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The light climate at high latitudes, in particular the extended twilight of winter and the reduced diel variation in light level in midsummer and midwinter, potentially constrains visual function and the synchronisation of temporal organisation in polar species. In this paper, we describe the temporal pattern and variation in the spectral composition and brightness of skylight (daylight, twilight, moonlight, starlight, airglow and aurorae) at high latitudes and review photoreception of reindeer/caribou (Rangifer tarandus; ‘Rangifer’) which is one of the few polar resident species for which data are available. Experimental data indicate that the rods of Rangifer may be stimulated by levels of ambient light lower than those occurring during astronomical twilight (solar angle <−18°). Several features of the eyes of Rangifer contribute to their visual capability under extended twilight. These include transmission of UV through the optical media, which enables the animals to exploit the shorter wavelengths characteristic of twilight, and a shift in the peak spectral reflectance of the tapetum lucidum (TL) from around 640 nm in summer to around 450 nm in winter, which increases retinal illumination at short wavelengths. Enhanced sensitivity to short wavelengths is likely to enhance the contrast of some objects and hence the ability of Rangifer to discriminate forage plants and to detect other animals (conspecifics or predators) against a snowy background under low illuminance. There is, nevertheless, currently no evidence of any specific boreal adaptation in their visual system: (i) The eyes of Rangifer, and by inference the area of the dilated pupil, are no larger than expected based on the allometry of eye size in ruminants. (ii) There is no evidence of a change in the spectral sensitivity of photoreceptors associated with detection of UVa. (iii) Transmission of UV through the anterior eye is not unique to Rangifer. (iv) The blue shift in the reflectance of the winter TL appears to be a passive response to prolonged dilation of the pupil and there is no a priori reason not to predict the same response in other large ungulates exposed to low light levels. (v) There is no conclusive evidence of a seasonal shift in absolute retinal sensitivity in Rangifer. (vi) Weak circadian organisation in Rangifer has tentatively been linked to mutations within the circadian molecular clockwork but it remains unclear to what extent this represents a specific adaptation to high latitude. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

The vertebrate retina is said to be inverted because the photoreceptors are oriented in the posterior direction and are thus unable to maximize photodetection under conditions of low illumination. The tapetum lucidum is a photoreflective structure located posterior to the photoreceptors in the eyes of some fish and terrestrial animals. The tapetum reflects light forward, giving incident photons a "second chance" to collide with a photoreceptor, substantially enhancing retinal photosensitivity in dim light. Across vertebrates (and arthropods), there are a wide variety of tapeta that vary in structure, chemical composition, and even tissue architecture, indicating repeated convergent evolution. To date, the tapetum has not been observed in any cephalopod, however, which also possess a camera‐like eye, but with the retinal photoreceptors oriented in the anterior direction. We therefore hypothesize that the tapetum lucidum is a compensatory adaptation for the suboptimal design of the inverted retina of vertebrates. [ABSTRACT FROM AUTHOR]

Reindeer are the only mammal known to seasonally adapt their eyes to the extremely blue colour of the extended twilight that occupies a large part of the winter 24 h cycle in their Arctic habitat. We describe the atmospheric phenomenon that results in this extreme spectral change in light environment. Reflectance spectroscopy is used to characterize the photonic nanostructure that generates the reflective region of the tapetum lucidum behind the retina. A model is proposed to explain the reversible reformatting of the reflector by seasonal changes in the volume of interstitial fluid within the two-dimensional photonic crystal of parallel collagen fibrils. This model is tested by allowing slow evaporation of the fluid from both summer and winter tapetum surfaces while monitoring changes in the reflectance spectrum. Coupled variations in the spacing and the degree of order of the fibril packing can transform the typical gold-turquoise colour of such a tapetal reflector to a deep blue that matches the peak spectral irradiance of twilight. The mechanism we describe might be applied by other animals with similar tapeta that experience prolonged changes in light environment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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Field investigations represent an excellent opportunity to integrate the Next Generation Science Standards to complement and enhance both classroom and laboratory instruction. This inquiry-based exercise is designed to introduce students to the basic anatomy, ecology, and natural history of a common backyard denizen, the wolf spider (Lycosidae). Students are charged with developing one or more testable hypotheses regarding wolf spiders in their own backyards. Wolf spiders are an ideal subject for field investigation because their secondary eyes possess a highly reflective layer called the tapetum lucidum. At night, this layer produces an unmistakable "eyeshine" when viewed with the beam of a flashlight. Playing the role of students, we tested the hypothesis that wolf spiders should occur at higher density in an undeveloped field than in a typical backyard. To test this, we utilized random quadrat sampling in both habitats using flashlights to detect nocturnal eyeshine. Students obtaining similar results would likely have concluded that wolf spiders were more abundant in natural habitats. [ABSTRACT FROM AUTHOR]

