The tapetal cell: a unique melanocyte in the tapetum lucidum cellulosum of the cat (felis domestica l.)

Multilayer subwavelength gratings or sandwiches with periodic structure shape light reflection in the tapetum lucidum of taxonomically diverse vertebrate animals

Eye shine in the dark has attracted many researchers to the field of eye optics, but the initial studies of subwavelength arrangements in tapetum began only with the development of electronic microscopy at the end of the 20th century. As a result of a number of studies, it was shown that the reflective properties of the tapetum are due to their specialized cellular subwavelength microstructure (photonic crystals). These properties, together with the mutual orientation of the crystals, lead to a significant increase in reflection, which, in turn, enhances the sensitivity of the eye. In addition, research confirmed that optical mechanisms of reflection in the tapetum are very similar even for widely separated species. Due to progress in the field of nano-optics, researchers now have a better understanding of the main principles of this phenomenon. In this review, we summarize electron microscopic and functional studies of tapetal structures in the main vertebrate classes. This allows data on the microstructure of the tapetum to be used to improve our understanding of the visual system.

Immunohistochemical Characteristics of Normal Canine Eyes

Immunohistochemistry is widely utilized in diagnostic laboratories to study neoplastic and nonneoplastic diseases. Knowledge of the immunohistochemical characteristics of normal tissue is essential for interpretation of immunoreactivity in pathologic conditions. In this study, immunohistochemistry was performed with a broad panel of diagnostically relevant antibodies on 4 normal canine globes—namely, vimentin, pan-cytokeratin (AE1/AE3), cytokeratin 7, cytokeratin 8/18, cytokeratin 20, α–smooth muscle actin, muscle specific actin, desmin, Melan-A, microphthalmia transcription factor, S-100, glial fibrillary acidic protein, triple neurofilaments, neuron-specific enolase, chromogranin A, synaptophysin, laminin and CD31. Results include cytokeratin immunoreactivity limited to the conjunctival epithelium, corneal epithelium, and retinal pigment epithelium; distinct patterns of immunopositivity of muscle markers; and widespread immunoreactivity for vimentin and most neural/neuroendocrine markers. These findings in normal eyes provide the basis for interpretation of ocular immunohistochemistry in dogs. Published immunophenotypes of primary ocular neoplasms are also reviewed.

Comparative morphology of the tapetum lucidum (among selected species)

OBJECTIVES

The phenomenon of 'eye-shine' is seen in a variety of animal species, and is generally thought to be related to the presence of an intraocular reflecting structure, the tapetum lucidum. The tapetum lucidum is a biologic reflector system that is a common feature in the eyes of vertebrates. It normally functions to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity at low light levels. The tapetum lucidum is presented here according to a classification based on the location, as well as the composition, of this reflective layer. Finally, the physical and chemical properties, as well as the origins of the different tapeta lucida, are discussed and compared.

METHODS

The anatomic and biochemical aspects of the tapetum lucidum in various vertebrates are examined. Morphologic observations were made from paraffin and plastic embedded specimens. Specimens were treated with traditional stains and observed by light and transmission electron microscopy.

RESULTS

Some species (primates, squirrels, birds, red kangaroo and pig) do not have this structure and they usually are diurnal animals. In vertebrates, the tapetum lucidum exhibits diverse structure, organization and composition. Therefore, the retinal tapetum (teleosts, crocodilians, marsupials, fruit bat), the choroidal guanine tapetum (elasmobranchs), the choroidal tapetum cellulosum (carnivores, rodents, cetacea), and the choroidal tapetum fibrosum (cow, sheep, goat, horse) are described.

CONCLUSIONS

The tapetum lucidum represents a remarkable example of neural cell and tissue specialization as an adaptation to a dim light environment and, despite these differences, all tapetal variants act to increase retinal sensitivity by reflecting light back through the photoreceptor layer. These variations regarding both its location and structure, as well as the choice of reflective material, may represent selective visual adaptations associated with their feeding behavior, in response to the use of specific wavelengths and amount of reflectance required.

