Ultrastructure of the eye of the amber snail Succinea putris (L.) (Gastropoda, Stylommatophora)]

The structure and some aspects of the development of the eye of Succinea putris were studied with the aid of the electron microscope. The eye is of the closed vesicle type and is composed of retina, cornea, vitreous body, lens and optic nerve. Three different types of cell are to be found in the retina: (1) the small elongated pigment cell with an avoid nucleus, many pigment granulae and short microvilli at the apical end of the cell; (2) the sensory cell type I with a large irregular nucleus, long microvilli, which extend to under the surface of the lens, a large number of light-cored vesicles, 700 A in diameter and the axon; (3) the elongated slender sensory cell type II with many dense cored vesicles, several pigment granulae in the distal region of the cell and short irregular microvilli at the apical end of the cell. This type is few in number. Two results of the study of the embryonic eye are described: the cornea cells differ from those in the adult eye in the nucleus-cytoplasm relation and the optic nerve is smaller than in the adult eye.     

In Vivo and in Vitro Experimental Analysis of Lens Regeneration in Larval Xenopus laevis

After lentectomy of larval Xenopus laevis , the outer cornea undergoes tissue transformation resulting in formation of a new lens. This lens regeneration is triggered and sustained by neural retina. In the present study, lens-forming transformation of the outer cornea was completed in vitro when the outer cornea was cultured within the lentectomized eye-cup. Well-differentiated lens fiber cells, which showed positive immunofluorescence for total crystallins, were also formed when the outer cornea was cultivated with the retina. No lens tissue was formed when the cornea was cultured alone. Lens-forming transformation, originating from the cornea three and five days after lentectomy, completely regressed when the tissue was isolated in vitro . Fom the present and previous findings, we concluded that, the interaction of corneal cells with the retina plays a decisive role in lens regeneration in situ .     

Alterations of the eyes during ontogenesis inAporrhais pespelecani (Mollusca,Caenogastropoda)

The cerebrally innervated eyes of metamorphically competent larvae, newly metamorphosed larvae, and adults ofAporrhais pespelecani are ultrastructurally investigated and compared. The eyes are composed of a lens, a cornea, and an everse retina. In adults, a humour is located behind the lens. The retina consists of two different types of cells: sensory cells and supportive cells. The present study confirms earlier results and demonstrates that the distal part of the sensory cells is altered during ontogenesis. In metamorphically competent larvae, the sensory cells are exclusively ciliary. In newly metamorphosed larvae and in adults, however, the sensory cells are of the mixed type, bearing both cilia and microvilli. Furthermore, the findings confirm that both the supportive and corneal cells, as well as the distal supportive cell processes which are restricted to the eyes of adults are involved in lens formation.     

The compound eye of<Emphasis Type="Italic"> Scutigera coleoptrata</Emphasis> (Linnaeus, 1758) (Chilopoda: Notostigmophora): an ultrastructural reinvestigation that adds support to the Mandibulata concept

The lateral compound eye of Scutigera coleoptrata was examined by electron microscopy. Each ommatidium consists of a dioptric apparatus, formed by a cornea and a multipartite eucone crystalline cone, a bilayered retinula and a surrounding sheath of primary pigment and interommatidial pigment cells. With reference to the median eye region, each cone is made up of eight cone segments belonging to four cone cells. The nuclei of the cone cells are located proximally outside the cone near the transition area between distal and proximal retinula cells. The connection between nuclear region and cone segment is via a narrow cytoplasmic strand, which splits into two distal cytoplasmic processes. Additionally, from the nuclear region of each cone cell a single cytoplasmic process runs in a proximal direction to the basement membrane. The bilayered rhabdom is usually made up of the rhabdomeres of 9–12 distal retinula cells and four proximal retinula cell. The pigment shield is composed of primary pigment cells (which most likely secrete the corneal lens) and interommatidial pigment cells. The primary pigment cells underlie the cornea and surround, more or less, the upper third of the crystalline cone. By giving rise to the cornea and by functioning as part of the pigment shield these pigment cells serve a double function. Interommatidial pigment cells extend from the cornea to the basement membrane and stabilise the ommatidium. In particular, the presence of cone cells, primary pigment cells as well as interommatidial pigment cells in the compound eye of S. coleoptrata is seen as an important morphological support for the Mandibulata concept. Furthermore, the phylogenetic significance of these cell types is discussed with respect to the Tetraconata.     

