Effect of dark and light adaptation on the retina and pecten of chicken

Pecten oculi of kestrel (Falco tinnunculus rupicolaeformes) and little owl (Athene noctua glaux): Scanning electron microscopy and histology with unique insights into SEM-EDX elemental analysis

There is scanty data about the comparative morphological features between the pecten oculi of two carnivorous birds with different visual active clock hours: the diurnal common kestrel and the nocturnal little owl. This study illustrated the comparative gross, scanning electron microscopy, and histological characteristics between pecten oculi of kestrel and little owl. This study first attempts to describe the scanning electron microscopy-energy dispersive x-ray (SEM-EDX) elemental analysis at the parts (apex, middle, and base) of the pecten oculi of these two birds. The present study results observed the same position, origin, directions, parts, convoluted outer pleat surfaces, and SEM-EDX elemental analysis, but there were some minor variations due to the different visual active clock hours. These minor variations were summarized in the following points: pleat number (21-23 in the kestrel and 10-11 in the owl), shape (fan rhomboid in the kestrel and accordion in the owl), inter-pleat spaces (wider in the kestrel than in the owl), pigmentations (highly black pigmented in the owl than in the kestrel), hyalocyte cell aggregations (highest in the middle and dwindling at the apex and base in the kestrel, while highest in the middle and base and dwindling at the apex in the owl), and SEM-EDX elemental analysis percentage. SEM/EDX elemental analysis confirmed the presence of oxygen (the highest one), carbon, nitrogen (the second one), nitrogen (the third one), and aluminum (the lowest one) in varying percentages within the pecten oculi; these findings contribute to our understanding of its structural, adaptations with different visual active clock hours, and functional characteristics. RESEARCH HIGHLIGHTS: This study compared the pecten oculi of two carnivorous birds with different visual active clock hours: the diurnal common kestrel and the nocturnal little owl. Anatomically, the characteristic features were similar in both the birds, but some minor variations were observed adapted to their visual active clock hours. The pecten oculi of both birds were analyzed using SEM-EDX for elemental analysis, and it revealed that oxygen was the highest elemental concentration, followed by carbon and nitrogen. Aluminum concentrations were small as per SEM-EDX analysis. The study suggested that the pecten oculi of these birds are related to their active visual clocks and adaptive nutritional mechanisms.

Morphological and Ultrastructural Studies of Pecten in the Eurasian Tree Sparrow

We studied the fine histological structures of pecten oculi of the Eurasian tree sparrow using various microscopy techniques. The pecten of the tree sparrow was found to be of a pleated type comprising of pleats, bridges, and base. The light microscopic study revealed further that the pleats consist of capillaries of varying sizes, blood vessels, and numerous pigmented cells that give them a black color. Histochemical studies of pecten showed a large deposition of lipid droplets, which were more abundant in the basal area. The transmission electron microscopy displayed capillaries and blood vessels that remain surrounded by a thick fibrous basal membrane. They are formed of endothelial cells having a large lumen and abluminal area with microfolds. Interstitial spaces were found filled with rounded melanocytes, electron-dense pigment granules, and mitochondria. Observations under the scanning electron microscope revealed the presence of a dense vascular network of capillaries and vessels. In addition, large hyalocytes were also observed on the surface of the pleats. The above observations suggest that the histological structure of the pecten of the tree sparrow resembles those present in the pecten of other diurnal birds. However, further investigation is required to ascertain its functional role in birds.