Drug-induced unique cytoplasmic vacuolation was found in the subchronic oral toxicity study of 4-dimethylamino-1-{3-(1-methyl-1H-imidazole-2-yl)propanoyl}piperidine (DMIP), a potential therapeutic agent for neuropathic pain, in beagle dogs. In the first study, DMIP was administered at a dose of 250, 500, or 1,000 mg/kg/day once daily for 14 days. Discoloration of tapetum lucidum accompanied by tapetal swelling was observed at ≥250 mg/kg/day. The tapetal swelling was correlated to the light microscopic observation of cytoplasmic vacuolation in tapetal cells, and similar vacuolation was observed in several other tissues, including the coronary artery and aortal arch, in a dose-dependent manner. Immunohistochemistry for lysosomal-associated membrane protein 2 indicated that the vacuoles were enlarged lysosomes. However, the nature of these vacuoles was different from that of phospholipidosis because no lamellar bodies were observed. In the second study, DMIP was administered at a dose of 10, 50, or 250 mg/kg/day once daily for 14 days followed by a 14-day recovery period. Tapetal changes and systemic vacuolation were not observed at ≤50 mg/kg/day, and vacuolation observed at 250 mg/kg/day was reversible. A few reports have described the enlargement of lysosomes not attributable to phospholipid accumulation. Our findings provide further information about the toxicological implications of drug-induced lysosomal swelling. [ABSTRACT FROM AUTHOR]

ABSTRACT Assessing the impact of human development on animals is complicated by the fact that overt effects may have covert causes. Cryptic impacts (sensu Raiter et al. 2014) can arise where sensory stimuli to which species respond fall outside the human sensory range. Ultraviolet (UV) light, which is detected by a range of nonprimate mammals, is a potential example. We review evidence that dark‐adapted eyes of reindeer–caribou Rangifer tarandus can detect light at 330–410 nm emitted by electrical corona on high‐voltage power lines, which is necessarily barely visible to humans. Based on this, we suggest that the superior ability of Rangifer to detect corona UV light may partly account for the tendency of the animals to avoid power lines. Rangifer has UV‐permissive ocular media that transmit approximately 15 times more corona light than human eyes. Retinal irradiance under full dilation is in the order of 7 times greater in Rangifer compared with humans. Seasonal transformation of the tapetum lucidum substantially increases retinal sensitivity in this species in winter. Threshold distances of detection of corona by Rangifer are in the order of hundreds of meters. Displays of corona may catch the animals' attention, and plume coronas, in particular, may induce the illusion of motion (the phi phenomenon), thereby falsely signaling the presence of potential predators. Both features are likely to increase wariness and cause animals to withdraw from the source of the stimulus. We suggest that spatial and temporal variability of corona contributes to substantial variation observed in the strength and persistence of avoidance responses in Rangifer at these structures. © 2016 The Authors. Wildlife Society Bulletin published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.

Eleven African giant rats were caught, bilaterally enucleated and the globes were routinely processed for light microscopy. Eye weight, vertical and horizontal corneal diameters, as well as vertical, horizontal and axial eye diameters were obtained from each globe. Observed external macroscopic ocular features were typical of mammalian eyes comprising cornea, sclera and attached extraocular muscles. The eye dimensions were however generally smaller than those of humans. The large mean corneal diameter to mean eye diameter ratio of 0.86 as well as the scanty retinal ganglion cells observed are associated with morphological adaptations for nocturnal vision. Pigments observed in the retinal epithelium suggests absence of tapetum lucidum, indicating a sub-optimal nocturnal visual capability. This study has provided scientific documentation of some ocular morphological characteristics of the giant rat and has shown that the eye of this species will be anatomically unsuitable for xenotransplantation in humans. [ABSTRACT FROM AUTHOR]

For explanation eye-shine phenomenon, we used both eyes of five healthy adult donkeys (Equus asinus), cats (Felis domestica) and one-humped camels (Camelus dromedarius). The eyes of a live animal of the three species were photographed under existing light and with a flash. The donkey's pupils appeared black centrally situated and horizontal in direction at daylight. Under flash, the light condensed centrally and the color changed to green or blue. The cat's pupils were oval vertically slit-like and colored yellow to red orange under flash. The camel's pupils characterized by presence of a small centrally spot of light with flash. Tapetum of the donkey was horizontal triangular in shape under the weak light colored indigo with dark blue spots changed into semicircular appearance under strong light. In the cat, Tapetum appeared semicircular in outline with yellow color under weak and strong light. The fundus of camel appeared divided into dark proximal half and lighter distal one under the weak light. Under flash, the two halves appeared transparent white. Microscopically, Tapetum lucidum of the donkey was fibrous in its texture while in cat, it was cellular. In camel, there was a brush's membrane and no Tapetum lucidum. In donkey, the thickness of the tapetal tissue and the degree of pigmentation in the retinal epithelium differed according to the region of Tapetum. The thick tapetal tissue and the unpigmented retinal epithelium combination created the greater reflectance of light. So the absence or presence of Tapetum, the tapetal tissue thickness, the degree of pigmentation in the retinal epithelium and the degree of illumination controlled the eye-shine phenomenon. [ABSTRACT FROM AUTHOR]

