Blood-eye barrier of the developing drosophila melanogaster (Diptera : Drosophilidae)

For the past quarter century, it has been known that a blood-eye barrier exists in adult insects. However, the life stage at which the barrier arises and the anatomical correlate of the barrier were not known. Compound eye development in Drosophila melanogaster (Diptera : Drosophilidae) is essentially complete at approximately 140 h after pupariation ; or about 20 h prior to eclosion. A search for a blood-eye barrier spanned late third-instar larvae, through late pupal life sampled at 40 and 140 h post-pupariation. No blood-eye barrier is present in the eye discs of last-instar larvae, based on the presence of lanthanum tracer among ommatidial cells and their processes. Pleated-sheet septate junctions, which link larval ommatidial cells, are not yet capable of totally blocking tracer from paracellular passage. The blood-eye barrier is constructed in the early phase (0–60 h) of pupal development in the wake of apoptosis and new cellular reorganization. In the developing compound eye, mature photoreceptor neurons must then be protected from the ionic vagaries of hemolymph to become electrophysically competent. First vestiges of a barrier with occluding septate junctions are seen in the 40-h old pupa, and by 140 h, the barrier is complete. The barrier prevails throughout adult life.     

Intensity and polarization of the eyeshine in butterflies

The compound eyes of most diurnal butterflies have a reflecting tapetum below the retina. Light that enters the eye is guided down the rhabdom, reflected by the tapetum, and then guided back up the rhabdom. The light that is not absorbed by the rhabdom is reemitted and gives rise to an eyeshine. We have measured the fraction of the incident light that is re-emitted, and also the degree to which this light retains its original polarization. The following conclusions are drawn:

Lanthanum and freeze fracture studies on the retinular cell junction in the compound eye of the housefly

Summary The retinular (R) cell junction between adjacent photoreceptor cells in the house-fly ommatidium was characterized by freeze fracture, thin section and tracer (lanthanum) studies. Focal tight junctions occur between cells, and some P face ridge-E face groove correspondences are present in this intramembranal area. When colloidal lanthanum was introduced into the extracellular space (ECS) of the peripheral retina of the housefly, this electrondense tracer moved from the ECS (extra-ommatidial space), through the R-cell junctions and belt desmosomes, into the ommatidial cavity (OC = intrarhabdomal space) of each ommatidium. In the OC, lanthanum outlined a meshwork structure that pervaded this space. The evidence of this tracer movement suggests that there may be ionic continuity between the traditional ECS and the fluid bathing the individual rhabdomeres. The volume of the OC is calculated and we suggest that this space is part of the ECS. The functional implications of this postulate are considered in the light of: (1) the different functions of the peripheral and central cells; (2) the dissimilarity of rhabdomal membrane surface facing the OC compared to the unmodified plasma membrane of the photoreceptor cell facing the extra-ommatidial cavity; (3) the permeability properties of the R cell junction; and (4) the total ECS containing an ion store capable of sustaining current for the generator potential.We gratefully acknowledge support from the N.I.H., National Eye Institute, EYO 1686 and from the College of Agricultural and Life Sciences, Hatch Project 2100. We thank Dr. Philippa Claude (UW Primate Research Center, Madison) for training in freeze fracture technique. This part of the work was supported by Grant RR00167 from the N.I.H. to the Primate Research Center. Dr. Robert Goy, Director of the Primate Research Center is acknowledged for his kind permission to use the Center's freeze fracture apparatus. Professor Stanley D. Beck, Department of Entomology, UW, Madison provided valuable advice for which we thank him heartily     

Organisation of the compound eye of a tipulid fly during the day and night

Summary The ultrastructure of the compound eye of the Australian tipulid fly,Ptilogyna spectabilis, is described. The ommatidia are of the acone type. The rhabdom corresponds to the basic dipteran pattern with six outer rhabdomeres from retinular cells 1–6 (R1-6) that surround two tiered central rhabdomeres from R7 and 8. Distally, for about 8 m, the rhabdom is closed. For the remainder, where the rhabdomere of R8 replaces that of R7, the rhabdom is open, and the rhabdomeres lie in a large central ommatidial extracellular space. In the proximal two thirds of the rhabdom, the central space is partitioned by processes from the retinular cells so that the individual rhabdomeres are contained in pockets.At night the rhabdom abuts the cone cells, but during the day it migrates some 20 m proximally and is connected to a narrow (1–2 m) cone cell tract. This tract is surrounded by two primary pigment cells, which occupy a more lateral position at night and thus act like an iris. Pigment in secondary pigment cells also migrates so as to screen orthodromic light above the rhabdom during the day. Between midday and midnight, the rhabdom changes in length and cross-sectional area as a result of asynchrony of the shedding and synthetic phases of photoreceptor membrane turnover. The effects of these daily adaptive changes on photon capture ability are discussed with regard to the sensitivity of the eye.     

