Iridescent Layer Research Articles

Morphological adaptations on the eye of the golden gray mullet (Chelon aurata): Using light and scanning electron microscopical study.

The present investigations were designed to describe the ultrastructural properties of the eye of the golden gray mullet (Chelon aurata). For this purpose, the eyes were examined grossly, and by light and electron microscope. The external layer consists of the cornea and the sclera. Three layers compose the cornea; the anterior stratified cuboidal epithelial; the anterior limiting (Bowman) membrane; and the thick dermal layer of the stroma. The mucoidal layer has small collagen fiber bundles embedded in the CT layer and located between the anterior portion of the scleral cornea and the dermal stroma, (or "substancia propria"). The iridescent layer is thin at the center and thick at the periphery. It contains a pigmented layer with many ossicles. SEM analysis reveals that the cornea consists of undetermined shaped cells joined together by numerous thread-like micro-ridges, with several micro-tubercles on the external surface. The photoreceptor layer had two types of cells: the rod-shaped and the cone-shaped cells. The cone cells differentiate into two types of cells: single and double cells. SEM analysis of the retina showed that rod cells appear as thin long uniform rod-like, while the cone cells appear as rod cells with ovoid bases. SEM analysis demonstrates that the inner side of the retinal epithelium appears to be wrapped around itself. The morphological appearance of the eye adapts to life in superficial aquatic conditions. In conclusion, the current findings provide morphologic evidence to better understand the mechanism of fish vision adaptation to environmental conditions. RESEARCH HIGHLIGHTS: The transparent cornea composed of three layers; anterior stratified cuboidal epithelial, Bowman's membrane, and a thick dermal stromal layer. The mucoidal layer is formed from small collagen fibers bundles embedded in the CT layer and located between the anterior portion of the scleral cornea and the dermal stroma. There are two types of photoreceptor cells: rod and cone cells. Cone cells can be single and double cells.

Visual adaptations of the eye of the gilthead sea bream (Sparus aurata).

The current investigation represents the morphological description of the adaptation of gilthead sea bream Sparus aurata to its environmental conditions. For the achievement of this study twelve mature fishes were used for gross, light and electron microscope examinations. The cornea is consists of three layers; an anterior stratified cuboidal epithelium, bowman layer, and a dermal stroma. A mucoid layer located between the dermal stroma and the anterior part of the scleral cornea, while the iridescent layer located between the anterior and the posterior scleral stroma. The retina is composed of ten layers. There are two areas of the retina; non-nervous and nervous. The non-nervous area is represented only by the thick pigmented epithelium layer. The retina has both rod and cone photoreceptors. The cones are differentiated into three types; single, double and triple cones. Rods inner segments contain numerous mitochondria while that of the cones has ellipsosomes. These ellipsosomes may function in condensing light in the outer segment. There is a clear correlation between ellipsosomes formation in the inner segment of cone and night condition.

Structure and function of the retinal pigment epithelium, photoreceptors and cornea in the eye of Sardinella aurita (Clupeidae, Teleostei)

The structure of the pigment epithelium, photoreceptors and the cornea in the eye of a teleost, Sardinella aurita was examined by light and electron microscopy. The retinal pigment epithelium forms a single layer of cells joined laterally by cell junctions. Centrally in the retina these cells are columnar, while more peripherally they become cuboidal in shape. The basal (scleral) border of the pigment epithelial cells is not infolded but is relatively smooth. Phagosomes containing lysosome-like bodies are also common features of the retinal pigment epithelium. Numerous melanosomes (pigment granules) are abundant throughout the epithelial cells. These melanosomes probably absorb light which has passed through the photoreceptor layer. Four photoreceptor cells were identified; rods, long single cones, short single cones and double cones. The presence of these types suggests a diversity of photoreceptor function. Square mosaic pattern of cones and well-developed choroid gland are also main features of the eye. The inner segment of rods and cones were rich in organelles indicating much synthetic activity. Calycal processes projecting from cone outer segments are also observed. The cornea includes an epithelium with a complex pattern of surface microplicae, a basement membrane, dermal stroma, an iridescent layer, scleral stroma, Descemet’s membrane and endothelium. The autochthonous layer which is seen in some teleosts has not been observed in the cornea of this species. These and other observations were discussed in relation to the photic environment and habits of this fish.