Visual adaptation and retinal characterization of the Garganey (Anas querquedula): Histological and scanning electron microscope observations

The present study was designed to provide a complete morphological description of the eye of the migratory Garganey duck (Anas querquedula) and its visual adaptation with the different surrounding environmental conditions during its migration journeys using a stereomicroscope, scanning electron microscope (SEM), and light microscope. The current work depends on six adult Garganey ducks that were captured from the area near and on the shores of Edku city. The obtained results revealed that the eye of the migratory Garganey duck has the features of both diurnal and nocturnal birds. The histological examination reveals that the pigmented epithelium of the retina has long prolongations filled with melanin. The cone is the dominant photoreceptor, but simple rods are present. The inner nuclear and ganglion cell layers are thick. SEM examination shows that the arrangement of the collagen fibrils on the external surface was reticular in shape. The radial folds present as pledged structures on the pigmented epithelium covered with circular structures. The main lens body has flat with hexagonal outlines fibers. The edges and surfaces of these hexagonal fibers were studded with protrusions or elevations (balls) and depressions (sockets). The sockets and balls were either rounded or ellipsoid in shape. The balls were more on the edges and the sockets on the surface. In conclusion, our findings indicated a higher degree of functional adaptation between the morphological structure of the eye and the surrounding environmental conditions.

Light and electron microscopic studies on the pecten oculi showing blood-retina barrier properties in Turkey's native Gerze chicken

The pecten oculi is a highly vascularized and pigmented organ that projects from the optic disc into the vitreous body in the avian eye. In this study, the pecten oculi of Turkey's native Gerze chicken was examined by light and scanning electron microscopy. Furthermore, the localization of some adherens junction components (E-cadherin and pan-cadherin) in intact vessels of the blood-retina barrier was investigated by immunohistochemistry. In the Gerze chicken, the pecten oculi was a thin structure, which was located over the head of the discus nervi optici and projected from the retina into the corpus vitreum. The pecten oculi consisted of 18-21 highly vascularized pleats, joined apically by a bridge and resembled an accordion in appearance. Hyalocytes and melanocytes were observed around the small and large vessels. The morphometric data of the pecten oculi showed that there were no statistical differences in terms of sex. The immunohistochemical analysis of the pecten oculi, which is used as a model for the investigation of the formation and maturation of the barrier properties in the central nervous system, revealed cytoplasmic E-cadherin and pan-cadherin immunoreactivity in the endothelial cells of the small, large and capillary vessels. These observations suggest that while the morphological and histological structure of the Gerze chicken's pecten oculi was generally similar to that of other diurnal domestic birds, the pecten oculi, a model system for vascular differentiation and the blood-retina barrier, expressed different cadherins.

Gross, histochemical and electron microscopical characterization of the Pecten oculi of Baladi ducks (Anas boschas domesticus)

Objective:

As pecten oculi had great functional significances for ornithology, pecten oculi of Baladi duck was well-deserving of intensive morphological study. So, the aim of this study was to throw light on some anatomical and histological formation of the pecten oculi of Baladi ducks as well as use of scanning electron microscopy.

Materials and Methods:

Twenty eyeballs of 10 adult Baladi ducks were used to fulfill this work. Ten eyes were used to study the gross anatomy of pecten oculi, including the location, shape, and numbers of pleats. Five samples were embedded at 10% neutral buffered formalin. The specimens were examined by regular histological procedures. The latter five samples were applied for electron microscopy.

Results:

Grossly, the pecten oculi is formed of three portions: the base, emerged from the optic disk; the pleats, sorted in fan shape; and the bridge. The essential histological ingredients of Baladi ducks’ pecten oculi are the blood vessels, lymph vessels, pigment cells, and hyalocytes.

Conclusion:

The current work explains the primary macro- and micro-morphological features of pecten oculi in Baladi duck and collates these features to those formerly explained in other birds. Generally, pecten oculi of Baladi duck was analogous to that of the diurnal birds.

Doppler ultrasonography of the pectinis oculi artery in harpy eagles (Harpia harpyja)

Twenty harpy eagles (Harpia harpyja) without systemic or ocular diseases were examined to measure blood velocity parameters of the pectinis oculi artery using Doppler ultrasonography. Pectinate artery resistive index (RI) and pulsatility index (PI) were investigated using ocular Doppler ultrasonography. The mean RI and PI values across all eyes were 0.44±0.10 and 0.62±0.20 respectively. Low RI and PI values found in the harpy eagle´s pectinis oculi artery compared with the American pekin ducks one and other tissue suggest indeed a high metabolic activity in pecten oculi and corroborates the hypothesis of a nutritional function and/or intraocular pressure regulation.