It has been shown that mammalian retinal glial (Müller) cells act as living optical fibers that guide the light through the retinal tissue to the photoreceptor cells (Agte et al., 2011; Franze et al., 2007). However, for nonmammalian species it is unclear whether Müller cells also improve the transretinal light transmission. Furthermore, for nonmammalian species there is a lack of ultrastructural data of the retinal cells, which, in general, delivers fundamental information of the retinal function, i.e. the vision of the species. A detailed study of the cellular ultrastructure provides a basic approach of the research. Thus, the aim of the present study was to investigate the retina of the spectacled caimans at electron and light microscopical levels to describe the structural features. For electron microscopy, we used a superfast microwave fixation procedure in order to achieve more precise ultrastructural information than common fixation techniques. As result, our detailed ultrastructural study of all retinal parts shows structural features which strongly indicate that the caiman retina is adapted to dim light and night vision. Various structural characteristics of Müller cells suppose that the Müller cell may increase the light intensity along the path of light through the neuroretina and, thus, increase the sensitivity of the scotopic vision of spectacled caimans. Müller cells traverse the whole thickness of the neuroretina and thus may guide the light from the inner retinal surface to the photoreceptor cell perikarya and the Müller cell microvilli between the photoreceptor segments. Thick Müller cell trunks/processes traverse the layers which contain light-scattering structures, i.e., nerve fibers and synapses. Large Müller cell somata run through the inner nuclear layer and contain flattened, elongated Müller cell nuclei which are arranged along the light path and, thus, may reduce the loss of the light intensity along the retinal light path. The oblique arrangement of many Müller cell trunks/processes in the inner plexiform layer and the large Müller cell somata in the inner nuclear layer may suggest that light guidance through Müller cells increases the visual sensitivity. Furthermore, an adaptation of the caiman retina to low light levels is strongly supported by detailed ultrastructural data of other retinal parts, e.g. by (i) the presence of a guanine-based retinal tapetum, (ii) the rod dominance of the retina, (iii) the presence of photoreceptor cell nuclei, which penetrate the outer limiting membrane, (iv) the relatively low densities of photoreceptor and neuronal cells which is compensated by (v) the presence of rods with long and thick outer segments, that may increase the probability of photon absorption. According to a cell number analysis, the central and temporal areas of the dorsal tapetal retina, which supports downward prey detection in darker water, are the sites of the highest diurnal contrast/color vision, i.e. cone vision and of the highest retinal light sensitivity, i.e. rod vision. [ABSTRACT FROM AUTHOR]

Two transferrin (Tf) glycan-isoforms were previously found in cerebrospinal fluid (CSF); one appears to be derived from serum (Tf-2) and the other from choroid plexus, a CSF-producing tissue (Tf-1). To analyse metabolic differences associated with the two isoforms, their ratio (Tf-2/Tf-1) was defined as the Tf index. Here we report that Tf indices of patients with tauopathies including Alzheimer's disease (2.29 + 0.64) were similar to those of neurological controls (2.07 + 0.87) (P = 0.147). In contrast, Tf indices with Parkinson's disease (PD, 3.38 ± 1.87) and multiple system atrophy (MSA, 3.15 ± 1.72) were higher than those of the controls (2.07 ± 0.87), the P-values being 5 0.001 and 0.024, respectively. Tf indices of PD and MSA did not appear to be normally distributed. Indeed, detrended normal Quantile--Quantile plot analysis revealed the presence of an independent subgroup showing higher Tf indices in PD and MSA. The subgroup of PD showed higher levels of CSF α-synuclein (38.3 ± 17.8ng/ml) than the rest (25.3 ± 11.3ng/ml, P = 0.012). These results suggest that PD (and MSA) includes two subgroups, which show different metabolism of CSF transferrin and α-synuclein. [ABSTRACT FROM AUTHOR]

