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The fine structure of the tapetum of the kitten eye as revealed by the electron microscope
Jo ULTRASTRUCTURE RESEARCH 1, 359-364 (1958)
359
The Fine Structure of the Tapetum of the Kitten Eye as Revealed by the Electron Microscope E. YAMADA
Department of Anatomy, School of Medicine, Kurume University, Kurume Received April 29, 1958 The cell of the tapetum of the kitten eye was observed with the electron microscope. Within the cell body, characteristic fibrous structures or filaments were observed. Each filament shows a cross-striation with a 45 ~ periodicity and is enclosed within a capsule. During a study of the kitten eye retina, cells of the tapetum were recognized in the choroid and their fine structure was observed with the electron microscope. In the case of the cat and carnivores in general, the tapetum is composed mainly of a mass of cellular elements i.e., the tapetal cells, and is located on the inner side of the choroid facing the stratum pigmenti of the retina. The tapetum of these animals is called tapetum cellulosum in comparison with tapetum fibrosum and guanin tapetum as found in other types of vertebrates. The tapetum is considered as a kind of reflecting plate and as a structure which increases photoreception of the retina. A special kind of structure within the cytoplasm of the tapetal cells would be anticipated, and in this paper a characteristic fibrous structure, which has not been revealed thus far in other cell types, is described.
MATERIAL AND METHODS Kitten eyes were fixed with a modified Palade's buffered osmium tetroxide solution for 30 to 60 minutes. After dehydration with ethanol, the tissues were embedded in n-butyl methacrylate. Sections were cut with a JCM microtome and were observed with a JEM4c electron microscope. OBSERVATIONS The cells of the tapetum of the kitten eye are large ovoid to round cells and their round nuclei with prominent nucleoli are usually located in the center of the cell body. Within the cell body, one can observe the usual cell organelles: mitochondria, Golgi apparatus, rough-surfaced endoplasmic reticulum with its associated particles
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E. YAMADA
(4). The mitochondria have slender rod- or filament-like shapes and show cristae mitochondriales (3). The Golgi apparatus is found near the nucleus and shows a structure similar to that described in other cell types (Fig. 2). The rough-surfaced endoplasmic reticulum is observable in considerable amounts mainly around the mitochondria (Figs. 1 and 3-6). The most conspicuous and characteristic feature of the tapetal cell of the kitten eye is the presence of numerous fibrous or filamentous structures within the cell body. Each filament appears to be almost straight and more than 2 # long as judged from single sections. It is ovoid or round in cross-section and about 300-370 A in diameter. The filaments occupy the majority of the cell body except for a small area where mitochondria and the Golgi apparatus usually can be observed. They show some tendency to gather in groups and, while each group runs in a different direction to each other, most groups are disposed parallel with the plane of retina, as shown in Fig. 1. Closer observation (Figs. 3-6) reveals that each filament is encapsulated by a thin single membrane. The membrane or the capsule of the filament shows dilatations or folds at certain places along the filament. The matrix limited by this membrane has a density which differs from that of the surrounding cytoplasm and the filament within it. The filament, itself, is much denser than the matrix. These findings are shown most clearly where dilatation of the capsule is observed. Another feature of the fine structure of the filament is a cross-striation or the axial periodicity along its length. The narrow cross-band, of high electron density, and the less dense interspace are almost the same width, measuring about 45 A wide (Figs. 5 and 6). DISCUSSION The most eminent example of fibrous structures within a cell body are the myofibrils of striated muscle. Recent electron microscopy has revealed filamentous structures within other cell types. For example, Porter (5) has described keratin filaments, which have a characteristic periodicity, within epidermal cells. The fibrous rootlet associated with the basal body of t h e cilium often shows cross-striation (1, 6). The findings presented here represent another example. However, the filaments of the
FIGS. 1 and 2. General surveys of the tapetal cell of the kitten eye. In Fig. 1, the majority of the cell body is seen to be full of filaments (f). They are running as groups in different directions but, in general, seem to be parallel to the plane of the retina. Well-developed Golgi bodies (G) in Fig. 2 near the vicinity of the nucleus (N) consist of the usual components. A part of the nucleolus (NI) extends towards the nuclear membrane (nm). Rough-surfaced endoplasmic reticulum (er) is found surrounding the mitochondria (m). × 20,000 and 33,000 respectively.