Retinal epithelial fine structure in the domestic cat (Felis catus)

The morphology of the retinal epithelium (RPE) and closely associated choriocapillaris and Bruch's membrane (complexus basalis) has been investigated in the eye of the domestic cat (felis catus) by light and electron microscopy. The RPE consists of a single layer of cuboidal cells joined laterally by apically-located junctional complexes. Basally (sclerally) these cells display numerous infoldings while apically (vitreally) two distinct types of processes enclose photoreceptor outer segments. Internally the large vesicular nucleus is centrally located. Smooth endoplasmic reticulum, mitochondria, polysomes and lysosome-like bodies are abundant. Rough endoplasmic reticulum while present is not plentiful. Over the tapetum lucidum, melanosomes are absent from the RPE cells while in non-tapetal locations melanosomes are large and plentiful. The endothelium of the choriocapillaris is highly fenestrated and these capillaries are deeply indented into the epithelial layer over the tapetum. Bruch's membrane (complexus basalis) is non-tapetal regions is the typical pentalaminate structure noted for most mammals. Over the tapetum however it is reduced to a trilaminate structure and when associated with the indented capillary profiles is further reduced to a single thickened basal lamina.

Histological study of choroidal melanocytes in animals with tapetum lucidum cellulosum

The distribution and morphology of choroidal melanocytes in dogs and cats which have a tapetum were compared with those of humans who do not. In dogs or cats, tapetal cell-like melanocytes were arranged in layers on the scleral side of the tapetum and underneath the choriocapillaris in the non-tapetal area. Although the tapetum of the dog occupied a smaller area than that of the cat, the tapetum plus the area of tapetal cell-like multilayered melanocytes occupied most of the fundus in the dog in almost the same way as in the cat. These multilayered melanocytes contained few intracytoplasmic organelles except for melanin granules, and some had regularly arranged melanin granules. In human eyes tapetal cell-like melanocytes were not found anywhere. It was concluded that the morphology and structural architecture of choroidal melanocytes of dogs or cats are different from those of human eyes and closely correspond to the tapetum.

Melanosome abnormalities of ocular pigmented epithelial cells in beagle dogs with hereditary tapetal degeneration

Eyes of laboratory beagle dogs with an inherited tapetal degeneration were abnormally lightly pigmented. The development of pigmentation was followed morphologically from 7 days postnatal to 9 years of age. At all postnatal ages the iris pigmented epithelia contained no normal melanosomes, only organelles resembling secondary lysosomes or residual bodies. The ciliary body pigmented epithelium contained a variety of melanosome organelles at the earliest stages examined, but in fewer numbers than in normal animals. These included premelanosomes, partially melanized and some fully melanized pigment granules. However, the melanin deposition was usually patchy and irregular. With time, many of these granules appeared to condense into residual bodies. The retinal pigmented epithelium in peripheral and inferior posterior regions of affected animals never contained normal appearing melanin granules at any stage of postnatal development. The iris and choroidal stroma had melanosomes of normal size and shape, but many fewer than in normal animals. These results imply that there is local cellular control over melanosome production and regression, since the melanosome abnormalities do not follow the anterior to posterior development of pigment in ocular epithelia. It is proposed that a defect in synthesis of the matrix component of melanosomes could result in absent or abnormal deposition of melanin and initiate a process of autophagy of these organelles.

Development of hereditary tapetal degeneration in the beagle dog

Laboratory beagle dogs with an apparent absence of a tapetum lucidum were identified by ophthalmoscopic examination. Breeding experiments demonstrated a probable autosomal recessive mutation. Studies of the development of the tapetal abnormality showed that up to postnatal day 21 the tapetum was normal by light and ultrastructural morphology. Subsequent to that time the tapetal rodlets failed to accumulate electron-dense material, did not accumulate zinc, and degenerated primarily into spherical inclusion bodies of varying electron density. In the early phases of the degeneration the rough endoplasmic reticulum formed large whorls of membrane denuded of ribosomes. With time, the inclusions became electron lucent, and the entire tapetal cell degenerated, ending with almost total loss of the tapetum lucidum by approximately one to two years of age. The structure of the retina was normal. Retinal function measured by electroretinography was normal except for a slight elevation of dark adapted white light thresholds. It is speculated that the hereditary defect may be defective synthesis of the tapetal rodlet matrix or of the zinc-complexing substance of the tapetum.

