Fine Structure of the Normal Trigeminal Ganglion in the Cat and Monkey*

Transcription

1 Fine Structure of the Normal Trigeminal Ganglion in the Cat and Monkey* DAVID S. MAXWELL, PH.D. Principal Contributor and Leader of Discussion HE inclusion of animal material m a y be justified as a means of obtaining optimal preservation of the ganglion. All ganglia were taken f r o m aldehyde-perfused animals, but some were less than optimally preserved by allowing p o s t m o r t e m changes to occur before perfusing and by perfusing with substantially hypertonic fixatives. We hoped that it would thus be possible to distinguish fixation artifacts f r o m pathological changes in the h u m a n material. The ganglion cells display a homogeneous nucleus and a large nucleolus. T h e nuclear cleft is prominent and continuous with the rough endoplasmic reticulum. The large surrounding expanses of cytoplasm contain p r o m i n e n t Nissl bodies, m a n y Golgi m e m brane accumulations, glycogen granules, numerous mitochondria, and extensive swirls of microtubules and fine filaments between the Nissl bodies. Clusters of lipofuscin granules m a y be observed in isolated patches. M a n y minute processes extend f r o m the surface of the ganglion cell and interlock with processes of the investing satellite cells. The single axon cannot be confused with these processes, due to its relatively enormous size, and its oriented microtubules and filaments. The axon is generally invested with several layers of satellite-cell cytoplasm, either f r o m multiple processes of a single cell in more distal regions of the axon or f r o m more than one satellite cell near the ganglion soma. The initial (unmyelinated) segment of the axon is commonly thrown into a tangled skein around the soma, according to metallic impregnation studies at the light microscopic level. This arrangement accounts for the presence of several axon profiles around a single ganglion cell in thin sections viewed with the electron microscope. The ganglion cells are totally wrapped in layered processes of satellite cells, arranged T * This work was done in collaboration with Drs. Anselmo Pineda and Lawrence Kruger. FIG. 21. Trigeminal ganglion, delayed fixation. Cat. Exsanguination and a 20-minute delay followed by glutaraldehyde-formaldehyde perfusion. An axon (A) at a node of Ranvier displays a not unusual herniation of axoplasm (arrows). The myelin sheath at the right (M) exhibits a slightly reduced osmiophilia, but the lamellar structure is well preserved. This is to be contrasted with the paranodal myelin which has lost its lamellar structure and its osmiophilia (E); 15,000. so that the processes nearest the soma interdigitate with the cytoplasmic evaginations of the ganglion cells. The satellite cells are in turn covered with a basal lamina which m a y extend between adjacent satellite cells or between the processes of a single satellite cell, but do not intervene between the ganglion cell or its processes and the satellite cells. The satellite cells display a degree of osmotic sensitivity. In ganglia perfused with fixative in a 0.15 M caeodylate buffer, these cells and their processes are distinctly shrunken away from the ganglion cell and f r o m each other, so that empty spaces a p p e a r between the processes and the neuron similar to those observed in Figure 11. As do Schwann ceils, satellite cells display some variation in nuclear morphology, especially in the c h r o m a t i n arrangement at the nuclear membrane, and some variability in cytoplasmic matrix density. I t seems reason127

2 128 Workshop on Trigeminal Neuralgia F1G. 22. Astrocytic dome in the root of the trigeminal ganglion. Japanese Ape, glutaraldehyde-formaldehyde. A myelinated axon emerges from the central portion of the root (below and left) at a node of Ranvier. The peripheral nerve myelin sheath on the axon is thicker than the central sheath, and the pattern of termination of each myelin lamella in the paranodal region is different in the central and peripheral internodes. The dome is characterized by numerous intertwined astrocytic processes, containing fibrils and glycogen. The basal lamina of the surface of the dome is continuous with that of the Schwann cell of the first peripheral internode; )< 14,000.