TAPETUM OF THE KITTEN EYE
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E. YAMADA
FI~s. 3 and 4. Parts of the cytoplasm of the tapetal cell. The rod-shaped mitochondria (m) and the rough-surfaced endoplasmic reticulum (er) are clearly shown. Each filament (f) is encapsulated by a thin membrane (c) which sometimes shows dilatations or folds. Some of the filaments are crosssectioned. Note the difference of density between the matrix limited by the membrane and the cytoplasm, x 50,000.
TAPETUM OF THE KITTEN EYE
363
FIGS. 5 and 6. The longitudinal profiles of filaments (f) are shown at high magnification, and the axial periodicity of about 45 ~ is observable in most of the filaments. The capsule (c) or membrane which encloses the filament is also demonstrated. The capsule seems to be continuous with the membranes of the rough-surfaced endoplasmic reticulum (er) to the right of both figures, x 90,000.
364
E. YAMADA
tapetal cells have features which distinguish them from the others. Firstly, the filament of the tapetal cell shows a striation with the periodicity of 45 ]k and secondly, the filament is enclosed within a membrane or capsule. From these findings, one can imagine that the filamentous structure might be differentiated from the substance or the matrix limited by the membrane within the cytoplasm of the cell of the tapetum. The fact that frame-works of filaments which run in different directions as a whole are parallel to the retinal plane seems to explain the reflecting nature which has been considered the characteristic function of the tapetal cell layer. Kolmer (2) described with the light microscope a net-like apparatus in the tapetal cell of cat. This may correspond to the filaments described here. The exact chemical nature and the morphogenesis of the filaments are not known and await elucidation by further investigations. REFERENCES 1. BRADFIEI.D,J. R. G., Symposia Soc. Exptl. Biol. 9, 306 (1955). 2. KOLMER,W., in v. M6LLENDORFF,W. (Ed.) Handbuch der mikroskopischen Anatomic des Menschen, Bd. 3, Teil 2 (1936). 3. PALADE, G. E., Anat. Record 114, 427 (1952). 4. - J. Biophys. Biochem. Cytol. 2, No. 4, Suppl., 85 (1956). 5. PORTER,K. R., Anat. Record 118, 433 (1954). 6. SJ6STRAND,F. S., J. Cellular Comp. Physiol. 42, 45 (1953).
359
The Fine Structure of the Tapetum of the Kitten Eye as Revealed by the Electron Microscope E. YAMADA
Department of Anatomy, School of Medicine, Kurume University, Kurume Received April 29, 1958 The cell of the tapetum of the kitten eye was observed with the electron microscope. Within the cell body, characteristic fibrous structures or filaments were observed. Each filament shows a cross-striation with a 45 ~ periodicity and is enclosed within a capsule. During a study of the kitten eye retina, cells of the tapetum were recognized in the choroid and their fine structure was observed with the electron microscope. In the case of the cat and carnivores in general, the tapetum is composed mainly of a mass of cellular elements i.e., the tapetal cells, and is located on the inner side of the choroid facing the stratum pigmenti of the retina. The tapetum of these animals is called tapetum cellulosum in comparison with tapetum fibrosum and guanin tapetum as found in other types of vertebrates. The tapetum is considered as a kind of reflecting plate and as a structure which increases photoreception of the retina. A special kind of structure within the cytoplasm of the tapetal cells would be anticipated, and in this paper a characteristic fibrous structure, which has not been revealed thus far in other cell types, is described.