ABSTRACT Assessing the impact of human development on animals is complicated by the fact that overt effects may have covert causes. Cryptic impacts ( sensu Raiter et al. 2014) can arise where sensory stimuli to which species respond fall outside the human sensory range. Ultraviolet (UV) light, which is detected by a range of nonprimate mammals, is a potential example. We review evidence that dark-adapted eyes of reindeer-caribou Rangifer tarandus can detect light at 330-410 nm emitted by electrical corona on high-voltage power lines, which is necessarily barely visible to humans. Based on this, we suggest that the superior ability of Rangifer to detect corona UV light may partly account for the tendency of the animals to avoid power lines. Rangifer has UV-permissive ocular media that transmit approximately 15 times more corona light than human eyes. Retinal irradiance under full dilation is in the order of 7 times greater in Rangifer compared with humans. Seasonal transformation of the tapetum lucidum substantially increases retinal sensitivity in this species in winter. Threshold distances of detection of corona by Rangifer are in the order of hundreds of meters. Displays of corona may catch the animals' attention, and plume coronas, in particular, may induce the illusion of motion (the phi phenomenon), thereby falsely signaling the presence of potential predators. Both features are likely to increase wariness and cause animals to withdraw from the source of the stimulus. We suggest that spatial and temporal variability of corona contributes to substantial variation observed in the strength and persistence of avoidance responses in Rangifer at these structures. © 2016 The Authors. Wildlife Society Bulletin published by Wiley Periodicals, Inc. on behalf of The Wildlife Society. [ABSTRACT FROM AUTHOR]

Arctic reindeer experience extreme changes in environmental light from continuous summer daylight to continuous winter darkness. Here, we show that they may have a unique mechanism to cope with winter darkness by changing the wavelength reflection from their tapetum lucidum (TL). In summer, it is golden with most light reflected back directly through the retina, whereas in winter it is deep blue with less light reflected out of the eye. The blue reflection in winter is associated with significantly increased retinal sensitivity compared with summer animals. The wavelength of reflection depends on TL collagen spacing, with reduced spacing resulting in shorter wavelengths, which we confirmed in summer and winter animals. Winter animals have significantly increased intra-ocular pressure, probably produced by permanent pupil dilation blocking ocular drainage. This may explain the collagen compression. The resulting shift to a blue reflection may scatter light through photoreceptors rather than directly reflecting it, resulting in elevated retinal sensitivity via increased photon capture. This is, to our knowledge, the first description of a retinal structural adaptation to seasonal changes in environmental light. Increased sensitivity occurs at the cost of reduced acuity, but may be an important adaptation in reindeer to detect moving predators in the dark Arctic winter. [ABSTRACT FROM AUTHOR]

The tapetum lucidum is a light-reflective tissue in the eyes of many animals. Many ungulates have a fibrous tapetum. The horse has one of the largest eyes of any living animal and also has excellent vision in low-light environments. This study aimed to clarify the macroscopic tapetal shape, relationship between the tapetal thickness and the degree of pigmentation of the retinal pigment epithelium ( RPE), spatial relationship between the visual streak and the tapetum, and wavelength of the light reflected from the tapetum in the horse. Macroscopically, weak light revealed the tapetum as a horizontal band located dorsal to and away from the optic disc. The tapetum expanded dorsally as the illumination increased. The tapetal tissue consisted of lamellae of collagen fibrils running parallel to the retinal surface; these spread over almost the entire ocular fundus and were thicker in the horizontal band dorsal to the disc. Only the horizontal band of the tapetum was covered by unpigmented RPE, suggesting that this band reflects light and is responsible for mesopic and scotopic vision. The visual streak was located in the ventral part of the horizontal band, ventral to the thickest part of the tapetum. The wavelength of the light reflected from the horizontal band of the tapetum was estimated from the diameter and interfibrous distance of the collagen fibrils to be approximately 468 nm. Therefore, the light reflected from the tapetum should be more effectively absorbed by rods than by cones, and should not interfere with photopic vision. [ABSTRACT FROM AUTHOR]

Objective To describe the gross, histopathological, and ultrastructural findings in a dog with bilateral tapetal dysplasia. Procedures The globes of a 15-year-old neutered male Swedish Vallhund dog with a ventrally displaced tapetum in both eyes were fixed in 10% formalin and submitted to the Comparative Ocular Pathology Laboratory of Wisconsin for histological evaluation. Sections were stained with hematoxylin and eosin, Masson's trichrome, and Melan- A immunohistochemistry ( IHC), and tissues were subsequently processed for transmission electron microscopy. Results Bilateral fundic and gross examination revealed a tapetal fundus inferior to the optic nerve head ( ONH) and a nontapetal fundus with mild scattering of tapetal tissue superior to the ONH. Histologically, there was decreased pigmentation of the retinal pigment epithelium with only a few melanin granules in the peripheral retina. The affected tapetum was relatively acellular and fibrous with occasional tapetal cells scattered throughout the inner choroid or displaced into the vascular outer choroid. Special stains revealed that the tapetum was mostly composed of collagen ( Masson's trichrome) and failed to express Melan- A ( IHC) unlike a normal canine control tapetum. Ultrastructurally, the tapetum was markedly dysplastic both superior and inferior to the ONH with no uniformly arranged tapetal cells. The few cells identified within the tapetum contained irregularly arranged and disorganized electron-dense structures within their cytoplasm, which were interpreted as dysplastic tapetal rodlets. Conclusions Based on microscopic and ultrastructural findings, this is the first report of tapetal dysplasia in a dog. [ABSTRACT FROM AUTHOR]

