- Email: [email protected]
On the distribution and probable origin of axonal bundles in the pigment epithelium of the eyecup
Developmental Brain Research, 17 (1985) 293-295 :Elsevier
293
BRD 60047
On the distribution and probable origin of axonal bundles in the pigment epithelium of the eyecup R. W. GUILLERY*, A. LYSAKOWSKI**and S. PRICE Department of Pharmacological and Physiological Sciences, The University of Chicago, 947 E. 58th St., Chicago, 1L 60637 (U.S.A.)
(Accepted August 21st, 1984) In one of his descriptions of the early development of the spinal cord, Cajal 6 gave a striking account of nerve cells which, ' . . . perhaps by some accident, have undergone an initial polar inversion. As a consequence of this disorientation, the growth cone, instead of proceeding towards the external limiting membrane, is directed towards the internal o n e . . . ' . He showed some of these axons entering the ventricular cavity, growing in the ventricular fluid for some distance and then re-entering the gray matter, to join other fibers taking a more direct course. In view of recent interest in apparently aberrant axonal pathways formed in the course of normal embryonic development1,3-5, it seemed worth investigating some evidence favoring the possibility that in the early eyecup there might be a significant number of misrouted axons comparable to those described by Cajal for the spinal cord. Our attention was first drawn to this possibility by the occurrence of several small bundles of axons lying within the pigment epithelium of the eyecup. Closer study of these bundles has suggested to us that they are not misrouted axons, but represent a very early invasion of the pigment epithelium by autonomic nerve fibers. Observations have been made on hamsters and mice fixed at various stages of intrauterine development between the 1 lth and the 14th days after conception. The fiber bundles described here were particularly evident in the hamsters fixed at 13 days and in the mice fixed at 13 days. The hamsters were normally pigmented, while the mice were albino mice of the C57BL strain. The pregnant females were anesthetized with sodium pentobarbital or chloral hy-
drate, and the fetal heads were fixed by immersion in a mixture of glutaraldehyde (2.0%) and paraformaldehyde (2.5%) made up in a cacodylate buffer at pH 7.6. The heads were hemisected, and the eyes were partially dissected free from surrounding tissues. The eyes were then rinsed in the buffer, treated for 1 h in 1% buffered osmium tetroxide, dehydrated and embedded in 'Durcupan'. Sections that included different levels of the whole eyecup were prepared for electron microscopic study. At the stages studied here, the pigment epithelium is readily distinguished from the neural retina. The former is a single epithelial layer, rich in melanosomes or premelanosomes (Figs. 1-4), while the latter is a relatively advanced neural tissue with a large number of well formed fiber bundles running next to the vitreal surface. Within the pigment epithelium many of the sections showed several small bundles of 3-10 axons (Figs. 1-3). These bundles lie close to the ventricular surface (Fig. 1) and are also seen next to the basal lamina at the external surface of the eyecup (Fig. 2) or between these two surfaces of the epithelial sheet (Fig. 3). Many of the pigment cells adjacent to the bundles form slender laminar extensions which partially surround the bundles as though starting to form an enwrapment (Fig. 1, small arrows). When we studied the distribution of these small bundles in the eyecup we found that they were most commonly encountered in the lateral half of the eyecup, the segment facing away from the optic stalk. In the medial half of the eyecup, where the neural retina shows its most advanced development, and where many axon bundles can be traced over the vitreal surface of the eyecup, the pigment epithelium contains
* Present address: Department of Human Anatomy, South Parks Road, Oxford, U.K. Present address: Department of Anatomy, University of Illinois Medical Center, Chicago, IL, U.S.A. Correspondence: R. W. Guillery. Present address: Department of Human Anatomy, South Parks Road, Oxford OX1 3QX, U.K. **
0165-3806/85/$03.30 © 1985 Elsevier Science Publishers B.V.
t-~
295 few bundles.
axons.