Rhodopsin regeneration: role of interaction between the photoreceptors and pigment epithelium cells]

Regeneration of rhodopsin has been studied in the eyecup, isolated retina and retinal homogenate of frog Rana temporaia as well as in the eyecup and isolated retina of fish-flounder Limanda aspera (Pallas). Rhodopsin has been found to regenerate only in the eyecup of frog, while isorhodopsin appeared to be the final product in the frog retinal homogenate. Decrease in rhodopsin regeneration level has been resulted from addition of inhibitors--theophyllin (2.10-2 M), papaverine (10-4--10-3 M) and strophantin (2.10-4 M) To the eyecup preparations (60, 20, 23%, consequently). A conclusion is made that structural connection between pigment epithelium cells and photoreceptors is necessary to provide regeneration of native rhodopsin.     

The fine structure of the pallial eyes of Laternula truncata (Bivalvia: Anomalodesmata: Pandoracea)

The structure of the pallial eyes of Laternula truncata (Lamarck 1818) has been studied using the light and electron microscopes. The eye is complex and can be- considered to be- the most advanced yet described for a bivalve mollusc. The cornea consists of modified flattened epithelial cells with an external border of microvilli. The cornea covers a large, circular, multinucleate lens. The lens comprises (1) centrally located translucent lens cells, (2) laterally located supporting cells from which cell processes interdigitate with processes from the lens cells. The retina is two layered and inverted. The proximal and distal retinae are made up of concentrically arranged laminae derived from the membranes of ciliary basal bodies. The cilia comprise a base and feet, but no root system and have a 9+0 arrangement of filaments.
The pigment cup or tapetum is bounded by a sclerotic coat and is three layered, each layer possessing characteristic pigment granules. From the base of the eye arises a large optic nerve.
The eye possesses an eye appendage, the epithelium of which is invaginated on its internal border to form a groove within which are found some 28 cilia. The cilia, it is thought, make contact with the microvilli of the epithelium when the appendage is touched. Such an action serves to protect the delicate eye from damage.
The structure of the eye is compared with that of other molluscs, particularly members of the Bivalvia.     

胭脂鱼眼早期形态发生研究

采用组织学方法观察了胭脂鱼(Myxocyprinus asiaticus) 眼的发生过程, 结果显示: 胭脂鱼眼的发育经历了眼原基形成期、眼囊形成期、视杯形成期、晶体板形成期、晶体囊形成期、角膜原基形成期、角膜上皮形成期、视网膜细胞增殖期、晶状体成熟期、眼色素形成期以及眼成型期等11个时期。视网膜发育最早, 起始于眼原基的形成, 直至眼成型期分化完成, 形成了厚度不一的8层细胞, 由内向外依次为神经纤维层、神经细胞层、内网层、内核层、外网层、外核层、视杆视锥层和色素上皮层, 且发育历时最长, 约264h。晶状体的发育在视网膜之后, 始于晶体板的形成, 于出膜前期成熟, 发育历时最短, 约74h。角膜发育最晚, 始于角膜原基的形成, 出膜1 d分化为透明的成熟角膜, 发育历时约96h。出膜4 d仔鱼眼色素沉积明显, 视网膜各层分化明显, 晶状体内部完全纤维化, 眼的形态结构基本发育完全。     

Metaplastic transformation of the tissue of the eye in tadpoles and adult Xenopus laevis frogs]

The pigment epithelium of the tadpoles and adults X. laevis, as well as of other anurans and cyprinids, is not capable of transformation into the retina without the special influences of agents produced by the retina. When implanting a layer of pigmented epithelium of tadpoles with the Bruch's membrane into the cavity of lensless eye of a tadpole, the transformation of pigment epithelium into retina proceeded in 40% of cases and when implanting the pigment epithelium of adults without the Bruch's membrane, the transformation proceeded in 68% of cases. The lens regeneration from the cornea which proceeds simultaneously under the retina influence exerted no effect upon the metaplasia of pigmented epithelium.     