Purpose In chicks, ocular growth inhibition is associated with choroidal thickening and growth stimulation with choroidal thinning, suggesting a mechanistic link between the two responses. Because muscarinic antagonists inhibit the development of myopia in animal models by a non-accommodative mechanism, we tested the hypothesis that agonists would stimulate eye growth and thin the choroid. We also hypothesized that the effective growth-inhibiting antagonists would thicken the choroid. Methods Chicks, age 12-16 days, were used. In vivo: Agonists: Single intravitreal injections (20 μL) of oxotremorine (oxo), pilocarpine (pilo), carbachol (carb), or arecaidine (arec) were given to otherwise untreated eyes. A-scan ultrasonography was done prior to injections, and at 3, 24, 48 and 72 h. Antagonists: −10D lenses were worn on one eye for 4 days. Atropine (atro), pirenzepine (pirz), oxyphenonium (oxy) or dicyclomine (dicy) were injected (20 μL) daily into lens-wearing eyes; saline injections were done as controls. Ultrasonography was done on d1 and on d4; on d4 measurements were done before and 3 h after injections. In vitro: Paired eyecups of retinal pigment epithelium ( RPE), choroid and sclera were made from 1-week old chicks. All drugs except atropine were tested on one eyecup, its pair in plain medium. Choroidal thickness was measured at various times over 48 h. Results Agonists: In vivo, oxotremorine caused an increase in the rate of axial elongation (drug vs saline: 24-72 h: 338 μm vs 250 μm; p < 0.001). All except pilocarpine caused choroidal thinning by 24 h (oxo, carb and arec vs saline: −25, −35 and −46 μm vs 3 μm). In vitro, all agonists thinned choroids by 24 h (oxo: −6 vs 111 μm; pilo: 45 vs 212 μm; carb: −58 vs 65 μm; arec: 47 vs 139 μm; p < 0.05). Antagonists: Atropine, pirenzepine and oxyphenonium inhibited the development of myopia in negative lens-wearing eyes, and also caused choroidal thickening (drug vs saline: 42, 80, 88 vs 10 μm per 3 h). In vitro, pirenzepine thickened choroids by 3 h (77 vs 2 μm, p < 0.01). Conclusions Muscarinic agonists caused choroidal thinning in intact eyes and eyecups, supporting a role for acetylcholine in the choroidal response to hyperopic defocus or form deprivation. Only oxotremorine stimulated eye growth, which is inconsistent with a muscarinic receptor mechanism for antagonist-induced eye growth inhibition. The dissociation between choroidal thinning and ocular growth stimulation for the other agonists in vivo suggest separate pathways for the two. [ABSTRACT FROM AUTHOR]

Purpose Young eyes compensate for the defocus imposed by spectacle lenses by changing their rate of elongation and their choroidal thickness, bringing their refractive status back to the pre-lens condition. We asked whether the initial rate of change either in the ocular components or in refraction is a function of the power of the lenses worn, a result that would be consistent with the existence of a proportional controller mechanism. Methods Two separate studies were conducted; both tracked changes in refractive errors and ocular dimensions. Study A: To study the effects of lens power and sign, young chicks were tracked for 4 days after they were fitted with positive (+5, +10 or +15 D) or negative (−5, −10, −15 D) lenses over one eye. In another experiment, biometric changes to plano, +1, +2 and +3 D lenses were tracked over a 24 h treatment period. Study B: Normal emmetropisation was tracked from hatching to 6 days of age and then a defocusing lens, either +6 D or −7 D, was fitted over one eye and additional biometric data collected after 48 h. Results In study A, animals treated with positive lenses (+5, +10 or +15 D) showed statistical similar initial choroid responses, with a mean thickening 24 μm h−1 over the first 5 h. Likewise, with the low power positive lenses, a statistically similar magnitude of choroidal thickening was observed across groups (+1 D: 46.0 ± 7.8 μm h−1; +2 D: 53.5 ± 9.9 μm h−1; +3 D 53.3 ± 24.1 μm h−1) in the first hour of lens wear compared to that of a plano control group. These similar rates of change in choroidal thickness indicate that the signalling response is binary in nature and not influenced by the magnitude of the myopic defocus. Treatments with −5, −10 and −15 D lenses induced statistically similar amounts of choroidal thinning, averaging −70 ± 15 μm after 5 h and −96 ± 45 μm after 24 h. Similar rates in inner axial length changes were also seen with these lens treatments until compensation was reached, once again indicating that the signalling response is not influenced by the magnitude of hyperopic defocus. In study B, after 48 h of +6 D lens treatment, the average refractive error and choroidal changes were found to be larger in magnitude than expected if perfect compensation had taken place, with a + 2.4 D overshoot in refractive compensation. Conclusion Taken together, our results with both weak and higher power positive lenses suggest that eye growth is guided more by the sign than by the magnitude of the defocus, and our results for higher power negative lenses support a similar conclusion. These behaviour patterns and the overshoot seen in Study B are more consistent with the behaviour of a bang-bang controller than a proportional controller. [ABSTRACT FROM AUTHOR]