Hemodynamics of the pectinis oculi artery in American pekin ducks (Anas platyrhynchos domestica)

OBJECTIVE

To measure blood velocity parameters of the main detectable branch of the pectinis oculi artery and compare with values found in other arteries in other species to form a hypothesis of the function of the pecten oculi in birds.

METHODS

Eleven American pekin ducks (Anas platyrhynchos domestica) without ocular diseases were examined with ocular Doppler ultrasonography. The pectinate artery resistive index (RI) and pulsatility index (PI) were calculated.

RESULTS

The mean of RI and PI values resulted, respectively, in: left eye (0.43 ± 0.07; 0.58 ± 0.13), right eye (0.37 ± 0.09; 0.47 ± 0.14), and both eyes (0.40 ± 0.08; 0.53 ± 0.14).

DISCUSSION

The low RI and PI values found in the main branch of the pectinis oculi artery compared with other arteries in other tissues suggest a high metabolic activity in the pecten oculi, and it could indicate a nutritional function and/or intraocular pressure regulation.

A comparative morphometrical study of the pecten oculi in different avian species

In this study was investigated the structure of pecten oculi in the ostrich, duck, pigeon, turkey, and starling. The pecten oculi of the ostrich was vaned type and made up primary, secondary, and few tertiary lamellae. However, duck, pigeon, turkey and starling had a pleated-type pecten oculi which displayed folded structure. The numbers of pleats of the pectens were 12, 13-14, 21-22, and 17 in duck, pigeon, turkey, and starling, respectively. Light microscopic investigation demonstrated that pecten oculi is basically composed of numerous capillaries, large blood vessels, and pigment cells in all investigating avian species. Capillaries were 20.23, 14.34, 11.78, 12.58, and 12.78  μ m in diameter in ostrich, duck, pigeon, turkey, and starling, respectively. The capillaries are surrounded by thick basal membrane, and pigmented cells were observed around the capillaries.

Effects of selective neurotoxins on eye growth in the young chick

We have determined the extent of retinal cell damage and eye growth after treatment with different neurotoxins. Day-old chicks received an intraocular injection containing 2, 10, 50, 100 or 200 nmol of kainic acid (KA), an excitotoxic analogue of glutamate. After 21 days, with 2 nmol KA damage was confined to a small proportion of bipolar cells, whereas with 10-200 nmol KA there was dose-dependent damage to amacrine cells, ganglion cells and photoreceptors. There was an increase in eye weight and size of the vitreal chamber with 10 nmol KA or more. In a similar protocol using 5, 50 or 200 nmol quisqualic acid (QUIS), there was massive loss of amacrine cells and slight loss of horizontal cells, and a large increase in the anterior chamber depth. A single dose of 200 nmol N-methyl-D-aspartate lesioned amacrine cells, but did not alter eye size. Thus, excitotoxins which have different retinotoxic effects also have different effects on eye size. In another study, we examined the effects of KA and QUIS on occlusion-induced eye enlargement. Treatment with 200 nmol QUIS alone resulted in increased depth of the anterior chamber. QUIS combined with occlusion for 3 weeks did not interfere with occlusion-induced growth of the vitreal chamber. Therefore, the loss of a large proportion of amacrine cells did not interfere with occlusion-induced growth. KA results in eye growth in its own right, but restricts occlusion-induced growth. These effects are associated with damage to photoreceptors, amacrine and bipolar cells and a small proportion of ganglion cells. Finally, we examined the effects of an intraocular injection of tunicamycin, a purported photoreceptor-specific neurotoxin in amphibia. Tunicamycin (0.1 or 1 mumg/eye) resulted in a flattened anterior chamber but did not affect growth of the vitreal chamber. Tunicamycin inhibited occlusion-induced growth, and treated retinae displayed massive disruption and loss of all cell types apart from a single row of pigmented retinal epithelium. We conclude that (a) normal and occlusion-induced eye growth may have separate mechanisms of action, (b) the vitreous and anterior chambers have different growth mechanisms, and (c) photoreceptors may play a critical role in occlusion-induced growth of the eye.

