Ultrastructure of the ocellar visual system in normal and mutant Drosophila melanogaster

Between the two compound eyes on the vertex on the adult head are the three simple eyes, ocelli. Transmission and scanning electron microscopy were used to investigate the corneal lenses, ocellar photoreceptors, and axonal projections in normal and mutant Drosophila melanogaster. In wild type flies, the cornea consists of about 45 lamellae. It has corneal nipples distally and is underlaid with a monolayer of corneagenous cells. Retinula cells have open rhabdomeres of about 2 microns (diameter) x 7 microns (length). Rhabdomeres extend to the distal extent of the cell and do not have caps. Microvilli have a rodlet within. Retinula cells are joined by belt desmosomes on the lateral borders. Eye color pigment granules are housed within the retinula cells of normal flies, not in accessory cells. The granules do not migrate in response to light. No screening pigment granules exist in the white mutant. Each ocellus has about 80 retinula cells whose axons project to corresponding ganglia from which 4 giant afferent interneurons (per ganglion) project to the brain. receptor terminals are invested with capitate projections from glia. Receptors synapse onto dyads of follower cells, usually interneuron processes, at sites of T shaped presynaptic ribbons. These "T bars" are surrounded by indistinct flattened vesicles. Interneurons make feed back synapses onto receptor terminals at T bars clustered with distinct round vesicles. Three mutants with abnormal ocelli were investigated. The none mutant has unusual compound eye and ocellar corneas. The compound eye is devoid of differentiated photoreceptors but some axons from undifferentiated cells from synapses. No receptors were found in the ocelli of none. The oc mutant has no ocelli, although sometimes an ocellar cornea like that of none is seen; the compound eye is normal. The rdo mutant is also specific to ocelli with smaller ocelli having half the wild type allotment of receptor cells; the number of giant afferents is unaffected. Mutants best known for their compound eye defects were examined. The norpA mutant loses its ocellar rhabdomeres with age but has normal feed forward and feed back synapses. This normal synaptology prevails despite the electrophysiological defects in norpA ocelli reported earlier. The rdgABS12 mutant has poorly formed ocellar receptors which show some degeneration with age but synapses survive. The trp. rdgBKS222 and rgdAPC47 mutants are essentially normal with respect to structure and survival of ocellar receptors and synapses.     

Ultrastructure of the retina of Drosophila melanogaster. the mutant ora (outer rhabdomeres absent) and its inhibition of degeneration in rdgB (retinal degeneration-B)

The Drosophila mutant ora lacks rhodopsin in the Rl-6 set of photoreceptors and has a diminution of the photopigment containing rhabdomeres of Rl-6. Newly emerged flies have rhabdomeres, albeit small, which extend from the distal rhabdomere cap to the proximal basement membrane. As the fly ages, these are reduced until only distal remnants remain. Carotenoid deprivation does not protect ora flies from rhabdomere loss. When first characterized, ora was designated as a non-formation mutant rather than a degeneration mutant. The truth lies between, since rhabdomeres diminish with age but cells do not die. The plasmalemma of Rl-6 have unusual dense striated areas like the “zippers” described earlier for the Drosophila mutant norpA. Similar membranes are also present within the receptor somata, especially in young flies. The latter probably become internalized from the plasmalemma. They are likely not related to the diminished rhabdomeres as claimed earlier. R7 and R8 have normal rhabdomeres, and in particular, they have normal coated vesicles and multivesicular bodies (MVB's), early steps in the degradation phase of normal rhabdomere maintenance. No MVB's are seen in the Rl-6 somata, indicating that the routes for rhabdomere degradation differ from those of normal receptors. However, some MVB-like structures are seen in the intraommatidial cavity. The compound mutant rdgB;ora has a phenotype just like that of ora. This means that ora protects rdgB from the light induced degeneration of Rl-6 which characterizes rdgB.     

Ultrastructure of the retina of Drosophila melanogaster: The mutant ora (outer rhabdomeres absent) and its inhibition of degeneration in rdgB (retinal degeneration-B)

The Drosophila mutant ora lacks rhodopsin in the R1-6 set of photoreceptors and has a diminution of the photopigment containing rhabdomeres of R1-6. Newly emerged flies have rhabdomeres, albeit small, which extend from the distal rhabdomere cap to the proximal basement membrane. As the fly ages, these are reduced until only distal remnants remain. Carotenoid deprivation does not protect ora flies from rhabdomere loss. When first characterized, ora was designated as a non-formation mutant rather than a degeneration mutant. The truth lies between, since rhabdomeres diminish with age but cells do not die. The plasmalemma of R1-6 have unusual dense striated areas like the “zippers” described earlier for the Drosophila mutant norpA. Similar membranes are also present within the receptor somata, especially in young flies. The latter probably become internalized from the plasmalemma. They are likely not related to the diminished rhabdomeres as claimed earlier. R7 and R8 have normal rhabdomeres, and in particular, they have normal coated vesicles and multivesicular bodies (MVB's), early steps in the degradation phase of normal rhabdomere maintenance. No MVB's are seen in the R1-6 somata, indicating that the routes for rhabdomere degradation differ from those of normal receptors. However, some MVB-like structures are seen in the intraommatidial cavity. The compound mutant rdgB;ora has a phenotype just like that of ora. This means that ora protects rdgB from the light induced degeneration of R1-6 which characterizes rdgB.     