The eyes of three adult male African elephants were examined, the retinas were whole-mounted, stained and analyzed to determine visual acuity. A range of small to large ganglion cell types were observed across the retinas. We observed three regions of high ganglion cell density, one in the upper temporal quadrant, a visual or horizontal streak and a smaller region at the nasal end of the horizontal streak. The peak density of ganglion cells observed was 5,280/mm2, and our calculations indicate that the elephant has a maximal visual acuity of between 13.16 and 14.37 cycles/degree. We observed a heterogeneous structure of a tapetum lucidum, the cells of which were found to be most strongly aggregated behind the temporal and nasal densities of retinal ganglion cells. The strength of the tapetum lucidum was weaker posterior to the density of ganglion cells forming the horizontal streak. The morphology of the elephant eye appears to be such that it reflects: (1) the importance of trunk–eye co-ordination for feeding; (2) the importance of 24-hour vigilance for either predators or conspecifics, and (3) the arrhythmic nature of the daily activity of this animal, being useful both diurnally and nocturnally. Copyright © 2010 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The article examines the relationship between the distribution of tapetum fibrosum and retinal pigment epithelium (RPE) in the sheep eye. The authors explain that the tapetum lucidum is a device that improves visual sensitivity in mesopic vision. They found that about 55% of the histological tapetal area is covered with the nonpigmental area and is functional under a natural light condition. They also state that the functional tapetal area was similar to the L-shaped high-density area of retinal ganglion cells.

Abstract: Histology and fine structure of three anchovy outer retinae is investigated by light- and electron microscopy and computer aided topographic mapping. They all possess novel linear triple cones with two large lateral components and one small central component. Engraulis encrasicolus has a polycone-dominated retina with two triple-cone sectors, the retinae of Cetengraulis mysticetus and Anchovia macrolepidota are triple-cone dominated. Interspecific differences in the configuration of the complex outer retina structures appear well adapted to the requirements of visual orientation in the respective photic habitats. In addition a survey on the occurrence of triple cones in teleosts is given. [Copyright &y& Elsevier]

The owl monkey, Aotus, is the only modern nocturnal anthropoid with monogamous social structure. It has been demonstrated by the fossil species, Aotus dindensis, discovered from La Venta, Colombia, that the Aotus lineage had emerged as early as the middle Miocene (12-15 Ma). The type specimen of A. dindensis, which was discovered in 1986, preserves extremely large orbits, indicating a nocturnal habit. However, a few anatomical traits in living Aotus, such as the lack of a tapetum lucidum, indicates that nocturnality is a secondary adaptation from diurnal ancestry in this genus. Here we report new fossil specimens of A. dindensis from La Venta. The specimens include maxillary teeth and a mandibular fragment preserving lower molars. The detailed analysis of the specimen suggests that A. dindensis exhibits strong sexual dimorphism in the maxillary canine and premolars, which is traditionally associated with intense intermale competition for mates and/or food resources in non-monogamous, diurnal societies. As a result, the new fossil materials of A. dindensis demonstrate the first osteological evidence for the diurnal ancestry of the night monkey, Aotus. Moreover, the coexistence of large orbits and canine dimorphism suggests the presence of mosaic evolution in the craniodental characters of the Aotus lineage. [ABSTRACT FROM AUTHOR]