Iridescence in Ancient Glass: A Morphological and Chemical Investigation

Visible damage on the surface of historic glass, iridescence, is developed by the influence of environmental factors during time. Iridescence is usually considered as an age indicator in historic glass. Samples from the ancient Sasanian historic site called “Tomb‐e Pargan” in the Boushehr Province of Iran were subjected to microscopic analyzing with ESEM, ICP‐OES, and polarization microscopy to characterize structural damages and its impact on the surface deposition. Chemical–mineralogical studies were also performed. The current investigation suggests that the iridescent layer is developed as the result of alkali ion depletion on the glass surface following the formation of a shelly layer. Due to the porosity of ancient glass surface, ions migrate and deposit continuously on the surface in the form of crust. The phenomenon causes fine lamellar structure on the surface. Samples from Tomb‐e Pargan characteristically contain potassium and magnesium.

Derivation of the stress-strain behavior of the constituents of bio-inspired layered TiO2/PE-nanocomposites by inverse modeling based on FE-simulations of nanoindentation test.

Owing to the apparent simple morphology and peculiar properties, nacre, an iridescent layer, coating of the inner part of mollusk shells, has attracted considerable attention of biologists, material scientists and engineers. The basic structural motif in nacre is the assembly of oriented plate-like aragonite crystals with a 'brick' (CaCO3 crystals) and 'mortar' (macromolecular components like proteins) organization. Many scientific researchers recognize that such structures are associated with the excellent mechanical properties of nacre and biomimetic strategies have been proposed to produce new layered nanocomposites. During the past years, increasing efforts have been devoted towards exploiting nacre's structural design principle in the synthesis of novel nanocomposites. However, the direct transfer of nacre's architecture to an artificial inorganic material has not been achieved yet. In the present contribution we report on laminated architecture, composed of the inorganic oxide (TiO2) and organic polyelectrolyte (PE) layers which fulfill this task. To get a better insight and understanding concerning the mechanical behaviour of bio-inspired layered materials consisting of oxide ceramics and organic layers, the elastic-plastic properties of titanium dioxide and organic polyelectrolyte phase are determined via FE-modelling of the nanoindentation process. With the use of inverse modeling and based on numerical models which are applied on the microscopic scale, the material properties of the constituents are derived.

Nacre from mollusk shells: a model for high-performance structural materials

Nacre is the iridescent layer found inside a large number of mollusk shells. This natural composite has a very high mineral content, which makes it hard and stiff. However it is the toughness of nacre which is the most impressive: it is three orders of magnitude tougher than the mineral it is made of. No manmade composite material can boast such amplification in toughness, and for this reason nacre has become a biomimetic model material. The mineral in nacre comes in the form of microscopic polygonal tablets, which have the ability to ‘slide’ on one another in large numbers when the material is loaded in tension. This key mechanism makes nacre a quasi-ductile material, which in turn greatly increases its toughness and makes it damage tolerant. Numerous ‘artificial nacres’ were developed in the past but none of them can truly duplicate the remarkable mechanism of tablet sliding. In this work selected structural features of nacre were implemented in a PMMA-based composite, which for the first time could replicate the collective tablet sliding mechanism. This material demonstrates that the powerful toughening mechanism operating in natural nacre can be duplicated and harnessed in engineering materials.

Making Mother of Pearl

Nacre is an iridescent layer of calcium carbonate lining the inside of shells of marine mollusks and is commonly known as “mother of pearl.” It is composed of layers of uniformly oriented crystals of aragonite (a metastable form of calcium carbonate) separated by layers of organic matrix. How the ordered structure of aragonite layers is achieved has been unclear. Suzuki et al. (p. [1388][1], published online 13 August 2009; see the Perspective by [Kroger][2] ) identified two acidic matrix proteins (Pif 97 and Pif 80) that regulate nacre formation in the Japanese pearl oyster. The proteins appear to form a complex in which Pif 80 binds to aragonite and Pif 97 binds to other macromolecules in the organic matrix. [1]: /lookup/doi/10.1126/science.1173793 [2]: /lookup/doi/10.1126/science.1177055

