The choroidal tapetum lucidum of Latimeria chalumnae

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.

The Organic Crystalline Materials of Vision: Structure-Function Considerations from the Nanometer to the Millimeter Scale

Vision mechanisms in animals, especially those living in water, are diverse. Many eyes have reflective elements that consist of multilayers of nanometer-sized crystalline plates, composed of organic molecules. The crystal multilayer assemblies owe their enhanced reflectivity to the high refractive indices of the crystals in preferred crystallographic directions. The high refractive indices are due to the molecular arrangements in their crystal structures. Herein, data regarding these difficult-to-characterize crystals are reviewed. This is followed by a discussion on the function of these crystalline assemblies, especially in visual systems whose anatomy has been well characterized under close to in vivo conditions. Three test cases are presented, and then the relations between the reflecting crystalline components and their functions, including the relations between molecular structure, crystal structure, and reflecting properties are discussed. Some of the underlying mechanisms are also discussed, and finally open questions in the field are identified.

The eyes of deep-sea fish. I: Lens pigmentation, tapeta and visual pigments

Deep-sea fish, defined as those living below 200 m, inhabit a most unusual photic environment, being exposed to two sources of visible radiation; very dim downwelling sunlight and bioluminescence, both of which are, in most cases, maximal at wavelengths around 450-500 nm. This paper summarises the reflective properties of the ocular tapeta often found in these animals, the pigmentation of their lenses and the absorption characteristics of their visual pigments. Deep-sea tapeta usually appear blue to the human observer, reflecting mainly shortwave radiation. However, reflection in other parts of the spectrum is not uncommon and uneven tapetal distribution across the retina is widespread. Perhaps surprisingly, given the fact that they live in a photon limited environment, the lenses of some deep-sea teleosts are bright yellow, absorbing much of the shortwave part of the spectrum. Such lenses contain a variety of biochemically distinct pigments which most likely serve to enhance the visibility of bioluminescent signals. Of the 195 different visual pigments characterised by either detergent extract or microspectrophotometry in the retinae of deep-sea fishes, ca. 87% have peak absorbances within the range 468-494 nm. Modelling shows that this is most likely an adaptation for the detection of bioluminescence. Around 13% of deep-sea fish have retinae containing more than one visual pigment. Of these, we highlight three genera of stomiid dragonfishes, which uniquely produce far red bioluminescence from suborbital photophores. Using a combination of longwave-shifted visual pigments and in one species (Malacosteus niger) a chlorophyll-related photosensitizer, these fish have evolved extreme red sensitivity enabling them to see their own bioluminescence and giving them a private spectral waveband invisible to other inhabitants of the deep-ocean.

Latimeria : a paradoxical fish

The traditional view of the phylogenetic position of Latimeria is based on osteological details and states that coelacanths are derivatives of rhipidistians and hence are the nearest living relatives of tetrapods. However, recent studies of the soft anatomy of Latimeria have highlighted many similarities with chondrichthyans. These similarities have led to the hypothesis that coelacanths are most closely related to cartilaginous fishes. The evidence for this hypothesis is questioned and the view that Latimeria is a bony fish is reaffirmed, with the suggestion that, among Recent fishes, it is the cousin of tetrapods.

Fine structure of the retinal epithelium and tapetum lucidum in the giant danio (danio malabaricus) (teleost)

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.

Studies on the eyes of anchovies Anchoa mitchilli and A. hepsetus (Engraulidae) with particular reference to the pigment epithelium

A study has been made of the eyes of the anchovies Anchoa mitchilli and A. hepsetus (Engraulidae) with particular reference to the pigment epithelium. Eyes of dark (d.a.) and light (l.a.) adapted fish were investigated by optical and electron microscopy. In light adapted eyes the pigment epithelium is radially extended with the vitread region of the cells inserted amongst the cones and lying in vertical rows above them. They are separated from adjacent rows of pigment epithelial cells by masses of rods arranged in similar vertical sheets. The vitread end of the cell, where it fits between the cones, is cuneate in shape. In l.a. eyes lateral lobes develop from the cuneate portion of the cell, extend into the spaces between the cone rows, and serve to shield the lateral margins of the cones. On dark adaptation of the eye the lateral lobes are withdrawn as the cell shortens radially. The cytoplasm of the pigment epithelium contains numerous myeloid bodies composed of stacks of smooth endoplasmic reticulum distributed throughout the cell. In l.a. eyes masses of less well ordered e.r. cisternae and tubules are also common, especially in the vitread region among the tapetal material and melanosomes. Mitochondria, some large vacuoles, lysosomes containing ingested portions of rod outer segments, and also occasionally melanosomes and tapetal material occur in the basal region of the cell in the vicinity of the nucleus. Structurally differentiated ground substance and ribosomes are poorly represented throughout all regions. Spherical and cylindrical melanosomes and tapetal material composed of guanine are plentiful within the cell. In the l.a. state melanosomes occur in the vitread region, at the level of the cones, but in the d.a. state they become concentrated in the basal region forming a dark scleral zone throughout the tissue. The tapetal material consists of approximately isodiametric crystallites, truncate needles and thin almost square platelets. All forms are strongly birefringent. The crystallites are distributed throughout the l.a. cell but in the d.a. condition they become concentrated mainly in the middle region forming a prominent whitish zone throughout the tissue vitread to the melanosome layer. In l.a. eyes the needles are arranged axially and are distributed mainly around the periphery of the cell in the vitread region. On dark adaptation of the eye some of the needles are dispersed amongst the crystallites in the middle region. The platelets are grouped into two stacks which remain essentially unaltered in organization and location during photomechanical movements of the eye. The stacks are grouped into a V in the vitread cuneate region with the platelets abutting the cone units and lying in the same vertical plane as the cone lamellae. Each stack is composed of several regularly spaced rows with each row made up of two files of platelets. The average thickness of platelets measured from vertical sections through stacks is about 82 nm and the distance between platelets of adjacent rows is about 112 nm. A cisterna of endoplasmic reticulum runs between each row and is connected to the membrane sacs of adjacent platelets by tubular connections. Non-membrane crossbridging material also links the cisterna and platelet sacs. The consequence of the organization of the platelet stacks as a system for the reflexion of light into the cones by constructive interference is discussed.

Fine structure of the retinal epithelium and tapetum lucidum of the opossum (Didelphis virginiana)

The fine structure of the retinal epithelium has been studied by electron microscopy in the opossum (Didelphis virginiana). The retinal epithelium, over most of the retina, is typical of that in other vertebrates and consists of a single layer of heavily pigmented, cuboidal cells. These cells display extensive basal (scleral) infoldings and numerous apical (vitreal) processes which enclose photoreceptor outer segments. A semicircular area of the retinal epithelium in the superior fundus is further specialized as a tapetum lucidum. The reflecting material consists of a large quantity of lipoidal spheres scattered throughout the epithelial cells. Centrally in the tapetal area very few or no melanosomes are found, indicating a non-occlusible tapetum. Peripherally in be tapetum, the epithelial cells contain both reflecting material and melanosomes. As in the non-tapetal area, the epithelial cells of the tapetum display large amounts of smooth endoplasmic reticulum and numerous mitochondria. Bruch's membrane everywhere displays the usual pentalaminate structure described for most vertebrates. The choriocapillaris is also typical, in that numerous fenestrations are present in the endothelium bordering Bruch's membrane.