MATERIAL AND METHODS Kitten eyes were fixed with a modified Palade's buffered osmium tetroxide solution for 30 to 60 minutes. After dehydration with ethanol, the tissues were embedded in n-butyl methacrylate. Sections were cut with a JCM microtome and were observed with a JEM4c electron microscope. OBSERVATIONS The cells of the tapetum of the kitten eye are large ovoid to round cells and their round nuclei with prominent nucleoli are usually located in the center of the cell body. Within the cell body, one can observe the usual cell organelles: mitochondria, Golgi apparatus, rough-surfaced endoplasmic reticulum with its associated particles
360
E. YAMADA
(4). The mitochondria have slender rod- or filament-like shapes and show cristae mitochondriales (3). The Golgi apparatus is found near the nucleus and shows a structure similar to that described in other cell types (Fig. 2). The rough-surfaced endoplasmic reticulum is observable in considerable amounts mainly around the mitochondria (Figs. 1 and 3-6). The most conspicuous and characteristic feature of the tapetal cell of the kitten eye is the presence of numerous fibrous or filamentous structures within the cell body. Each filament appears to be almost straight and more than 2 # long as judged from single sections. It is ovoid or round in cross-section and about 300-370 A in diameter. The filaments occupy the majority of the cell body except for a small area where mitochondria and the Golgi apparatus usually can be observed. They show some tendency to gather in groups and, while each group runs in a different direction to each other, most groups are disposed parallel with the plane of retina, as shown in Fig. 1. Closer observation (Figs. 3-6) reveals that each filament is encapsulated by a thin single membrane. The membrane or the capsule of the filament shows dilatations or folds at certain places along the filament. The matrix limited by this membrane has a density which differs from that of the surrounding cytoplasm and the filament within it. The filament, itself, is much denser than the matrix. These findings are shown most clearly where dilatation of the capsule is observed. Another feature of the fine structure of the filament is a cross-striation or the axial periodicity along its length. The narrow cross-band, of high electron density, and the less dense interspace are almost the same width, measuring about 45 A wide (Figs. 5 and 6). DISCUSSION The most eminent example of fibrous structures within a cell body are the myofibrils of striated muscle. Recent electron microscopy has revealed filamentous structures within other cell types. For example, Porter (5) has described keratin filaments, which have a characteristic periodicity, within epidermal cells. The fibrous rootlet associated with the basal body of t h e cilium often shows cross-striation (1, 6). The findings presented here represent another example. However, the filaments of the
FIGS. 1 and 2. General surveys of the tapetal cell of the kitten eye. In Fig. 1, the majority of the cell body is seen to be full of filaments (f). They are running as groups in different directions but, in general, seem to be parallel to the plane of the retina. Well-developed Golgi bodies (G) in Fig. 2 near the vicinity of the nucleus (N) consist of the usual components. A part of the nucleolus (NI) extends towards the nuclear membrane (nm). Rough-surfaced endoplasmic reticulum (er) is found surrounding the mitochondria (m). × 20,000 and 33,000 respectively.
TAPETUM OF THE KITTEN EYE
361
362
E. YAMADA
FI~s. 3 and 4. Parts of the cytoplasm of the tapetal cell. The rod-shaped mitochondria (m) and the rough-surfaced endoplasmic reticulum (er) are clearly shown. Each filament (f) is encapsulated by a thin membrane (c) which sometimes shows dilatations or folds. Some of the filaments are crosssectioned. Note the difference of density between the matrix limited by the membrane and the cytoplasm, x 50,000.
TAPETUM OF THE KITTEN EYE
363
FIGS. 5 and 6. The longitudinal profiles of filaments (f) are shown at high magnification, and the axial periodicity of about 45 ~ is observable in most of the filaments. The capsule (c) or membrane which encloses the filament is also demonstrated. The capsule seems to be continuous with the membranes of the rough-surfaced endoplasmic reticulum (er) to the right of both figures, x 90,000.
364
E. YAMADA
tapetal cells have features which distinguish them from the others. Firstly, the filament of the tapetal cell shows a striation with the periodicity of 45 ]k and secondly, the filament is enclosed within a membrane or capsule. From these findings, one can imagine that the filamentous structure might be differentiated from the substance or the matrix limited by the membrane within the cytoplasm of the cell of the tapetum. The fact that frame-works of filaments which run in different directions as a whole are parallel to the retinal plane seems to explain the reflecting nature which has been considered the characteristic function of the tapetal cell layer. Kolmer (2) described with the light microscope a net-like apparatus in the tapetal cell of cat. This may correspond to the filaments described here. The exact chemical nature and the morphogenesis of the filaments are not known and await elucidation by further investigations. REFERENCES 1. BRADFIEI.D,J. R. G., Symposia Soc. Exptl. Biol. 9, 306 (1955). 2. KOLMER,W., in v. M6LLENDORFF,W. (Ed.) Handbuch der mikroskopischen Anatomic des Menschen, Bd. 3, Teil 2 (1936). 3. PALADE, G. E., Anat. Record 114, 427 (1952). 4. - J. Biophys. Biochem. Cytol. 2, No. 4, Suppl., 85 (1956). 5. PORTER,K. R., Anat. Record 118, 433 (1954). 6. SJ6STRAND,F. S., J. Cellular Comp. Physiol. 42, 45 (1953).