Ultrastructural and histochemical observations on the pigmented eyes of the oncomiracidium of Entobdella soleae,a monogenean skin parasite of the common sole,Solea solea

Summary The pigmented eyes of the oncomiracidium of the monogenean skin parasite Entobdella soleae are rhabdomeric in nature, the smaller anterior eyes each containing a single rhabdomere and the larger posterior eyes each containing two rhabdomeres. Each eye has a lens and a single cup-shaped pigment cell. There are differences in the arrangement of the microvilli in the rhabdomeres of the anterior and posterior eyes. Microvilli from each side of the anterior retinular cell are convergent. In each rhabdomere of the posterior eyes most of the microvilli lie parallel to each other but their direction is perpendicular to that of the microvilli of the adjacent posterior rhabdomere. The retinular cell leaves the eye between the pigment cell and the lens, and the retinular cell nucleus lies outside the eye. The darkbrown pigment in the pigment cell was identified as melanin. The lens contains carbohydrate and possibly protein but tests for fat were negative.Key to Lettering of Figs. 1–7 ae anterior eye - ar retinular cell of anterior eye - d desmosome-like thickenings - e eye - ex retinular cell exit from posterior eye - f possible muscle fibre - h haptor - l lens - m mitochondrion - np nucleus of pigment cell - nr nucleus of the retinular cell - pc pigment cell - pe posterior eye - pr retinular cell of posterior eye - rh rhabdomere     

Die projektion der optischen umwelt auf das raster der rhabdomere im komplexauge von Musca

Summary The dioptrics of the Musca ommatidium acts as an inverting lens system. The distal endings of the rhabdomeres at the basis of the dioptric apparatus are separated and arranged in a typical asymmetric pattern. The optical axes of the individual rhabdomeres of one ommatidium are the geometric projections of the distal rhabdomere endings into the environment, inverted by 180° by the dioptric apparatus. The divergence angles between the optical axes of the rhabdomeres of one ommatidium correspond to divergence angles between the appropriate set of ommatidia in such a way, that seven rhabdomeres of seven ommatidia are looking at one point in the environment (in the intermediate region between dorsal and ventral part of the eye: eight to nine rhabdomeres of a set of eight to nine ommatidia). These facts were established from sections of living eyes and confirmed by using special optical methods in the intact animal.The pattern of the decussation of individual retinulacell axons between retina and lamina was predicted adopting the hypothesis that the fibres of retinulacells number one to six, whose rhabdomers are looking at one point in the environment, project into a single cartridge in the lamina. These predicted connections were confirmed by other investigators. We have therefore a one to one correspondence between a lattice of points in the environment and the lattice of cartridges in the lamina.It is shown that the unfused rhabdomeric structure of the Musca ommatidium increases the effective entrance pupil of the eye by a factor of seven (resp. eight to nine in the intermediate region between dorsal and ventral part of the eye) compared to the classical apposition eye. —The Musca compound eye can be regarded as a neural superposition eye.

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CD138/syndecan-1 and SSEA-1 mark distinct populations of developing ciliary epithelium that are regulated differentially by Wnt signal

Ciliary epithelium (CE), which consists of nonpigmented and pigmented layers, develops from the optic vesicle. However, the molecular mechanisms underlying CE development have not been closely examined, in part because cell-surface markers suitable for specific labeling of subregions of the retina were unknown. Here, we identified CD138/syndecan-1 and stage specific embryonic antigen-1 (SSEA-1) CD15 as cell-surface antigens marking nonpigmented and pigmented CE, respectively. During retinal development, both CD138 and SSEA-1 were expressed in the early stage, and segregation of these markers in the tissue began at around embryonic day (E) 10. As a result, CD138-positive (CD138+) cells were found at the most distal tip of the retina, and SSEA-1+ cells were found in the periphery adjacent to the area of CD138 expression. In vitro characterization of isolated CD138+ or SSEA-1+ cell subpopulations revealed that CD138+ cells lose their retinal progenitor characteristics between E13 and E16, suggesting that they commit to becoming nonpigmented CE cells within this period. By in vivo mouse models, we found that stabilized beta-catenin expanded the area of CD138+ nonpigmented CE and that elimination of beta-catenin inhibited development of nonpigmented CE cells. These findings are the first to use cell-surface markers to ascertain the spatial and temporal transitions that occur in developing CE.     