Tapetal degeneration in cats with Chediak-Higashi syndrome

The Chediak-Higashi syndrome (CHS) is a genetic disorder of man, cats, and four other animal species. Enlarged cytoplasmic granules, including lysosomes and melanosomes, characterize the syndrome. Cats affected with CHS lack funduscopically visible tapeta. In normal cats, the tapetum is the light reflecting cellular layer located in the choroid. The tapetal cells contain bundles of parallel cytoplasmic rods. In this study, eyes from CHS and control cats were examined by light microscopy and transmission electron microscopy. The CHS kittens up to 14 days of age had tapeta which appeared similar to those of the controls. By 28 days of age some of the CHS tapetal rods had degenerated. Degeneration of the tapetal rods progressed rapidly and by 56 days of age there was a dramatic difference in the ultrastructural appearance of the tapetal cells. All the rods had degenerated and the contents of the tapetal cells were disorganized. The tapetal layer gradually thinned over a period of several months until the layer was absent or nearly so in CHS cats over one year of age. This study demonstrated that there is a previously overlooked degenerative component of the Chediak-Higashi syndrome.

Ocular melanin pigmentation anomalies in cats, cattle, mink, and mice with Chediak-Higashi syndrome: histologic observations

The Chediak-Higashi syndrome (CHS) is a hereditary disorder of man, with the homologous condition reported in five animal species. Multiple defects, including oculocutaneous hypopigmentation, are present in individuals with this syndrome. Giant cytoplasmic granules, including melanosomes and lysosomes, are characteristic. In this study, eyes from CHS affected and control cats, cattle, mink, and mice were examined histologically to determine: 1) degree of pigmentation; 2) structure and distribution of melanin granules; and 3) morphology of cells and tissues containing melanin. The CHS cattle were found to be the most ocularly hypopigmented species, whereas CHS mouse eyes contained considerably more melanin than those of the other species. Melanin granules of abnormal sizes and shapes were present in neuroepithelial and uveal tissues of CHS animals of all four species. Depigmentation apparently had occurred in the CHS eyes, since less melanin was present in eyes of old CHS animals of each species than was present in eyes of young animals. In addition, melanin containing macrophages were common in CHS eyes, and the numbers of melanocytes and pigmented epithelial cells were decreased in older CHS eyes.

Tapetum lucidum in the pigmented and albino ferret

Light and electron microscopy showed that the tapetum lucidum in the pigmented ferret is morphologically indistinguishable from that in the albino ferret. The matrix of the rods of the tapetal cells was strongly osmiophilic, but glutaraldehyde fixation before osmium tetroxide treatment caused a dissolution of the matrix material. It has been proposed that the tapetal cells are modified melanocytes and that the tapetal rods are composed of melanin, but it can be concluded from our data that the matrix of the tapetal rods is not melanin. Further studies by plasma-atomic emission spectrometry showed that the tapetal cells are very rich in zinc, with similar levels in pigmented and albino ferrets. Excessive concentrations of other metals were not observed. Histochemical demonstration of heavy metal showed that the zinc is present in the tapetal rods and indicated a localization mainly in the rod membranes.

Hereditary abnormality in tapetum lucidum of the Siamese cats. A histochemical and quantitative study

Gross examination showed a weaker reflection (less shining) of the tapetum lucidum of the Siamese cats compared with common cats. Toluidine blue sections revealed that many tapetal cells were weakly stained and giving vacuolated appearance under high magnification. Further examination with electron microscope showed that those weakly stained cells were filled with disrupted tapetal rods. In these affected cells, the arrangement of the tapetal rods was no longer regular. The membranes of the tapetal rods were either enlarged or disrupted. Some of them appeared to be myelin-like structures. The cores of the tapetal rods were either empty or filled with electron-dense materials which may be the remnant of the original cores. The severity of this type of abnormality or degeneration in the tapetum varied from layers to layers. Those layers closer to the retina showed a greater number of cells with degeneration. Quantitative analysis of histochemical detection of zinc showed a significantly smaller amount of zinc in tapetal rods of the Siamese cats as compared with common cats. Less zinc and disruption of the regular arrangement of the tapetal rods may result in weaker reflection of light by Siamese cat tapetum. In four of the nine Siamese cats studied, this type of abnormality was observed. It suggests that it is a hereditary disorder of relatively high frequency.

Histochemical and cytochemical demonstration of zinc cysteinate in the tapetum lucidum of the cat

Zinc cysteinate is shown histochemically and cytochemically in the tapetum lucidum of the cat. Heavy metal is demonstrated in the paraplasmic rods of the tapetal cells with a sulphide silver method whereas no such reaction can be found in the pigmented epithelium of the retina. These rods are also stained with silver methenamine indicating the presence of reducing groups which probably appeared after hydrolysis of cysteine.