The morphology of the pecten oculi of the ostrich, Struthio camelus

The pecten oculi is a structure peculiar to the avian eye. Three morphological types of pecten oculi are recognized: conical type, vaned type and pleated type. The pleated type has been well studied. However, there exists only scanty data on the morphology of the latter two types of pectens. The structure of the vaned type of pecten of the ostrich, Struthio camelus was investigated with light and electron microscope. The pecten of this species consists of a vertical primary lamella that arises from the optic disc and supports 16-19 laterally located secondary lamellae, which run from the base and confluence at the apex. Some of the secondary lamellae give rise to 2 or 3 tertiary lamellae. The lamellae provide a wide surface, which supports 2-3 Layers of blood capillaries. Pigmentation is highest at the distal ends of the secondary and tertiary Lamella where blood capillaries are concentrated and very scanty on the primary and the proximal ends of the secondary lamella where the presence of capillaries is much reduced. In contrast to the capillaries of the pleated pecten, the endothelium of the capillaries in the pecten of the ostrich exhibits very few microvilli. These observations suggest that the morphology of the pecten of the ostrich, a flightless ratite bird is unique to the pleated pecten and is designed to meet the balance between optimal vision and large surface area for blood supply and yet ensuring it is kept firmly erect within the vitreous.

Saccadic oscillations facilitate ocular perfusion from the avian pecten

THE evolution of the eye is constrained by two conflicting requirements--good vascular perfusion of the retina, and an optical path through the retina that is unobstructed by blood vessels. Birds are interesting in that they have higher metabolic rates and thicker retinas than mammals, but have no retinal blood vessels. Nutrients and oxygen must thus reach the neurons of the inner retina either from the choroid through 300 micron of metabolically very active retina, or from the pecten, a pleated vascular structure protruding from the head of the optic nerve into the vitreous chamber, and more than a centimetre away from some retinal neurons. Despite the diffusional distance involved, several lines of evidence indicate that the pecten is the primary source of nutrients for the inner retina: the presence of an oxygen gradient from pecten to retina, the large surface area produced by macroscopic folds and by microscopic infoldings of the luminal and external surfaces of the capillary endothelium, extrusion of circulating fluorescein, high content of carbonic anhydrase and alkaline phosphatase, and retinal impairments after pecten ablation. Another peculiarity of birds, their saccadic oscillations, occur with a large cyclotor-sional component during every saccadic eye movement. In different species, saccades, which occur at intervals of 0.5-40 s, have up to 13 oscillations with frequencies of 15-30 Hz and ampliá-tudes of about 10 degrees. Therefore, as much as 12% of some birds' total viewing time may be subject to the image instability caused by the oscillations. Using fluorescein angiography, we show here that during every saccade, the pecten acts as an agitator which propels perfusate towards the central retina much more effectively than is observed during intersaccadic intervals.

Fine structure of the pecten oculi of the emu (Dromaius novaehollandiae)

The pecten oculi of the emu (Dromaius novaehollandiae) has been examined by light and electron microscopy. In this species the pecten is small relative to the size of the globe and is of a primitive pleated type. It consists of only 3-4 loose folds that are joined apically by a bridge of tissue which holds the pecten in a fan-like shape widest at its base. Each fold is quite thick (100-120 microns) and has a central core of mostly unpigmented cell processes. In this central region are supply or drainage vessels while numerous melanocytes and pecteneal capillaries are only located at the periphery of the folds. The capillaries are extremely specialized for transport functions and for the most part display extensive microfolds on both their luminal (inner) and abluminal (outer) borders although capillaries with very few microfolds are also noted. An unusual feature of some capillaries is luminal folds of the cell body with further luminal microfolds superimposed on them. Except for the nuclear region which contains most of the organelles, the endothelial cell bodies are extremely thin. These capillaries are surrounded by thick fibrillar basal laminae which are felt to be structurally important. Pericytes are a common feature within the basal lamina of these capillaries. The numerous peripherally located melanocytes which more or less surround the capillaries are also presumed to be important for structural support of the pecten. The large number of cell processes forming the central core of each fold are felt to be unpigmented processes of the melanocytes.