An Acidic Matrix Protein, Pif, Is a Key Macromolecule for Nacre Formation

The mollusk shell is a hard tissue consisting of calcium carbonate crystals and an organic matrix. The nacre of the shell is characterized by a stacked compartment structure with a uniformly oriented c axis of aragonite crystals in each compartment. Using a calcium carbonate-binding assay, we identified an acidic matrix protein, Pif, in the pearl oyster Pinctada fucata that specifically binds to aragonite crystals. The Pif complementary DNA (cDNA) encoded a precursor protein, which was posttranslationally cleaved to produce Pif 97 and Pif 80. The results from immunolocalization, a knockdown experiment that used RNA interference, and in vitro calcium carbonate crystallization studies strongly indicate that Pif regulates nacre formation.

Nanomechanical properties of nacre

Nacre, the inner iridescent layer of seashells is a model biomimetic system composed of 95% of inorganic (aragonite) phase and 5% of organic phase. Nacre exhibits an interlocked layered “brick and mortar” structure where the bricks are made up of aragonitic calcium carbonate and mortar is an organic phase. Here, we report the role of indentation load and penetration depth on measurement of nanomechanical properties of nacre. A range of loads from 10 μN to 10,000 μN were applied to obtain the response from different depths of nacre. The values of hardness and elastic modulus decrease with increasing load (i.e., increase in penetration depth). The variation in these values is significant at lower loads and decreases with increase in indentation load. From our results, it appears that the nanoindentation tests done at lower loads are highly influenced by micro and nanostructure in nacre. The indentation experiments performed at low loads indicate an elastic modulus of about 15 GPa for the organic phase. The low load, low penetration experiments appear to be better indicators of nanomechanical behavior. Also, we have observed a step-like behavior in the load-displacement curves at high load indentations on nacre. These features are attributed to the organic layer between the aragonite platelets. The indentation tests with penetration depths more than ∼250-300 nm often disrupt the organic layer and the behavior is not recovered in the unloading part of the curve. The microarchitecture and the composition of nacre contribute to the decrease in hardness values with increasing depth along with the indentation size effects.

Periodic Structure of Blaze Grating in Abalone Shell

Periodic structures, such as photonic crystals and gratings, offer the ability to control light propagation and produce structural colors (not produced by pigments). Structural colors are quite widespread in nature, such as in opals and some animals. The abalone shells attract much attention due to its fracture resistant. In present work, we report on the microstructure of the surface area in where the iridescence color appears found in abalone shell shows a groove structure of blaze grating. The scanning electron microscopy (SEM) images of its iridescent layer show layers of platelets. The abalone shell has a statistically regular arranged tile structure. This specific tile structure serves as a two-dimensional grating to cause the iridescence phenomenon. Unlike opals, the iridescence color of the abalone shells is not directional due to the two-dimensional arrangement of the reflection grating structure and the convex shape of the tiles.

Mechanical Properties of Biological Nanocomposite Nacre: Multiscale Modeling and Experiments on Nacre from Red Abalone

AbstractNacre, the inner iridescent layer of mollusks shell is a bio-nanocomposite with the mineral aragonite as a major constituent and 2-5% of organics mainly in the form of proteins. Our multiscale modeling and experimental studies reveal that the microstructure and the small weight percent of organics are the key parameters attributed to the extreme toughness of nacre. We report that the presence of platelet interlocks nacre have a significant role in the enhancement of mechanical properties. Molecular simulation study is conducted to understand the behavior of aragonite-organic interface. The mechanical behavior of organics and inorganics in presence of each other is described using steered molecular dynamics simulations. This provides some understanding on the deformation mechanisms of the protein present between the aragonite layers. Our nanoindentation results indicate that the elastic modulus and hardness of nacre decreases as it is exposed to a denaturing temperature for proteins. The changes in the organic inorganic interaction have been experimentally described using Fourier Transform Infrared Spectroscopy. This work gives insight into the contribution of the various factors existing at different length scales on the overall mechanical behavior of nacre.