Electrophoretic and immunofluorescent study of lactate dehydrogenase isoenzymes during eye regeneration in adult tritons]

The spectrum of LDH isozymes was studied at the successive stages of retinal regeneration from the pigment epithelium and lens cells from the iris margin in the adults Pleurodeles waltlii. The combination of two methods, electrophoresis and immunofluorescence, has revealed the slow and rapid LDH isozymes with different intensity of histochemical staining in cells of the tissues under study (pigment epithelium, retina, iris and lens). During the regeneration the spectra of LDH isozymes peculiar to the pigment epithelium and iris and characterized by the predominance of slow forms were substituted by those peculiar to the retina and iris and characterized by the predominance of rapid forms. The rearrangement is realized in the proliferative phase during the transformation of one cell type into another.     

Ontogenetic changes and environmental effects on ocular transmission in four species of coral reef fishes

Filtration by the humors, cornea and lens limits the spectrum of light available for vision as blocking compounds prevent some wavelengths from reaching photo-sensitive cells of the retina. The visual ecology of fishes is dependent upon factors changing with size and/or habitat. We predicted that ontogeny and habitat depth would affect ocular transmission for four fishes, Mulloidichthys flavolineatus, Parupeneus multifasciatus, Acanthurus triostegas, and Naso lituratus. We measured ocular transmission in specimens from a range of sizes (juvenile-adult) and capture depths (<3-37 m), and used the wavelength (nm) where transmission was reduced 50% as our comparative measure (T(50)). We modeled lens transmission varying pigment concentrations and pathlength, and compared predicted versus measured results. P. multifasciatus, M. flavolineatus, and N. lituratus showed a significant increase in short-wavelength blocking with size. A. triostegas were constant across sizes, and showed a slight but significant effect with depth. Comparisons of predicted versus observed transmission values suggest that pigment concentrations are held constant with age for all species, but species- and family-level differences emerge. The accumulation of blocking compounds in ocular tissues is a contributing means for balancing the costs and benefits of admitting short-wavelength radiation to the retina.     

Elimination of egg pigment from developing ocular tissues in the frog Rana pipiens

The method by which egg pigment is eliminated from the developing retina, corneal epithelium and lens in Rana pipiens was studied with light and electron microscopy. The retina expells egg pigment into the space between the retina and pigment epithelium. This pigment is then engulfed by the pigment epithelial cells. The corneal epithelium eliminates egg pigment directly to the outside via the free surface of the epithelial cells. Egg pigment accumulates in a few cells in the lens. These cells probably degenerate and are extruded. These ectodermal derivatives in the eye are free of egg pigment long before ectodermal derivatives in other parts of the embryo lose their pigment. The early elimination of egg pigment from ocular tissues may related to the fact that these tissues must be transparent in order that light may pass freely to the photoreceptors.     

Transformation of Iris into Lens in vitro and its Dependency on Neural Retina

Upon lentectomy of adult newt eyes, the dorsal iris epithelium produces a cell population that develops into a new lens. The tissue transformation can be completed not only in the isolated lentectomized eye cultured as a whole, but also in the isolated newt normal dorsal iris combined with the retina of frog larvae in vitro. In this study, 93% of such cultures produced lens tissue made up of newt cells. Well-differentiated lens fibre cells were formed which showed positive immunofluorescence for gamma crystallins. When the isolated dorsal iris epithelium was cultured under the same conditions, well-differentiated lens tissue was again formed in 95% of the cases, suggesting that iris epithelial cells and not iris stromal cells are responsible for lens formation. In contrast, the combination of newt ventral iris with frog retina did not produce any newt lens tissue. No lens tissue was produced when the dorsal iris was cultured with newt spleen or lung, although a considerable number of iris epithelial cells became depigmented. Isolated normal dorsal iris or normal dorsal iris epithelium cultured alone infrequently produced a population of depigmented cells but failed to form lens tissue. On the basis of the present and earlier data, it is concluded that in Wolffian lens regeneration in situ , interaction of the iris epithelial cells with the retina plays a decisive role. However, it is suggested that the iris epithelial cells may have an inherent tendency towards lens formation, and that the factor(s) from the retina facilitates the realization of this tendency, rather than instructing the cells to produce lens. The reported experiments provide the first direct evidence for the existence of cellular metaplasia by demonstrating transformation of fully differentiated iris epithelial cells into lens cells.     