Purpose To carry out a descriptive investigation of the capybara ( Hydrochaeris hydrochaeris) eye and to perform selected ophthalmic diagnostic tests with the aim of establishing normal physiological reference values for this species. Method A total of 22 healthy, capybaras were used to test most of the parameters in this investigation. Ages varied from 2 to 4 years of age. Selected diagnostic ocular tests were performed including Schirmer tear test, tonometry using an applanation tonometer (Tonopen®), central corneal thickness using an ultrasonic pachymeter (Sonomed, Micropach®, Model 200P +), axial globe length and culture of the normal conjunctival bacterial flora. Results and discussion Capybara's normal ocular features include: dorsal and ventral puncta, vestigial third eyelid, true cilia only at the upper eyelid margins. The bulbar conjunctiva is noticeably densely pigmented with a brown to bronze color. The capybara's pupil is oval in shape and vertical in position No tapetum lucidum is present in this species and the retinal blood vessels are almost absent. Results for selected ocular diagnostic tests investigated were: Intraocular pressure: 18.4 ± 3.8 mmHg; Schirmer tear test: 14.9 ± 5.1 mm/min; Central corneal thickness: 0.46 ± 0.03 mm; Axial globe length: 22.20 ± 1.71 mm. No statistically significant differences between ages or genders were found for any of the results. Corynebacterium sp., Micrococcus sp., Bacillus sp. and Staphylococcus sp. were isolated from healthy conjunctiva, suggesting they are normal constituents of the conjunctival flora of the capybara eye. The corneal epithelium of the capybara possesses a thin and discrete Bowman's layer. Results and parameters obtained in this investigation exposed unique anatomic features of the capybara eye and will help veterinary ophthalmologists to more accurately diagnose discrete or unusual pathological changes of the capybara eye. Furthermore, corneal thickness and axial length of the capybara are similar to that of human beings, revealing that the world's largest living rodent might be an excellent biological model for ophthalmic studies. [ABSTRACT FROM AUTHOR]

Abstract: Blood flow in the choroid of eyes with retinal degeneration is normally reduced. In the Abyssinian cat, where no changes in choroidal blood flow were observed recently, a closer morphological analysis revealed that two factors might have lead to this finding. (a) The cat''s eye consists of a tapetum lucidum where 15.5μm microspheres cannot reach the precapillary branching in contrast to the non-tapetal areas, where microspheres were located right next to the choriocapillaris. This fact has not yet been described and might be important for further physiological measurements in eyes of species with a tapetum lucidum. (b) The tapetal cells themselves might have a protective role for the choriocapillaris and RPE during retinal degeneration in this specific animal model. Even in late stages of retinal degeneration the RPE and choriocapillaris in areas covered by the tapetum lucidum were not affected whereas non-tapetal areas showed loss of RPE and capillaries. [Copyright &y& Elsevier]

The tarsier skull has been of particular interest in studies of primate taxonomy and functional morphology for several decades. Despite this, there remains no comprehensive data on how the tarsier skull develops, especially in relation to the soft-tissues of the head. Here we have documented for the first time fetal development of the skull and brain as well as the nasal septum and eyes in T. bancanus. We have also tested for the possible influence of these tissues in shaping skull architecture. Nineteen post-mortem specimens were imaged using high-resolution magnetic resonance imaging and magnetic resonance microscopy. Landmarks and volume data were collected and analysed. Findings demonstrated massive increases of brain size and eye size as well as flattening of the midline cranial base, facial projection and orbital margin frontation. Little evidence was found to support the notion that growth of the brain or nasal septum physically drives the observed changes of the skull. However, increases in the size of the eyes relative to skull size were associated with orbital margin frontation. With the possible exception of the results for eye size, the findings indicate that rather than forcing change the soft-tissues form a framework that physically constrains the morphogenetic template of the skeletal elements. This suggests, for example, that the degree of cranial base angulation seen in adulthood is not directly determined by brain expansion bending the basicranium, but by brain enlargement limiting the extent of cranial base flattening (retroflexion) in the fetus. [ABSTRACT FROM AUTHOR]

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. 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. 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. 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. [ABSTRACT FROM AUTHOR]

The ultrastructure and chemical composition of reflective organelles in the anterior pigment epithelium of the iris of the European starling Sturnus vulgaris were examined. The reflective organelles produced a diffuse white reflectance at the iris mid-section which was visible only when the stroma was removed. The pigment granules were clear, angular, and birefringent under the light microscope. In electron micrographs the granules were irregular in shape and density, sometimes crystalline in appearance, but more often they were lost during sectioning or staining. Guanine was abundant in the modified pigment epithelium of the starling, but not in the pigment epithelia of other birds that lacked birefringent granules. Pteridines, such as xanthopterin and leucopterin, were present in small amounts. Pteridines were also present in the iris stroma which had no reflective organelles. The reflective organelles in the starling pigment epithelium resemble both the reflecting platelets of lower vertebrate chromatophores and the reflective granules in the tapeta of various vertebrates. Possible derivation of the organelles from these sources is discussed. [ABSTRACT FROM AUTHOR]