Mechanical Properties of Nacre Constituents: An Inverse Method Approach

ABSTRACTNacre, also known as mother-of-pearl, is the iridescent layer found inside some mollusk species such as oyster or abalone. It is made of relatively weak materials, but its hierarchical microstructure is so well optimized that its macroscopic mechanical properties are far superior to those of its constituents. For this reason there is a great interest in nacre as a source of inspiration for novel designs of composites. Despite many years of research on nacre, an accurate characterization of its constituents is lacking. In this work nacre was tested as a layered composite material using low depth indentation and uniaxial compression. The first test was modeled using finite element analysis and the second test was modeled as a Reuss composite in compression. A micromechanical model of the interface was also pursued to gain insight on the relevance of the interface features such as tablet roughness and biopolymer hydrated response. The results of the two experiments were combined to solve an inverse problem that yielded the needed properties for tablets and interfaces. These findings are expected to make possible computational models of nacre with a new degree of accuracy and therefore contribute to a better understanding of the mechanisms leading to its remarkable properties.

Iridescent Art Nouveau glass – IBA and XPS for the characterisation of thin iridescent layers

The external proton beam of the Tandem accelerator of the Research Centre in Rossendorf/Germany was used to carry out non-destructive particle-induced X-ray emission (PIXE) particle-induced gamma-ray emission (PIGE) and Rutherford backscattering (RBS) measurements simultaneously on Art Nouveau artifacts produced around 1900 by Tiffany/USA and Loetz/Austria. These studies should proof the technology of producing an iridescent layer on a glass surface. By means of the yield ratio Y(Si-K)/ Y(Si-γ) of both characteristic X-radiation (Si-K) and γ-radiation (Si-γ) of the element silicon it could be shown that a thin top layer is present on the glass surface due to the treatment of the heated artifacts (about 500°C) with an alcoholic solution of SnCl 2 [1]. Combined evaluation of the PIXE and RBS spectra resulted in a thickness of 20–300 nm for this top layer. In addition, a transition region between the iridescent layer and the bulk glass was obtained by RUMP simulations. Approximately 80% of the total amount of the Sn were found to be present in this transition layer and only 10–20% in the outermost surface region. XPS studies showed that the outermost layer consists of SnO 2. The formation of other Sn compounds in the outermost near-surface region based on Sn–Si–O during the manufacturing process can be excluded.

Photoreception in the planktonic larvae of two species of pullosquilla, a lysiosquilloid stomatopod crustacean

Optical microscopy, electron microscopy and microspectrophotometry were used to characterize pigments in the eyes of planktonic larvae of two species of the lysiosquilloid stomatopod Pullosquilla, P. litoralis and P. thomassini, which live sympatrically in French Polynesia. In contrast to the adult retina, which contains a diverse assortment of visual pigments in the main rhabdoms, the principal photoreceptors throughout the larval eyes of both species were found to contain a single rhodopsin with an absorption maximum (<IMG src="/images/symbols/&lgr ;.gif" WIDTH="8" HEIGHT= "12" ALIGN="BOTTOM" NATURALSIZEFLAG="3">max) close to 446 nm. The expression of this visual pigment may survive metamorphosis, since several adult rhodopsins occur at a similar spectral position. The retinas of these planktonic larvae also contain a novel yellow photostable pigment, which is arrayed in a regular pattern at the distal margin of the larval retina. The absorption spectrum of this pigment is well matched to the larval rhodopsin, suggesting that it acts to screen the rhabdoms from stray light. By replacing opaque, black screening pigment, the transparent yellow pigment may act together with a blue iridescent layer in the larval retina to reduce the visual contrast of the larval eye against downwelling and sidewelling light, while simultaneously acting as a retinal screen.

The fine structure of the cornea of the salamanderfish, Lepidogalaxias salamandroides (Lepidogalaxiidae, Teleostei).

The salamanderfish, Lepidogalaxias salamandroides, is a small freshwater fish that is endemic to southwestern Australia. Its phylogeny is still uncertain, but it belongs to a monotypic family that is as primitive as any living teleost. Able to survive during periods of drought by burrowing into the substrate and estivating for many months, this species has evolved a unique visual system, which is highly adapted to its changing environment. Thought to use cutaneous respiration during periods of dormancy, it secretes a mucus sheath over the eyes and genital openings to inhibit desiccation. The cornea is split into dermal and scleral components, separated by an iridescent layer and a mucoid layer. The dermal cornea comprises an epithelium containing both goblet cells and mucus-secreting granules in the central region, which overlies a thick stroma of collagen lamellae. The dermal stroma possesses numerous sutural fibers and flattened keratocytes and collections of three types of pigment granulates in the periphery. Posterior to the dermal stroma lies an iridescent layer consisting of five flattened cell processes interspersed with membranous extensions of smooth endoplasmic reticula. The mucoid layer is composed of numerous membrane-bound structures that allow movement of the scleral cornea beneath the dermal cornea or secondary spectacle. The scleral stroma is thin (six to seven lamellae) contains keratocytes, and overlies a relatively thin and irregular Desçemet's membrane and a monolayered endothelium. In the iridocorneal angle and situated between Desçemet's membrane and the endothelium is an "annular ligament," which is composed of flattened cells with lobulated nuclei and a fine granular cytoplasm with a few mitochondria. The numerous corneal inclusions are discussed with respect to their phylogeny and function.