Cultivation of the retinal pigment epithelium in the cavity of the lentectomized eye of newts

A study was made of proliferative activity and transdifferentiation of the cells of retinal pigment epithelium (RPE) cultivated in the cavity of the lensectomized eye of adult newt. Implantation of the newt RPE together with vascular membrane and scleral coat resulted in the regeneration of retina. In this process the character of changes in the proliferative activity of RPE and differentiation of retinal cells were the same as in the regeneration of retina in situ. RPE implanted with the vascular membrane alone, despite a high level of proliferation during the first ten days of cultivation, no differentiated retina was formed. Possible causes of these differences are discussed, and the comparison is made of the data obtained with those on RPE cultivation in vitro. After lens removal, with RPE implants present in the eye cavity, in addition to the regenerated lens, 2-3 extra lenses and retina were formed from the cells of the inner layer of the recipient's dorsal iris. Also some cases were revealed of lens formation from the cells of ventral iris. With a complete detachment of the recipient's retina (an after-effect of transplantation) a second differentiated retina regenerated in situ from the recipient's RPE cells.     

Fine structure of light- and dark-adapted eyes of desert ants,Cataglyphis bicolor (Formicidae,Hymenoptera)

Among ants, Cataglyphis bicolor shows the best performance in optical orientation. Its eye is of the apposition type with a fused rhabdom. Morphological studies on the general struture of the eye as well as the effect of light have been carried out with transmission and scanning electron microscopy. An ommatidium is composed of a dioptric apparatus, consisting of a cornea, corneal process and a crystalline cone, the sensory retinula, which is made up of eight retinula cells in the distal half and of an additional ninth one in the proximal half. The ommatidia are separated from each other by two primary pigment cells, which surround the crystalline cone and an average of 12 secondary pigment cells, which reach from cornea to the basement membrane. The eye of Cataglyphis bicolor possesses a light intensity dependent adaptation mechanism, which causes a radial and distal movement of the pigment granules within the retinula cells and a dilatation of cisternae of the ER along the rhabdom. Until now, no overall order in arrangement of retinula cells or direction of microvilli has been found from ommatidium to ommatidium. Such an order, however, must exist, either on the retina or the lamina level, since we have proven the ant's capacity for polarized light analysis.     

Cellular and ultrastructural features of the adult and the embryonic eye in the marine gastropod,Ilyanassa obsoleta

Light and electron microscopic techniques show that the eye of the marine prosobranch gastropod, Ilyanassa obsoleta, is composed of an optic cavity, lens, cornea, retina, and neuropile, and is surrounded by a connective tissue capsule. The adult retina is a columnar epithelium containing three morphologically distinct cell types: photoreceptor, pigmented, and ciliated cells. The retina is continuous anteriorly with a cuboidal corneal epithelium. The neuropile, located immediately behind the retina, is composed of photoreceptor cell axons, accessory neurons, and their neurites. The embryonic eye is formed from surface ectoderm, which sinks inward as a pigmented cellular mass. At this time, the eye primordium already contains presumptive photoreceptor cells, pigmented retinal cells, and corneal cells. Several days later, just before hatching, the embryonic eye remains in intimate contact with the cerebral ganglion. It has no ciliated retinal cells, neuropile, optic nerve, or connective tissue capsule and its photoreceptor cells lack the electron-lucent vesicles and multivesicular bodies of adult photoreceptor cells. As the eye and the cerebral ganglion grow apart, the optic nerve, neuropile, and connective tissue capsule develop.     

GROWTH OF LENS AND OCULAR ENVIRONMENT: ROLE OF NEURAL RETINA IN THE GROWTH OF MOUSE LENS AS REVEALED BY AN IMPLANTATION EXPERIMENT

The lens of 6-day-old normal mouse was implanted into the lentectomized eye of adult mouse to examine the effect of retina upon the growth of the implanted lens in vivo. The implanted lens grew normally and its transparency was kept for more than 5 months after implantation. The connection between the implanted lens and the ciliary part of the recipient iris was well established with the regeneration of zonular fibers from the recipient. In young lenses implanted reversely into adult eyes, the epithelial cells facing the retina elongated and a new epithelium was formed on the corneal side of the lens within 5 days. Young lenses implanted either in normal or reverse orientation into eyes from which the retina was previously removed did not grow. The cells of the original lens epithelium of these lenses were randomly accumulated beneath the posterior lens capsule, while the anterior portion of the implanted lenses became an epithelial structure without cell elongation. These results suggest that the growth of the implanted lens may be dependent on the retina of the adult eye.     

Observations on the morphology of the developing primate cornea: epithelium, its innervation and anterior stroma.