Fine structure of the pecten oculi in the American crow (Corvus brachyrhynchos)

The pecten oculi of the American crow (Corvus brachyrhynchos) has been examined by both light and electron microscopy. In this species the pecten is very large and of the pleated type. It consists of 22-25 accordion folds that are joined apically by a bridge of tissue which holds the pecten in a fan-like shape widest at its base. Within each fold are numerous capillaries, larger supply and drainage vessels and many melanocytes. The capillaries are extremely specialized for transport functions and display extensive microfolds on both their luminal and abluminal borders. Except for the nuclear region which contains most of the organelles, the endothelial cell bodies are extremely thin. These capillaries are surrounded by thick fibrillar basal laminae which are conjectured to be structurally important. Pericytes are a common feature of these capillaries. The numerous pleomorphic melanocytes interspersed between the capillaries are also felt to be important in structural support of the pecten. The pecten is considered to be comparable to the falciform process of some teleosts, the conus papillaris of reptiles, the supraretinal vessels of amphibians and teleosts and the intraretinal vessels of mammals which are all felt to be alternative methods of bringing nutrients to the inner retina.

Fine structure of the pecten oculi of the red-tailed hawk (Buteo jamaicensis)

The pecten oculi of the red-tailed hawk (Buteo jamaicensis) has been examined by light and electron microscopy. In this species the pecten is very large and of the pleated type. It consists of 17-18 accordion folds which are joined apically by a heavily pigmented bridge of tissue which holds the pecten in a fan-like shape, widest at its base. It is situated over the optic nerve head and extends into the vitreous. Within each fold are numerous capillaries, larger supply and drainage vessels and melanocytes. The capillaries are specialized and display extensive microfolds on both the luminal and abluminal borders. The endothelial cell bodies are thin with most organelles present in a paranuclear location. The capillaries are surrounded by thick fibrillar basal laminae which are probably structurally important and which often enclose pericytes. The melanocytes which are most plentiful in the bridge region and peripherally in the pecten, form an incomplete sheath around the capillaries and other blood vessels. These melanocytes are also felt to be fulfilling a structural role within the pecten. The morphology of the pecten of the red-tailed hawk is indicative of a heavy involvement in the transport of materials to the avascular avian retina.

Alterations in the differentiation of chick retina caused by an intraocular injection of an alkaline phosphatase inhibitor

Intense alkaline phosphatase (ALPase) activity has been localized in the outer plexiform layer of the developing chick retina. To elucidate the functional significance of this enzymatic activity, we have injected an ALPase inhibitor, levamisole, into embryonic eyes on either the 13th or 15th day of incubation. The retina was fixed between the 15th and 20th day of incubation and examined by electron microscopy. Levamisole injection on the 13th day caused various morphological alterations in retinal development, including the appearance of solitary photoreceptor cells in the subretinal space as well as folding of both the outer plexiform and outer nuclear layers. Pedicles of photoreceptor cells in the outer plexiform layer displayed rather smooth configurations with a reduced number of invaginations by post-synaptic neurites. The outer plexiform layer was thinned and the neuritic extensions in this layer appeared much less developed than in the control (PBS-injected) retina. Photoreceptor outer segments were seldom observed. Besides these alterations, layers of optic fibers and ganglion cells were also affected, as shown by evidence of degeneration in the ganglion cells and thinning of the nerve-fiber layer. Injection of levamisole into day 15 embryonic eyes exerted less influence on retinal development, but some photoreceptor cells were still found in the subretinal space. Some of these observations have been reported in the retinas of aged normal animals or in retinas with hereditary or induced retinal dystrophy. It is suggested that ALPase activity in the outer plexiform layer of the developing chick retina may be important for the onset of normal development of synapses in the outer plexiform layer and differentiation of the photoreceptor cells.