A Single Intravenous AAV9 Injection Mediates Bilateral Gene Transfer to the Adult Mouse Retina

Widespread gene delivery to the retina is an important challenge for the treatment of retinal diseases, such as retinal dystrophies. We and others have recently shown that the intravenous injection of a self-complementary (sc) AAV9 vector can direct efficient cell transduction in the central nervous system, in both neonatal and adult animals. We show here that the intravenous injection of scAAV9 encoding green fluorescent protein (GFP) resulted in gene transfer to all layers of the retina in adult mice, despite the presence of a mature blood-eye barrier. Cell morphology studies and double-labeling with retinal cell-specific markers showed that GFP was expressed in retinal pigment epithelium cells, photoreceptors, bipolar cells, Müller cells and retinal ganglion cells. The cells on the inner side of the retina, including retinal ganglion cells in particular, were transduced with the highest efficiency. Quantification of the cell population co-expressing GFP and Brn-3a showed that 45% of the retinal ganglion cells were efficiently transduced after intravenous scAAV9-GFP injection in adult mice. This study provides the first demonstration that a single intravenous scAAV9 injection can deliver transgenes to the retinas of both eyes in adult mice, suggesting that this vector serotype is able to cross mature blood-eye barriers. This intravascular gene transfer approach, by eliminating the potential invasiveness of ocular surgery, could constitute an alternative when fragility of the retina precludes subretinal or intravitreal injections of viral vectors, opening up new possibilities for gene therapy for retinal diseases.     

Optics of the butterfly eye

The afocal apposition optics of butterfly eyes was examined from both a geometrical optics and a wave optics point of view. We used several different species of butterfly but put special emphasis on a common Australian nymphalid,Heteronympha merope. From the anatomy of the retina, the optics of isolated components of the eye and the ophthalmoscopy of the intact living eye we derived the following.

Arhabdomeric cells of the median eye retina of scorpions

Summary Based on reconstructions from serial thin sections, arhabdomeric cells within the retina of the median eyes of the scorpion,Androctonus australis, are identified. Each retinula unit (formed by mainly five retinula cells with a fused rhabdom) is associated with one arhabdomeric cell. Extending distally from its soma which is located close to the postretina, the arhabdomeric cell bears an up to 80 m long dendrite that ends at the base of the fused rhabdom. The most noteworthy morphological feature of the dendrite is the presence, at the distal dendrite tip, of numerous finger-like or bulbous evaginations that extend into every one of the five visual cells forming a retinula unit. These and other characteristics strongly suggest that the arhabdomeric cell represents an intrinsically photoinsensitive second neuron involved in visual information processing.This study was supportet by a grant from the Deutsche Forschungs-gemeinschaft (F1 77/8).     

α-Methyl-p-tyrosine inhibits the production of free arachidonic acid and diacylglycerols in brain after a single electroconvulsive shock

A single electroconvulsive shock (ECS) significantly enlarged the free fatty acid (FFA) pool of the mouse brain, arachidonic acid being the most actively released FFA (48% over controls). In animals pretreated with -methyl-p-tyrosine (-MT), the endogenous levels of FFA (25 sec after decapitation) were decreased and the effect of ECS was completely abolished. The most pronounced inhibition took place in free arachidonic acid (42% and 52% under controls in nonstimulated and ECS-stimulated mice, respectively). A similar tendency, although lower and less specific than the one taking place in FFA, was observed in mouse brain diacyl-glycerols (DG). In contrast to ECS, -MT treatment did not affect the marked release of FFA and DG taking place 3 min after decapitation. Taking into account the specific inhibitory action of -MT on tyrosine hydroxylase activity, the present findings provide experimental in vivo evidence about the relationship between biogenic amines and membrane lipid breakdown during electrical stimulation and suggest an involvement of FFA and DG in neurotransmission.     