In order to determine how these axons enter the
We conclude that these nerve fibers do not repre-
pigment epithelium, we searched for similar bundles on each side of the pigment epithelium. If the axons represent misdirected axons comparable to those
sent retinofugal axons that have crossed the ventricular cleft in error. Instead, they may represent something almost equally interesting: a group of peripher-
seen by Cajal in the spinal cord, they must cross the ventricular cleft, and lie amongst the cells of the neu-
rived from the neural tube and then go on to inner-
ral epithelium next to the ventricular cleft. We have
vate smooth muscle 7. Within the epithelial sheet they
al nerve fibers that grow into an epithelial sheet de-
seen no axon bundles in these regions. Instead, there
assume the relationships of central rather than pe-
are several small axon bundles adjacent to the outer
ripheral nerve fibers, the cells of the pigment epithe-
surface of the eyecup (Fig. 4). These are accompanied by Schwann cells and by a poorly defined basal
lium providing the 'glial' e n v i r o n m e n t and in some places even providing a poorly differentiated wrap-
lamina, and are separated from the pigment epitheli-
ping for the small axon bundles. This change from pe-
um by the basal lamina of the eyecup. The bundles outside the pigment epithelium generally contain more axons than those seen amongst the pigment cells, suggesting that the bundles of peripheral nerves enter the pigment epithelium at a few points and then break up into small bundles in their course through the pigment epithelium. We consider that these peripheral nerves are autonomic fibers destined to innervate the muscles of the iris, which are derived from the cells of the pigment epithelium2,8, and we suggest that as they enter the pigment epithelium the axons lose their associated Schwann cells and basal lamina, taking on the appearance of central 1 Bunt, S. M., Lund, R. D. and Land, P. W., Prenatal development of the optic projection in albino and hooded rats, Develop. Brain Res., 6 (1983) 149-168. 2 Haggqvist, G., Gewebe und Systeme der Muskulatur. In W. von MOllendorff (Ed.), Handbuch der Mikroskopischen Anatomie des Menschen, Vol. 2, Springer, Berlin, 1931, pp. 1-247. 3 Innocenti, G. M., Fiore, L. and Caminiti, R., Exuberant projection into the corpus callosum from the visual cortex of newborn rats, Neurosci. Lett., 4 (1977) 237-242. 4 McLoon, S. C. and Lund, R. D., Transient retinofugal pathways in the developing chick, Exp. Brain Res., 45 (1982) 277-284. 50'Leary, D. D. M., Gerfen, C. R. and Cowan, W. M., The
ripheral to central relationships is comparable to the relationships established by dorsal root fibers entering the central nervous system, except that the fibers we have described are apparently destined to innervate muscle, they probably reacquire a Schwann sheath before their termination and they pass through pigment epithelium not central nervous tissue. This study was supported by Grants NS-14283 and EY-02374 from the National Institutes of Health. We thank Mi Young Park for typing the manuscript.
development and restriction of the ipsilateral retinofugal projection in the chick, Develop. Brain Res., 10 (1983) 93-109. 6 Ramon y Cajal, S., NouveUes observations sur l'dvolution des neuroblasts, avec quelques remarqes sur l'hypothdse neurogenetique de Hensen-Held., Anat. Anz., 32 (1908) 1-25 and 65-87, Translated by L. Guth. In Studies of Vertebrate Neurogenesis by S. Ramon y Cajal, Thomas., Springfield, IL, 1960. 7 Retzius, G., Zur Kenntniss vom Bau der Iris, Biol. Untersuch. N.F., 5 (1893) 43-47. 8 Tamura, T. and Smelser, J. K., Development of the sphincter and dilator muscles of the iris, Arch. Ophthalmol., 89 (1973) 332-339.
Figs. 1-3. Show small fiber bundles lying amongst the cells of the retinal pigment epithelium of a 13 day hamster fetus (see methods). Some of the melanosomes are identified by large arrows. The small arrows (Fig. 1) show cytoplasmic tongues that extend from the pigment epithelial cells to provide a partial enwrapment of the axon bundles. The asterisks in Fig. 1 indicate the ventricular cleft, identified by the junctional specializations. The arrow heads (Fig. 2) show the basal lamina (BL) of the eyecup. Notice that the bundles are found close to the external surface, under the basal lamina (Fig. 2), and also adjacent to the ventricular cleft (Fig. 1). These axon bundles are not accompanied by Schwann elements or by a basal lamina. Instead they have the relationships characteristic of central axons. Not uncommonly they are accompanied by small round (vesicular?) profiles as in Fig. 3. Magnifications: Fig. 1 x21,000; Fig. 2 x20,000; Fig. 3 x27,500. Fig. 4. Shows a bundle of peripheral axons adjacent to the pigment epithelium. Portions of Schwann cytoplasm (SC) can be seen and the basal lamina of the eyecup (BL) is indicated by arrow heads. The large arrows show a melanosome (left) and a premelanosome (right). Magnification: x 13,000.