The choroidally located tapetum lucidum of the Port Jackson shark ( Heterodontus phillipi) was examined by light and electron microscopy in light-adapted specimens. In this species the tapetum consists of a single layer of overlapping cells oriented at an angle of about 30° to the incoming light and situated immediately external to the choriocapillaris. These tapetal cells alternate with and are separated from one another by melanocytes which extend beyond the tapetal cells to intervene between the tapetal cells and the incoming light. The tapetal cells and the melanocytes are flattened plate-like cells with their widest dimension facing the retina. Internally the tapetal cells display a peripherally located vesicular nucleus with most organelles in a paranuclear location. The bulk of a tapetal cell is packed with regularly spaced crystals reported to be guanine. The size and spacing of these reflective crystals is commensurate with the principles of constructive interference. In light adaptation, the melanosomes of the intervening melanocytes are widely dispersed and for the most part block the passage of light to the tapetal cells. Although dark-adapted specimens were not available, it seems reasonable to assume that in dark adaptation these melanosomes will retreat to unmask the tapetum and allow it to function as a known reflective layer. [ABSTRACT FROM AUTHOR]

The fine structure of the tapetum lucidum of the domestic ferret was studied by both light and electron microscopy. In this species the reflective layer is a tapetum cellulosum situated in the choroid and restricted to the superior fundus. Posteriorly in its thickest region, the tapetum is composed of 10-12 layers of flattened polygonal cells. The number of layers gradually decreases peripherally and eventually the tapetum disappears. The nucleus of the tapetal cell is usually centrally located. Small mitochondria are scattered peripherally while the few other cell organelles observed are normally clustered near the nucleus. The dominant cytological feature of these cells is numerous electron dense rodlets whose long axes are perpendicular to the oncoming light. These rodlets are the reflective material of the tapetum. The orientation of these rodlets is normally uniform within each tapetal cell but may vary between adjacent cells. The diameter and spacing of these rodlets are consistent with the principles of constructive interference. Blood vessels penetrate the tapetal layer at right angles to supply the choriocapillaris. The choriocapillaris is indented into the retinal epithelial layer to give a flat reflecting surface to the tapetum. [ABSTRACT FROM AUTHOR]

The normal morphology of the retinal epithelium and tapetum lucidum has been studied by transmission electron microscopy in the eye of a teleost; the giant danio ( Danio malabaricus). The retinal epithelium forms a single layer of cells joined laterally by cell junctions. Centrally in the retina these cells are tall columnar, while more peripherally they become more cuboidal in shape. Apical processes of these cells enclose photoreceptor inner and outer segments. Few basal (scleral) infoldings are seen in the epithelial cells. Throughout the epithelial layer the cells display smooth endoplasmic reticulum, numerous mitochondria and phagosomes. Within the epithelial cells centrally located in the retina are found numerous lipid spheres which act as a tapetum lucidum. Only at the extreme periphery are these reflective spheres scarce or absent from the epithelial cells. Melanosomes, although found throughout the epithelial layer, are scarce centrally and more numerous peripherally. A population of wandering phagocytes appears to be a normal feature within the retinal epithelial layer of this species. Bruch's membrane shows a trilaminate composition rather than the usual pentalaminate structure seen in most vertebrates. The choriocapillaris endothelium facing Bruch's membrane is very thin but only minimally fenestrated. [ABSTRACT FROM AUTHOR]

The postnatal development of the tapetum lucidum cellulosum of the cat was studied with cytochemical, histochemical, microspectrofluorimetric, and chemical methods. At birth, the prospective tapetal cells exhibit a cytoplasmic, formaldehyde-induced catechol fluorescence which, by spectrographical criteria, is due to the presence of a thioether of DOPA, most likely 5-S-cysteinyldopa. These cells show a positive DOPA-reaction in the GERL, a Golgi-associated system of smooth endoplasmic reticulum. Beyond the third postnatal week, tyrosinase activity is found in the GERL and in the premelanosomal-like tapetal rods. Shortly thereafter, cells in the innermost layers of the tapetum reveal a change in the colour of the fluorophore from green (formaldehyde-induced) to orange-yellow (autofluorescent). Concomitant with the appearance of the yellow fluorophore (having spectral characteristics similar to authentic riboflavin), the DOPA-reactivity in the tapetal cells is no longer detectable and the cells attain the feature of mature tapetal elements with typical elongated, parallelly arranged, electron-dense rods. The increases and decreases in tyrosinase activity are parallelled by similar time-dependent changes in DOPA and cysteinyldopa content. Both DOPA and cysteinyldopa are considered as potential precursors of melanin polymers. Thus, these data support the concept that the tapetal cells represent uniquely differentiated and highly specialized choroidal melanocytes. The compact arrangement of these cells in the tapetum lucidum renders it an ideal tissue for studying the developmental biology of melanocytes. [ABSTRACT FROM AUTHOR]

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