contain additional arrays in membranes not displaying gap junctions. Arrays are especially numerous on astrocytic perivascular end-feet in both nuclei and in the laminations of the pial-glial limitans ventral to the SON. Ependymal cells near the PVN show arrays both on their lateral surfaces (displaying gap junctions) and on their apical surfaces (facing the CSF). Tight junctions are not noted on astrocytes or ependymal cells, but are noted on both the somas and myelin lamellae of oligodendroglia. Both of these latter membranes occasionally contain gap junctions as well; however, orthogonal arrays are never noted on oligodendroglia. The plasma membranes of pituicytes in the neurohypophysis display gap junctions, complex junctions, and tight junctions. Orthogonal arrays are noted near the first two of these, but not near the last. Arrays in the neutral lobe appear most dense on membranes adjacent to subpial or perivascular spaces. Pituicyte membranes containing orthogonal arrays appear infrequently near the neural stalk, increasing towards the distal end of the neural lobe. The distribution of orthogonal arrays in this system, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.

The filum terminale of the frog spinal cord

The filum terminale, or terminal portion of the spinal cord, was studied in normal adult frogs (Rana pipiens) bu means of light and electron microscopy. Astroglial cells are the predominant elements in this region. The rostral portion of the filum terminale consists mainly of (1) a peripheral dense ring of myelinated and some unmyelinated nerve fibers, and processes of astrobytes terminating at the subpial space; (2) an intermediate zone, in which astrocytes are the main cellular elements in addition to a few degenerated neurons; and (3) a central region where the central canal is lined by dark and light ependymal cells. In the caudal portion of the filum terminale, the amount of neuropil is greatly reduced. This region os formed mainly by astrocytic glial cells and very few neuronal elements. The central canal in the caudal portion is located ventrally and contains a lining consisting almost exclusively of dark ependymal cells.

Longitudinal spread of intraneurally injected local anesthetics. An experimental study of the initial neural distribution following intraneural injections.

Unexpected spinal anesthesia, occurring after peripheral nerve blocks close to the spine, may be caused by a centripetal spread of the local anesthetic along the injected nerve to the spinal cord. In order to analyze the pathway of such a spread, a radioactive local anesthetic mixed with a fluorescent dye was injected into difrerent compartments of the rabbit sciatic nerve, and the early distribution of these tracers was studied by scintillation counting and fluorescence microscopy. Epineurial (extrafascicular) injections were of low injection pressure (25-60 mmHg) (3.3-7.9 kPa) and limited spread, while endoneurial (intrafascicular) injections reached higher pressures (300-750 mmHg) (39.9-99.7 kPa) and caused a rapid spread over long distances within the fascicle. The sacral plexus seemed difficult to pass. However, 20% of endoneurial injections reached the spinal cord, where the injectate primarily spread in the thin subpial space. Our experimental findings suggest that intraneural injections of local anesthetics are responsible for the reported cases of unexpected spinal anesthesia due to inadvertent intrafascicular spread. Although intrafascicular injections are rarely made, we recommend that intraneural injections of local anesthetics or other solutions close to the spine should be avoided, as they may cause unexpected spinal anesthesia or lesion of the cord.

Scanning electron microscopy of the subarachnoid space in the dog. V. Macrophages challenged by bacillus calmette‐GUERIN

Mongrel dogs were anesthetized intraperitoneally with pentobarbitol. One cc of cerebrospinal fluid was drawn through a needle inserted into the cisterna magna and mixed with 1 cc (4-9 million viable BCG organisms) of freeze-dried bacillus Calmette-Guerin. One minute later this mixture was injected by the same needle into the cisterna magna. At 1 and 12 days postinjection, experimental animals were perfused with buffered aldehydes. Samples of the leptomeninges were post-fixed in OsO4 and routinely prepared for scanning and transmission electron microscopy. Leptomeningeal samples of untreated, control animals were similarly prepared. Scanning and transmission microscopy confirm that free cells resting on the subarachnoid linings and within the subpial connective tissue space of control animals possess the morphology of macrophages (Malloy and Low, '76). Viable BCG in the subarachnoid space produces a 3-fold increase in the free cell population of the leptomeninges in 24 hours and a 10-fold increase in 12 days. These cells tend to form associations varying from loose aggregates to tight clusters. Approximately 80% of these free cells express macrophage morphology, with abundant plasma-lemmal microappendages and cytoplasmic vacuoles. Transmission electron microscopy of the free cell population of BCG-stimulated animals reveals at least two other members of the leukocyte series on the leptomeningeal linings.