Retinal ultrastructure in the dorsal rim and large dorsal area of the eye of Aglais urticae (Lepidoptera)

Summary The ommatidia in the two dorsal rows at the rim of the eye of Aglais urticae differ from all the other ommatidia of the large dorsal area, in rhabdom structure, length, and configuration of the ninth retinula cell. The type of rhabdom in this dorsal rim zone provides the structural prerequisites for the reception of polarized light; functional subdivision of the retina into two parts is indicated.     

The fine structure of the visual system of Lycosa (Araneae: Lycosidae)

Summary Wolf spiders have four pairs of eyes distributed in three rows. The first row which lie in the frontal region of the caparace, just above the chelicera, contains four eyes: a medial pair known as the anterior medial eyes (AM eyes or principal eyes) and two smaller eyes known as the anterior lateral eyes (AL eyes). The second row which is located also in the frontal region of the prosoma consists of two big eyes. These are the posterior median eyes (PM eyes). The third row contains the posterior lateral eyes (PL eyes) which lie in the flanks of the prosomal caparace. The AL, PM and PL eyes are the so-called secondary eyes.The electron microscope shows that the AM eye photoreceptor cells have the rhabdomere in their distal segment, just behind the vitreous body. The rhabdomere consists of closely packed microvilli about 0.5 long exhibiting a uniform diameter of 500 Å. Each rhabdom consists of two rhabdomeres. The distal segment of the photoreceptor has a prismatic shape with four or five faces depending of their location within the retina.The distribution of the rhabdoms follows two different patterns or organization. In the peripheral portion of the retina they lie oriented either parallel or perpendicular to the retinal radii. In this zone most cells have four sides while in the central region five sided cells are predominant. These cells bear microvilli in three of their five faces and the rhabdoms show no preferential mode of orientation. Each retina contains approximately 450 photoreceptors. In the secondary eyes the rhabdoms lie far from the vitreous body behind the level of the cell nuclei. A light reflecting layer or tapetum is present in the three pairs of secondary eyes. The microvilli forming the rhabdomeres of the AL eyes are 0.5 long and 500 Å wide, while the microvilli of the rhabdomeres in the PM and PL eyes are longer and thicker (1.5 long and 550–660 Å wide). In these eyes the rhabdomeres are surrounded by abundant extracellular material. Like in the principal eyes each rhabdom consists of two rhabdomeres.In the AL eyes the photoreceptor cells send out collateral branches which end, without any specialization, in contact with other photoreceptors. Clear fibers running parallely to the tapetum have been found in the secondary eyes. These fibers show specialized regions corresponding to the zones of contact with the photoreceptor cells. These areas are characterized by an increased density of the membranes and groups of vesicles (the vesicles lie within the fibers).The optic nerves consist of photoreceptor axons, glial cells and a fibrous perineural sheath. The AM and AL eyes are connected to the CNS by a single compact optic nerve while in the PM and PL eyes the optic nerve consists of several individual bundles. The total number of optic fibers entering into the brain is about 12.000.A layer of glial cytoplasm covers each photoreceptor axon and the mesaxons appear as double lines which bifurcate frequently.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR Grant Nr. 618-64.     

Degeneration of the compound eye of the termite Neotermes jouteli (Isoptera) in darkness during the phase of reproduction

Summary Long-term light deprivation of the royal pair of Neotermes jouteli during the phase of reproduction leads to a dramatic change in the organization within the compound eye. In a swarming alate, investigated with scanning and transmission electron microscopy, the eye consists of about 200 ommatidia. No differences between male and female eyes are observed. Each ommatidium is composed of a biconvex cornea, a cone of the eucone type, and a rhabdom which is located directly beneath the Semper cells. The rhabdom consists of eight rhabdomeres which are fused along the ommatidial axis. In the central part of the compound eye the rhabdom measures roughly 20 m in length. Concealed life of the imagines causes a dismantling of the cone and the rhabdom until complete destruction. This is accompanied by an increase in the number of pigment granules and a decrease in the number of mitochondria.     

