Volume 14 Reports 927 Number 12 around the central matrix. The outer diameters From the Section of Tumor Research and Pa- of the circular profile of filaments seen surround- thology, Department of Ophthalmology and Visual ing the electron-lucent matrix in transverse sec- Science, Yale University School of Medicine, New tions were the same as in the longitudinal sec- Haven, Conn. Supported in part by National Institutes of Health Crant EY 00108-06. Submitted tion, with the single exception of the smaller for publication Aug. 12, 1975. Reprint requests: incomplete forms (Fig. 2, A). The outer diameters J. Craft, Department of Ophthalmology and in this case were approximately 740 A. It is Visual Science, Yale University School of Medi- interesting to note that the cylinder structures cine, 333 Cedar St., New Haven, Conn. 06510. were always enveloped by the cell processes. Discussion. The organization of the vertebrate REFERENCES retina has become increasingly well recognized 1 1. Hogan, M. J., Alvarado, J. A., and Weddell, in recent years. ' " The present paper describes ]. E.: Histology of the Human Eye, Phila- in detail for the first time the ultrastructural delphia, 1971, W. B. Saunders Company. characteristics of cylinder-like structures (termed 2. Radnot, M., and Lovas, B.: Die Ultrastruktur the cylinder organelle) present in the horizontal der Photoreceptor-Synapsen in einem Falle cell processes. Previous reports of this structure von Sehnervenatrophie, Albrecht v. Craefes in the human retina have included only a partial Arch. Klin. Exp. Ophthalmol. 173: 56, 1967. description of its ultrastructural detail.1- 2 3. Hebel, R.: Uber ein Korperchen mit regel- Certain other specialized structures in this area massiger Innenstruktur in einer. Synapse der have been described but appear morphologically ausseren plexiformen Schicht des Hundeauges, 1 Albrecht v. Graefes Arch. Klin. Exp. Ophthal- distinct from the cylinder organelle: Evans found mol. 180: 38, 1970. parallel fibers of electron-dense material in chick 5 4. Evans, E. M.: On the infrastructure of the rod endings. In 1967, Matsusaka described a synaptic region of visual receptors in certain structure in the synaptic cytoplasm of chick retina vertebrates, Z. Zellforsch. Mikrosk. Anat. 71: cone. He found stacks of paired membranes and 499, 1966. called them lamellar bodies. In serial sections, 5. Matsusaka, T.: Lamellar bodies in the synaptic Mountford" described a structure in the receptor- cytoplasm of the accessory cone from the bipolar synaptic area of the guinea pig retina chick retina as revealed by electron microscopy, J. Ultrastr. Res. 18: 55, 1967. and described it as spindle-shaped with cross 7 6. Mountford, S.: Filamentous organelles in re- striations. Pedler and Tilly observed a banded ceptor-bipolar synapses of the retina, J. Ultrastr. structure in Rhesus macaque monkeys; they re- Res. 10: 207, 1964. ported that it extended for approximately 37 sec- 7. Pedler, C. M. H., and Tilly, R.: Eye Structure, tions, having a length of approximately 3 /*. Eleventh Symposium, Stuttgart, 1965, Schat- In a study by Leuenbergers paracrystalline inclu- taver Verlag. sions were observed within the inner segments 8. Leuenberger, P. M.: Mikrofibrillare Kristal- and the synaptic pedicles of the human photo- lartige Einschliisse in den Photoreceptorsynap- sen der menschlichen Netzhaut, J. Microscopic receptor cells. These inclusions are formed by 15: 79, 1972. microfilaments, 150 A in diameter and showing 9. Dowling, J. E.: Organization of vertebrate regular spacing. retinas, INVEST. OPHTHALMOL. 9: 655, 1970. We would speculate that the cylinder structure apparently has gone unnoticed in previous studies, perhaps in part because of different orientations of the specimen block. The structure may also be Crossing axons in the third nerve nucleus. more observable under certain pathologic con- DON C. BIENFANG. ditions, and there may be some significance in the fact that all of the present specimens having The research presented in this paper studied the cylinder structure were from eyes with mela- the pathway taken by the crossed fibers of the nomas. In the present cases, however, the mela- third nerve nucleus in an animal whose nucleus nomas were distant from the areas where the has been well mapped and found to correlate well cylinder organelles were seen. We do not believe with higher mammals and man. Autoradiography the cylinder organelles to be artifacts, as we have using tritiated amino acid labeled the cell bodies not seen it in many other animal or human and axons of the left side of the oculomotor retinas, other ocular tissues or tumors similarly nucleus of the cat. Axons so labeled could be seen processed. emerging from the ventral portion of the left nucleus through the median longitudinal fasciculus At the present time the origin and function of (mlf) to join the left oculomotor nerve. Labeled the cylinder-like structure is unknown. Its close axons were also seen to emerge from the medial physical relationship with the synaptic vesicles border of the caudal