3 Anatomy of Trigeminal Ganglia 129 able that these morphological variations within a cell type are related to levels or types of activities, rather than to irregularities in preservation. The cytoplasm of the satellite cell contains numerous free ribosomes and an extensive rough endoplasmic reticulum, microtubules and filaments, and glycogen granules. The filaments and tubules are usually oriented parallel to the axon in satellite cells associated with the initial segment, but seem randomly dispersed elsewhere in the cells. The basal lamina of the satellite cell investment is continuous across the minute space separating adjacent cells, and is continuous with the basal lamina of the Schwann cell of the first myelin segment. Schwann cells in the ganglion are not remarkably different from those found in other peripheral nerves. The cytoplasm commonly contains osmiophilic laminated inclusions in the form of long bars or rods. The lamellae do not exhibit an identical repeat period from one inclusion to another; usually, however, the period is substantially less than that of myelin. These inclusions are found in Schwann cells associated with myelin, but not in those wrap- ping unmyelinated fibers. Schwann cell microtubules and filaments are oriented parallel to the axons associated with them. The plasma membrane of Schwann cells frequently displays rows of pinocytotic vesicles. Several unmyelinated axons may be embedded in a Schwann cell, but a Schwann cell which forms myelin myelinates only a single segment of a single axon. The presence of more than one myelinated profile in a Schwann cell is most likely due to cutting a curved axon, or cutting through a myelin redundancy or fold, especially common in nodal regions. A 20-minute delay in perfusion-fixation of the ganglion has remarkably little effect on the cytoplasm of the three cell types described. Generally, the glycogen is gone, and the mitochondria may display moderate swelling. The relationships between the satellite cells and the ganglion cells, and between the Schwann cells and the axons, are apparently undisturbed. The chromatin in the satellite cells and" the Schwann cells is distinctly clumped, but in the ganglion cells is relatively unaffected. The only other change FIG. 23. Astrocytic "island" in the trigeminal ganglion. Rhesus monkey, glutaraldehyde-formaldehyde. Below the intertwined processes of astrocytic processes are seen to be filled with fibrils and glycogen. The island is separated by a basal lamina from the rest of the ganglion which exhibits histological features of peripheral nerve. Above and right, fibroblast process, collagen and a myelin sheath; X25,000.

4 130 Workshop on Trigeminal Neuralgia FIG. 24. Myelin in the astrocytic dome of the root of the trigeminal ganglion. Japanese Ape, glutaraldehydeformaldehyde. An axon (A) is surrounded by a myelin sheath displaying extensive splitting and lamellar separation; X 50,000.

5 immediately evident occurs in the myelin. There are patches of greatly reduced osmiophilia in which the lamellar structure of the sheath is absent or disrupted (Fig. 21). This alteration is especially noted in Schmidt- Lantermann clefts and at nodes of Ranvier. Our tentative interpretation is that this alteration results from activation of lipolytic enzymes associated with Schwann cell cytoplasm trapped in clefts and at nodes. Such an interpretation is consistent with the appearance of myelin sheaths after lipid extraction. In the root of the ganglion there is an abrupt transition from central to peripheral nervous system features (Fig. 22). On the central side of the astrocytic dome, there is a feltwork of astroeytic processes through which the axons must pass to a limiting basal lamina separating the two regions. The processes are filled with astrocytic fibrils and contain glycogen granules; they appear no different, except in number, from astrocytic cytoplasm in other white matter in the central nervous system. Scattered among them are oligodendroeytes, which are presumably responsible for the myelin of the axons in this region. As the axons emerge from the dome, a node separates the central from the peripheral myelin. The Anatomy of Trigeminal Ganglia 131 basal lamina of the dome is continuous with the basal lamina investing the Schwann cell of the first peripheral internode. Patches of astrocytic processes can be found distal to the dome, appearing as apparently detached "islands" of central nervous system glia (Fig. 23). These islands can be found even as far distal as the ganglion. Axons are found in these islands but we have not observed ganglion cells. The islands are separated from the rest of the ganglion by a typical basal lamina. It is common in the islands and in the dome to find the myelin in a poor state of preservation. The lamellae are often split apart, and the sheath may display a severely distorted appearance, with folds in the myelin and a blistered or vacuolated appearance to the sheath generally (Fig. 24). The reason for the difficulty in preserving the myelin in these regions is obscure, but it could be that at the dome at least, the myelin is subject to the most stress during mechanical manipulation of the ganglion during tissue preparation. Alternatively, the central and peripheral portions of the nerve may be subject to different rates or degrees of shrinkage. A less likely interpretation is that this disrupted appearance represents the state of the sheath in life.