The Cornea of the Sand Lance, Limnichthyes Fasciatus (Creeiidae)

The sand lance, Limnichthyes fasciatus, is a small fish, 15-30 mm in length, found on the Great Barrier Reef. The eyes, which are not covered by a spectacle, are externally placed, move independently, and measure about 1 mm in diameter. The cornea, unusual even in the teleost world, is about 0.14 mm thick, which represents one-seventh of the length of the eye. The layers of the cornea include an epithelium with a complex pattern of surface microplicae, a grossly enlarged basal cell layer, and a thick basement membrane. Structures (iridophores), which may represent the vestigial remnants of a secondary spectacle, are present in the peripheral corneal epithelium. The stroma, which has no Bowman's layer, is composed of lamellae of collagen fibrils but contains no keratocytes. Posterior to the stroma is a thick (0.1 mm) cellular layer that may represent the autochthonous layer seen in some teleost species. An iridescent layer consisting of approximately 70 parallel cytoplasmic plates oriented at right angles to the visual axis lies anterior to a thick Descemet's membrane. There is a single layer of endothelial cells on the posterior surface.

The structure and function of iridescent corneas in teleost fishes

Many fishes possess an iridescent layer in the cornea which is visible when the eye is illuminated from above. Corneal iridescence is most common among shallow water marine teleosts that live among rocks or on the bottom. Iridescence is produced when light traverses a regular multilayer stack, the thickness of each layer being of the order of a quarter of the wavelength of visible light. However, the components of these stacks have different structures in different groups of fishes and may be formed from endoplasmic reticulum, plates of connective tissue, protoplasm, or regular arrays of collagen fibrils. Reflexion and transmission measurements indicate that too little light is lost by reflexion for the cornea to act as an effective colour or polarizing filter to light near normal incidence. When the angle of light incidence is oblique, the plates become highly reflecting and it is thought that very bright downwelling light that reaches the cornea at large angles of incidence is reflected out of the eye and does not contribute to intra-ocular flare. At angles of incidence near to the normal, it is possible that the plates may be so spaced that there is destructive interference of the reflected light and light losses through reflexion would be very small. The intra-ocular flare resulting from such layers has been measured and it is calculated that the iridescent layer gives a significant increase in visual range underwater without sacrifice of sensitivity. The possibility that corneal iridescence may help to camouflage the pupil is discussed.

Iridescent corneas in fishes.

DURING the course of underwater studies of fishes in the English Channel, Mediterranean and Indian Ocean, it was noticed that a coloured iridescent layer was often present on the inner surface of the cornea extending over the pupillary area (Fig. 1). Iridescence has been observed in a variety of animal tissues1,2 and is caused by regular multilayer structures stacked in a medium of different refractive index. Iridescent tissues already known in fish include the tapetum in which the principal material may be guanine1 or collagen3 and the silvery iris and scales of teleost fish where the principal material is guanine4.

Crystals of quartz with iridescent faces

The paper is a study of the iridescence exhibited by some of the pyramidal faces of two crystals of transparent quartz in the possession of the author. Spectroscopic examination shows the reflections and the corresponding extinctions to be monochromatic. The thickness of the iridescent layer may be directly observed, and the number of laminations inferred to be present is sufficient to explain the observed characters of the iridescence. The nature of the stratifications in the crystal giving rise to the iridescence is discussed. Photographs of one of the crystals and of the spectral character of the reflections and extinctions illustrate the paper. *** DIRECT SUPPORT *** A00OC061 00002