left nucleus, cross the midline, might suggest that it is involved in the synaptic and pass through the right nucleus and the right functions of the retina rod cells or horizontal mlf to pin the right oculomotor nerve. These latter cell processes. axons must be the crossed axons of the superior Downloaded from iovs.arvojournals.org on 09/28/2021 928 Reports Investigative Ophthalmology December 1975 rectus and levator palpebrae subnuclei. Since the head holder and an occipital craniotomy was per- path of these crossed axons is through the caudal formed. A small amount of the tip of the bony portion of the nucleus of the opposite side, the tentorium was removed to allow the stereotactic destruction of one lateral half of the octdomotor placement of a glass micropipette into the left nucleus would result in a bilateral palsy of the third cranial nerve nucleus. The micropipettes had crossed subnuclei. Bilateral palsy of the superior previously been loaded with 1 to 2 juCi of des- rectus and bilateral assymetrical palsy of the sicated tritiated proline or leucine. The dry amino levator palpebrae muscles would result. acid was redissolved in a small amount of normal saline at the time of the experiment. The bolus Previous investigations in the cat and the mon- of approximately 0.5 to 1.0 /tliters of solution was key have established that the branch of the third injected into the region of the left third nerve nerve to the superior rectus consists entirely of nucleus through the hydraulic pressure created by crossed fibers, and that the branch to the levator a Hamilton syringe connected to the micropipette palpebrae consists of both crossed and uncrossed with polyethylene tubing. 1 3 fibers. " This suggests that all the axons from the The scalp wound was sutured and the animals superior rectus and some from the levator were killed after one week. The brains were fixed palpebrae must cross the mesencephalic midline. in 10 per cent formaldehyde buffered to pH 7.4, The location of this crossing and the subsequent and the brain stem was processed for paraffin path taken by these crossed fibers has not been sectioning. Sections were cut at 10 micra and one well studied though some suggestive information section of every twenty was mounted. The is available.4' 5 mounted sections were processed down to the Stains for central nervous system myelin or for water stage and then dipped in % strength Kodak neuron fibers easily demonstrate an abundance of NTB-2 photographic emulsion. They were stored nerve fibers crossing the midline between the in dry, light-free boxes in a freezer at —13° C. paired third nerve nuclei of the cat. Though for six weeks and developed in Dektol. The sec- present throughout the rostro-caudal extent of the tions were lightly stained with toluidine blue. nucleus, they are most prominent in the caudal Results. In three of the five cats the caudal one-third, suggesting that they are axons of the portion of the third nerve nucleus was labeled. superior rectus, inferior oblique, and levator These three cats demonstrated radiolabeled axons palpebrae subnuclei located there.2- 3 that originated in the left third nerve nucleus, However, Cajal'1 pointed out on the basis of crossed the midline, and entered the right third Golgi studies of the third nerve nucleus that there nerve after passing through the right third nerve are two other potential sources for crossing fibers nucleus. Since proline seemed to have a greater in this nucleus, namely axons of afferents to the tendency to label small cells than leucine, and nucleus, e.g., from the vestibular nuclei,7 and hence showed less contrast, the results were more dendrites of the cell bodies of the third nerve striking with leucine. This differential uptake has nucleus. been noted in other areas of the brain.9 With Positive identification of these crossed fibers as both amino acids, an abundance of radioactive the axons of the cell bodies of the opposite side fibers can be shown exiting from the ventral por- could be made with a whole neuron marking tech- tion of the left nucleus destined for the muscles nique such as autoradiography. innervated by uncrossed axons. On the opposite Radioactive amino acids, when injected in the side, exiting from the ventral portion of the right vicinity of neurons, are taken up by the soma, nucleus are radioactive fibers that have had their incorporated into the protein of the cell, and origin in cell bodies on the left side of the mid- ultimately distributed along the course of the line (Figs. 1 and 2). entire cell thus labeling the cell body and the The details of these crossing fibers is shown in course of its axon.s This new tracing device offers Fig. 3. It should be noted that these radioactive great promise for neuro-anatomic mapping since axons cross the midline in the dorsoventral center the amino acid is not taken up by axons passing of the nucleus and plunge into the mass of cell through the region of the injection but only by bodies of the opposite side before exiting through cell bodies at the site of the injection.