293
BRD 60047
On the distribution and probable origin of axonal bundles in the pigment epithelium of the eyecup R. W. GUILLERY*, A. LYSAKOWSKI**and S. PRICE Department of Pharmacological and Physiological Sciences, The University of Chicago, 947 E. 58th St., Chicago, 1L 60637 (U.S.A.)
(Accepted August 21st, 1984) In one of his descriptions of the early development of the spinal cord, Cajal 6 gave a striking account of nerve cells which, ' . . . perhaps by some accident, have undergone an initial polar inversion. As a consequence of this disorientation, the growth cone, instead of proceeding towards the external limiting membrane, is directed towards the internal o n e . . . ' . He showed some of these axons entering the ventricular cavity, growing in the ventricular fluid for some distance and then re-entering the gray matter, to join other fibers taking a more direct course. In view of recent interest in apparently aberrant axonal pathways formed in the course of normal embryonic development1,3-5, it seemed worth investigating some evidence favoring the possibility that in the early eyecup there might be a significant number of misrouted axons comparable to those described by Cajal for the spinal cord. Our attention was first drawn to this possibility by the occurrence of several small bundles of axons lying within the pigment epithelium of the eyecup. Closer study of these bundles has suggested to us that they are not misrouted axons, but represent a very early invasion of the pigment epithelium by autonomic nerve fibers. Observations have been made on hamsters and mice fixed at various stages of intrauterine development between the 1 lth and the 14th days after conception. The fiber bundles described here were particularly evident in the hamsters fixed at 13 days and in the mice fixed at 13 days. The hamsters were normally pigmented, while the mice were albino mice of the C57BL strain. The pregnant females were anesthetized with sodium pentobarbital or chloral hy-
drate, and the fetal heads were fixed by immersion in a mixture of glutaraldehyde (2.0%) and paraformaldehyde (2.5%) made up in a cacodylate buffer at pH 7.6. The heads were hemisected, and the eyes were partially dissected free from surrounding tissues. The eyes were then rinsed in the buffer, treated for 1 h in 1% buffered osmium tetroxide, dehydrated and embedded in 'Durcupan'. Sections that included different levels of the whole eyecup were prepared for electron microscopic study. At the stages studied here, the pigment epithelium is readily distinguished from the neural retina. The former is a single epithelial layer, rich in melanosomes or premelanosomes (Figs. 1-4), while the latter is a relatively advanced neural tissue with a large number of well formed fiber bundles running next to the vitreal surface. Within the pigment epithelium many of the sections showed several small bundles of 3-10 axons (Figs. 1-3). These bundles lie close to the ventricular surface (Fig. 1) and are also seen next to the basal lamina at the external surface of the eyecup (Fig. 2) or between these two surfaces of the epithelial sheet (Fig. 3). Many of the pigment cells adjacent to the bundles form slender laminar extensions which partially surround the bundles as though starting to form an enwrapment (Fig. 1, small arrows). When we studied the distribution of these small bundles in the eyecup we found that they were most commonly encountered in the lateral half of the eyecup, the segment facing away from the optic stalk. In the medial half of the eyecup, where the neural retina shows its most advanced development, and where many axon bundles can be traced over the vitreal surface of the eyecup, the pigment epithelium contains
* Present address: Department of Human Anatomy, South Parks Road, Oxford, U.K. Present address: Department of Anatomy, University of Illinois Medical Center, Chicago, IL, U.S.A. Correspondence: R. W. Guillery. Present address: Department of Human Anatomy, South Parks Road, Oxford OX1 3QX, U.K. **
0165-3806/85/$03.30 © 1985 Elsevier Science Publishers B.V.
t-~
295 few bundles.
axons.