Septate junctions are required for ommatidial integrity and blood-eye barrier function in Drosophila

The anatomical organization of the Drosophila ommatidia is achieved by specification and contextual placement of photoreceptors, cone and pigment cells. The photoreceptors must be sealed from high ionic concentrations of the hemolymph by a barrier to allow phototransduction. In vertebrates, a blood-retinal barrier (BRB) is established by tight junctions (TJs) present in the retinal pigment epithelium and endothelial membrane of the retinal vessels. In Drosophila ommatidia, the junctional organization and barrier formation is poorly understood. Here we report that septate junctions (SJs), the vertebrate analogs of TJs, are present in the adult ommatidia and are formed between and among the cone and pigment cells. We show that the localization of Neurexin IV (Nrx IV), a SJ-specific protein, coincides with the location of SJs in the cone and pigment cells. Somatic mosaic analysis of nrx IV null mutants shows that loss of Nrx IV leads to defects in ommatidial morphology and integrity. nrx IV hypomorphic allelic combinations generated viable adults with defective SJs and displayed a compromised blood-eye barrier (BEB) function. These findings establish that SJs are essential for ommatidial integrity and in creating a BEB around the ion and light sensitive photoreceptors. Our studies may provide clues towards understanding the vertebrate BEB formation and function.     

Effects of photic and thermal stress on distal and proximal rhabdomeres in the crayfish eye: why are the visual membranes of the 8th retinula cell more resilient than the others?

Summary Visual membranes of the crayfish eye either belong to the small, distally placed rhabdomere of retinula cell R8 or are part of the much more voluminous proximal rhabdom, made up of rhabdomeres belonging to cells R1–R7. Under various conditions of environmental stress (e.g., prolonged darkness, elevated temperature, bright light with and without a concomitant rise in temperature, flickering lights) the visual membranes of R8 prove far more resistant to structural damage than those of R1–R7. Membrane damage is known to occur when dormant lipoxygenases become activated, for example through heat. Since R8 is the only type of visual cell in the crayfish retina that does not contain grains of screening pigment, the view that screening-pigment granules could aggravate or even trigger membrane damage in times of stress is strengthened. Functionally, R8's strong resistance to physical damage when exposed to flickering lights points to a role of the distal rhabdom in the movement detection system of the crayfish eye.     

Neurosecretory fibres in the median eyes of the scorpion,Androctonus australis L.

Summary The retina of the median eyes of the North African scorpion, Androctonus australis L., is supplied with numerous neurosecretory nerve fibres which establish synaptoid contacts on the retinula cells. The number of fibres or profiles of varicosities of fibre terminals associated with a retinular unit (five retinula cells with a fused rhabdom) varies between 10 and 20. Electron-opaque vesicles with a diameter of 80–100 nm are abundant within the axonal profiles. The synaptoid junctions are characterized by postsynaptic electron-dense material on the inner leaflet of the retinula cell membrane and, frequently, presynaptic submembranous dense material. Because of these ultrastructural features, the junctions observed here resemble typical interneuronal synaptic contacts. Hence this kind of neurosecretory junction appears to be unique among arthropods.It is suggested that the neurosecretory fibres within the retina represent the efferent pathways for the control of the circadian pigment movements within the retinula cells.Supported by the Deutsche Forschungsgemeinschaft (F1 77/7)     

The Development of the Rhabdom and the Appearance of the Electrical Response in the Insect Eye

Electron microscopic studies on the development of the rhabdom in the compound eye of the silkworm moth and pupa (Bombyx mori) were carried out in parallel with the recording of the electrical response to photic stimulation. No electrical response to photic stimulation was recorded from the pupal compound eye which had no trace of differentiation of the rhabdom. With the differentiation of development of the rhabdom in the pupal compound eye, electrical responses could be recorded, and the amplitude of such electrical responses increased with the progress of development of the rhabdom. These observations suggest that the rhabdom is probably the site of the photochemical reaction which leads to the generation of the slow retinal action potentials.     

An ultrastructural study of the eye of Gomphiocephalus hodgsoni, a collembolan from Antarctica

The collembolan Gomphiocephalus hodgsoni is one of a few hexapods occurring in Antarctica. Male and female individuals do not differ with regard to their eyes. Both possess eight single-lens eyes. In the adults, each lens has a diameter of 10–12 m and covers an almost spherical crystalline cone made up of four unequal moieties. When we accepted homogenous refractive indices, known from pterygote insect eyes, and then calculated the focal length of the dioptric apparatus, we found that light could be focused on the retina. The retinal cells of each eye are grouped in two tiers and surround the centrally fused rhabdom. Rhabdomeric microvilli, approximately 1.3 m long, possess a diameter of 80 nm and are typically aligned in one direction. More proximally, however, two rhabdomeres with microvilli perpendicular to the larger, more distal, rhabdom appear. Signs of light-induced damage, despite the bright summer radiation in Antarctica, were not seen. Ricegrain-like screening pigment granules, measuring 0.8×0.45 m in dimensions, densely surround each rhabdom and shield the cytoplasm of the surrounding cells. The orthogonal arrangement of the microvilli suggests that G. hodgsoni could be sensitive to light-polarization. This ability might be of importance in detecting tiny amounts of meltwater in close proximity to the insect.The order of the authors names reflects the sequence in which the individuals joined in this project.     