A survey is made of some ultrastructural features of the developing cornea of Macaca mulatta. The observations are confined to the anterior central area, starting with the lens vesicle stage and progressing through midgestation, when the morphologic characteristics of the cornea are fully established. Subepithelial filaments and some partially aggregated collagen fibrils are present in the earliest embryo and are of a size and appearance similar to those in the future vitreous cavity. Epithelial secretory activity points to, but does not prove direct contribution to the deposition of the acellular matrix components beneath it. No trace of a structured, orthogonal collagenous stroma can be visualized. The primitive endothelium forms prior to the fibroblast invasion of the distended filamentous matrix. Bowman's layer has undoubted epithelial contributions. Its aggregated collagen fibrils have approximately the same diameter as those of the anterior stroma. Intraepithelial appearance of single nerve fibers and fascicles takes place during the first trimester of gestation, as soon as the two continuous epithelial layers are formed. Terminal areas approach closely to the basal cell's nucleus, without touching it. The plasmalemma of the invaginating nerve fiber is surrounded by that of the epithelial cell in a mesaxon-like manner, with occasional gap junctions uniting adjoining neural and epithelial cell membranes. The fetal neurites contain microtubules, some clear vesicles and dense vacuoles resembling those of mature monamine and non-monamine neurons. Mitochondria are small and compact, their presence indicating a high rate of metabolic activity in the immature terminal area.     

Neural retinal regeneration in the anuran amphibian Xenopus laevis post-metamorphosis: transdifferentiation of retinal pigmented epithelium regenerates the neural retina

In urodele amphibians like the newt, complete retina and lens regeneration occurs throughout their lives. In contrast, anuran amphibians retain this capacity only in the larval stage and quickly lose it during metamorphosis. It is believed that they are unable to regenerate these tissues after metamorphosis. However, contrary to this generally accepted notion, here we report that both the neural retina (NR) and lens regenerate following the surgical removal of these tissues in the anuran amphibian, Xenopus laevis, even in the mature animal. The NR regenerated both from the retinal pigment epithelial (RPE) cells by transdifferentiation and from the stem cells in the ciliary marginal zone (CMZ) by differentiation. In the early stage of NR regeneration (5-10 days post operation), RPE cells appeared to delaminate from the RPE layer and adhere to the remaining retinal vascular membrane. Thereafter, they underwent transdifferentiation to regenerate the NR layer. An in vitro culture study also revealed that RPE cells differentiated into neurons and that this was accelerated by the presence of FGF-2 and IGF-1. The source of the regenerating lens appeared to be remaining lens epithelium, suggesting that this is a kind of repair process rather than regeneration. Thus, we show for the first time that anuran amphibians retain the capacity for retinal regeneration after metamorphosis, similarly to urodeles, but that the mode of regeneration differs between the two orders. Our study provides a new tool for the molecular analysis of regulatory mechanisms involved in retinal and lens regeneration by providing an alternative animal model to the newt, the only other experimental model.     

Regional differences in a nematoceran retina (Insecta,Diptera)

Summary The compound eye of Psychoda cinerea comprises two types of ommatidia, arranged so as to divide the retina into distinct dorsal and ventral regions. The P-type ommatidium, in the ventral part of the eye, differs fundamentally from the other dipteran ommatidia so far described, and is regarded as a primitive ommatidium. The acone dioptric apparatus is the same in both types, with a spherical lens and four Semper cells, the processes of which expand below the rhabdom to form a ring of pigment sacs. Only the distal region of the rhabdom is surrounded by a continuous ring of screening pigment, formed by 2 primary and 12–16 secondary pigment cells. The highly pigmented retinula cells penetrate the basement membrane proximally at about the level of their nuclei; in this region they are separated from the hemolymph by glial elements. The rhabdomeres R1–6 are fused to form a tube. The two types of ommatidia are defined by the arrangement of the retinula cells R7/8: in the T type the central rhabdomeres are one below the other, in the usual tandem position, whereas in the P type only R8 is central, with R7 in the peripheral ring. In the proximal region of the retina, retinula cells with parallel microvilli in neighboring ommatidia are joined in rows by lateral processes from the R8 cells. All the rhabdomeres are short and not twisted, which suggests that the retinula cells are highly sensitive to direction of polarization. The eye can adapt by a number of retinomotor processes. These findings, together with observations of behavior, imply that the psychodids have well-developed visual abilities.