In order to determine how these axons enter the
We conclude that these nerve fibers do not repre-
pigment epithelium, we searched for similar bundles on each side of the pigment epithelium. If the axons represent misdirected axons comparable to those
sent retinofugal axons that have crossed the ventricular cleft in error. Instead, they may represent something almost equally interesting: a group of peripher-
seen by Cajal in the spinal cord, they must cross the ventricular cleft, and lie amongst the cells of the neu-
rived from the neural tube and then go on to inner-
ral epithelium next to the ventricular cleft. We have
vate smooth muscle 7. Within the epithelial sheet they
al nerve fibers that grow into an epithelial sheet de-
seen no axon bundles in these regions. Instead, there
assume the relationships of central rather than pe-
are several small axon bundles adjacent to the outer
ripheral nerve fibers, the cells of the pigment epithe-
surface of the eyecup (Fig. 4). These are accompanied by Schwann cells and by a poorly defined basal
lium providing the 'glial' e n v i r o n m e n t and in some places even providing a poorly differentiated wrap-
lamina, and are separated from the pigment epitheli-
ping for the small axon bundles. This change from pe-
um by the basal lamina of the eyecup. The bundles outside the pigment epithelium generally contain more axons than those seen amongst the pigment cells, suggesting that the bundles of peripheral nerves enter the pigment epithelium at a few points and then break up into small bundles in their course through the pigment epithelium. We consider that these peripheral nerves are autonomic fibers destined to innervate the muscles of the iris, which are derived from the cells of the pigment epithelium2,8, and we suggest that as they enter the pigment epithelium the axons lose their associated Schwann cells and basal lamina, taking on the appearance of central 1 Bunt, S. M., Lund, R. D. and Land, P. W., Prenatal development of the optic projection in albino and hooded rats, Develop. Brain Res., 6 (1983) 149-168. 2 Haggqvist, G., Gewebe und Systeme der Muskulatur. In W. von MOllendorff (Ed.), Handbuch der Mikroskopischen Anatomie des Menschen, Vol. 2, Springer, Berlin, 1931, pp. 1-247. 3 Innocenti, G. M., Fiore, L. and Caminiti, R., Exuberant projection into the corpus callosum from the visual cortex of newborn rats, Neurosci. Lett., 4 (1977) 237-242. 4 McLoon, S. C. and Lund, R. D., Transient retinofugal pathways in the developing chick, Exp. Brain Res., 45 (1982) 277-284. 50'Leary, D. D. M., Gerfen, C. R. and Cowan, W. M., The
ripheral to central relationships is comparable to the relationships established by dorsal root fibers entering the central nervous system, except that the fibers we have described are apparently destined to innervate muscle, they probably reacquire a Schwann sheath before their termination and they pass through pigment epithelium not central nervous tissue. This study was supported by Grants NS-14283 and EY-02374 from the National Institutes of Health. We thank Mi Young Park for typing the manuscript.
development and restriction of the ipsilateral retinofugal projection in the chick, Develop. Brain Res., 10 (1983) 93-109. 6 Ramon y Cajal, S., NouveUes observations sur l'dvolution des neuroblasts, avec quelques remarqes sur l'hypothdse neurogenetique de Hensen-Held., Anat. Anz., 32 (1908) 1-25 and 65-87, Translated by L. Guth. In Studies of Vertebrate Neurogenesis by S. Ramon y Cajal, Thomas., Springfield, IL, 1960. 7 Retzius, G., Zur Kenntniss vom Bau der Iris, Biol. Untersuch. N.F., 5 (1893) 43-47. 8 Tamura, T. and Smelser, J. K., Development of the sphincter and dilator muscles of the iris, Arch. Ophthalmol., 89 (1973) 332-339.
Figs. 1-3. Show small fiber bundles lying amongst the cells of the retinal pigment epithelium of a 13 day hamster fetus (see methods). Some of the melanosomes are identified by large arrows. The small arrows (Fig. 1) show cytoplasmic tongues that extend from the pigment epithelial cells to provide a partial enwrapment of the axon bundles. The asterisks in Fig. 1 indicate the ventricular cleft, identified by the junctional specializations. The arrow heads (Fig. 2) show the basal lamina (BL) of the eyecup. Notice that the bundles are found close to the external surface, under the basal lamina (Fig. 2), and also adjacent to the ventricular cleft (Fig. 1). These axon bundles are not accompanied by Schwann elements or by a basal lamina. Instead they have the relationships characteristic of central axons. Not uncommonly they are accompanied by small round (vesicular?) profiles as in Fig. 3. Magnifications: Fig. 1 x21,000; Fig. 2 x20,000; Fig. 3 x27,500. Fig. 4. Shows a bundle of peripheral axons adjacent to the pigment epithelium. Portions of Schwann cytoplasm (SC) can be seen and the basal lamina of the eyecup (BL) is indicated by arrow heads. The large arrows show a melanosome (left) and a premelanosome (right). Magnification: x 13,000.