Polarization sensitivity of individual retinula cells

Summary This paper elucidates the influence of the structure of a rhabdom on the polarization sensitivity of its retinula cells. The terminology polarization sensitivity (PS) and dichroic sensitivity () needs clarification. expresses the directional property of the local microvillar medium and is independent of the gross morphology of the rhabdom. The PS of a retinula cell is that found by single cell electrophysiology and depends strongly on the gross morphology of the rhabdom. Both and PS are ratios of the effects of theE vector of linear polarized light parallel to, to that perpendicular to the microvilli. From the theoretical analysis and its correlation with experiments the following is concluded.     

FINE STRUCTURE OF THE RETINULAE IN THE COMPOUND EYE OF THE HONEY-BEE

The retinula of the compound eye of the worker honey-bee has been examined with the electron microscope. The rhabdom lies on the ommatidial axis; it is usually cylindrical in shape, about 3 to 4 µ in diameter, and lacks an axial cavity. Cross-sections show it to be four parted, although it is formed from eight retinular cells (Figs. 2, 3). Each quadrant of the rhabdom consists of a closely packed parallel array of tubules with long axes perpendicular to the axis of the rhabdom. The tubules in adjacent quadrants of the rhabdom are mutually perpendicular. At the distal end of the ommatidium these tubules are seen to be microvilli of the retinular cells. Immediately surrounding the rhabdom, the cytoplasm of the retinular cells contains a membranous endoplasmic reticulum which is oriented approximately radially with respect to the axis of the ommatidium. Farther away from the rhabdom the cytoplasm contains numerous mitochondria.     

The eyes of mesopelagic crustaceans: I. Gennadas sp. (Penaeidae)

Summary The eye of the deep-sea penaeid shrimp Gennadas consists of approximately 700 square ommatidia with a side length of 15 n. It is hemispherical in shape and is located at the end of a 1.5 mm long eye stalk. The cornea is extremely thin, but the crystalline cone is well-developed. A clear zone between dioptric structures and the rhabdom layer is absent. A few pigment granules are found within the basement membrane; otherwise they, too, are absent from the eye of Gennadas. The rhabdom is massive and occupies 50 % of the eye. It consists of orthogonally oriented microvilli (the latter measuring 0.07 m in diameter) and is 75 m long. In cross sections adjacent rhabdoms, all approximately 8 m in diameter, form an almost continuous sheet and leave little space for retinula cell cytoplasm. In spite of a one h exposure to light, rhabdom microvilli show no disintegration or disruption of membranes. Vesicles of various kinds, however, are present in all seven retinula cells near the basement membrane. Bundles of seven axons penetrate the basement membrane. On their way to the lamina they often combine and form larger aggregations.The authors wish to thank the director of the Meat Industry Research Institute in Hamilton and his staff for the use of their electron microscope facilities     

Analog of vertebrate anionic sites in blood-brain interface of larval Drosophila

The blood-brain barrier ensures brain function in vertebrates and in some invertebrates by maintaining ionic integrity of the extraneuronal bathing fluid. Recent studies have demonstrated that anionic sites on the luminal surface of vascular endothelial cells collaborate with tight junctions to effect this barrier in vertebrates. We characterize these two analogous barrier factors for the first time on Drosophila larva by an electron-dense tracer and cationic gold labeling. Ionic lanthanum entered into but not through the extracellular channels between perineurial cells. Tracer is ultimately excluded from neurons in the ventral ganglion mainly by an extensive series of (pleated sheet) septate junctions between perineurial cells. Continuous junctions, a variant of the septate junction, were not as efficient as the pleated sheet variety in blocking tracer. An anionic domain now is demonstrated in Drosophila central nervous system through the use of cationic colloidal gold in LR White embedment. Anionic domains are specifically stationed in the neural lamella and not noted in the other cell levels of the blood-brain interface. It is proposed that in the central nervous system of the Drosophila larva the array of septate junctions between perineurial cells is the physical barrier, while the anionic domains in neural lamella are a charge-selective barrier for cations. All of these results are discussed relative to analogous characteristics of the vertebrate blood-brain barrier.