Effect of elastase on the histochemical demonstration of alkaline phosphatase activity in chick pecten

The effect of elastase on alkaline phosphatase activity in the chick pecten capillaries was studied electron histochemically. The pecten was treated with elastase before incubation in the medium for alkaline phosphatase. Inactive alkaline phosphatase in the chick pecten capillary under dark adaptation could be demonstrated electron histochemically after treatment with elastase. Under light adaptation, the elastase-treated pecten demonstrated more intense alkaline phosphatase activity in the same localization as in the untreated pecten. Biochemical data also supported histochemically demonstrated enhancement effect of elastase on alkaline phosphatase activity in the chick pecten oculi. Elastase is expected to be useful for demonstrating enzymatic activity which is otherwise hard to detect.

Electron microscopic histochemistry of alkaline phosphatase activity in the developing chick retina

The present paper describes ultrastructural localization of non-specific alkaline phosphatase (ALPase) activity in the chick retina during late embryonic development and after hatching. In the newly-hatched chick retina, intense ALPase activity was found in the outer plexiform layer (OPL), and a less intense reaction was found in the ganglion cell layer and at the outer limiting membrane. Electron microscopic histochemistry revealed that bipolar and horizontal dendrites within the OPL displayed intense reaction product along their plasma membranes. Only horizontal cells showed reaction product on almost their whole circumference. Müller cells also showed reaction product on the plasma membrane in the ganglion cell layer and on the microvilli at their apical portion. The reaction product in the OPL was first recognizable in the retina of the 13- to 14-day-old embryonic chick, and its intensity increased during further development. Little reaction product was found in Müller cells at the embryonic stage. The function of ALPase activity is discussed in relation to the development of the OPL and the nutrient supply for the neural retina.

Electron histochemical study of alkaline phosphatase activity in the pecten oculi of the chick

Alkaline phosphatase activity in the pecten of chicks was studied electron histochemically. Alkaline phosphatase activity was located in the plasma membrane of the cytoplasmic processes in the luminal and basal portions of the endothelial cells of the capillary, in the plasma membrane of cells with pigmented granules and in the plasma membrane of cells in the vitreous in contrast with the basement lamina of the cells with pigmented granules. The cytoplasmic processes of the luminal and basal portions of the capillary endothelium and the plasma membrane of the cells with pigmented granules may play a role in the exchange of metabolites between the pecten and the vitreous. In the pecten of dark adapted eye, alkaline phosphatase activity could not be demonstrated histochemically. Thus, darkness may make the exchange of metabolites between the pecten and the vitreous less active.

Comparative electron-microscopic studies on the conus papillaris and its relationship to the retina in night and day active geckos

The ultrastructure of the capillary endothelium of the conus papillaris within the vitreous body was studied in night, day, and mixed (night and day) active geckos (Homopholis wahlbergi, Gekko gecko, Pachydactylus bibronii, Tarentola mauritanica, Lygodactylus conraui, Phelsuma guimbeaui, Phelsuma madagascariensis). Capillary endothelial cells were poorly developed in night and mixed active animals, whereas in day active animals they were highly organized. In particular the number and height of the luminal and basal microvilli and the frequency of micropinocytotic vesicles were distinctly increased in day active geckos. It is assumed that these structural mechanisms improve the transendothelial transportation capacity of metabolic substances. When the thickness of the retinal layers was compared, we found that the inner retinal layers of those geckos in which the conus capillaries were poorly developed, were approximately 60% of the thickness of the inner retina in the day active geckos. The results indicate that the structural organization of conal endothelial cells is related primarily to the retinal structure rather than to the animals' daytime behavior. Furthermore, our observations support the theory that the conus papillaris of lizards, like the pecten oculi of birds, has a primary function in the nutrition of the avascular retina and/or is involved in the exchange of fluid in the vitreous.