Changes in the microvillus cytoskeleton during rhabdom formation in the retina of the crayfishProcambarus clarkii

Summary Changes in the microvillus cytoskeleton during the formation of the light-receptive rhabdom in the crayfish retina were examined at four structurally distinct stages. The cytoskeleton of microvilli in early rhabdoms is composed of a regularly packed bundle of 12–25 actin filaments. The polarity of S1 decorated filaments indicates that the plus end of the actin filaments is located at the microvillus tip. The hexagonal packing of filaments within the bundle, their spacing, and the presence of cross-striations along the bundle in longitudinal sections indicate the filaments are held together by cross-linking proteins. Electron microscopic observations and data from three-dimensional reconstructions of individual microvilli indicate that the filaments arise from a concentration of dense material at the tip of the microvillus and extend into the cytoplasm as a rootlet. Over the four developmental stages examined there is an increase in the number of microvilli forming the rhabdomeres and a 50% decrease in the mean cross-sectional area of individual microvilli. During this same period the number of actin filaments forming the microvillus cytoskeleton also decreases. Following this decline, microvilli of late stage rhabdoms, which are structurally similar to adults, contain only two to four filaments. These changes are discussed in relation to the three phases of growth described for stereocilia and brush border microvilli.     

Regeneration of axons in the optic nerve of the adult Browman-Wyse (BW) mutant rat

Summary We have studied the regeneration of axons in the optic nerves of the BW rat in which both oligodendrocytes and CNS myelin are absent from a variable length of the proximal (retinal) end of the nerve. In the optic nerves of some of these animals, Schwann cells are present. Axons failed to regenerate in the exclusively astrocytic environment of the unmyelinated segment of BW optic nerves but readily regrew in the presence of Schwann cells even across the junctional zone and into the myelin debris filled distal segment. In the latter animals, the essential condition for regeneration was that the lesion was sited in a region of the nerve in which Schwann cells were resident. Regenerating fibres appeared to be sequestered within Schwann cell tubes although fibres traversed the neuropil intervening between the ends of discontinuous bundles of Schwann cell tubes, in both the proximal unmyelinated and myelin debris laden distal segments of the BW optic nerve. Regenerating axons never grew beyond the distal point of termination of the tubes. These observations demonstrate that central myelin is not an absolute requirement for regenerative failure, and that important contributing factors might include inhibition of astrocytes and/or absence of trophic factors. Regeneration presumably occurs in the BW optic nerve because trophic molecules are provided by resident Schwann cells, even in the presence of central myelin, oligodendrocytes and astrocytes. All the above experimental BW animals also have Schwann cells in their retinae which myelinate retinal ganglion cell axons in the fibre layer. Control animals comprised normal Long Evans Hooded rats, BW rats in which both retina and optic nerve were normal, and BW rats with Schwann cells in the retina but with normal, i.e. CNS myelinated, optic nerves. Regeneration was not observed in any of the control groups, demonstrating that, although the presence of Schwann cells in the retina may enhance the survival of retinal ganglion cells after crush, concomitant regrowth of axons cut in the optic nerve does not take place.     

Evidence that migratory oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells are kept out of the rat retina by a barrier at the eye-end of the optic nerve

Summary There is evidence that oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells migrate along the developing rat optic nerve from the chiasm toward the eye before differentiating into oligodendrocytes that myelinate the retinal ganglion cell axons in the nerve. Why, then, do these progenitor cells not migrate into the eye, differentiate into oligodendrocytes and myelinate the nerve fibre layer of the retina? Myelination would opacify the neural retina and thereby severely impair vision. Here we provide evidence that there is a barrier at the eye-end of the rat optic nerve that prevents the migration of O-2A progenitor cells into the retina. Our findings in the rat support a previous hypothesis that such a barrier keeps myelin-forming glial cells out of the human retina.     

A direct projection from the nucleus oculomotorius to the retina in rats

The centrifugal projection to the eye has been studied in rats with anterograde and retrograde tracing techniques. As a retrograde tracer Nuclear Yellow (NY) was used. Following NY injections into the vitreous body of the eye, labeled neurons were exclusively found bilaterally in nucleus oculomotorius. The course and termination site of the retinopetal fibers were studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Iontophoretic injections of PHA-L in nucleus oculomotorius resulted in labeling of retinopetal fibers which reach the eye via the optic tract and optic nerve. Preterminal arborizations were found in the inner nuclear layer of the retina. In addition, labeled fibers have been observed which seem to terminate within the optic tract and optic nerve. It is suggested that the projection from the nucleus oculomotorius to the retina constitutes a link in the multisynaptic efferent pathway from the visual cortex to the eye, by which the visual cortex can influence the functioning of the retina.     

Parasympathetic postganglionic cells in the glossopharyngeal nerve trunk and their relationship to unmyelinated nerve fibers in the fungiform papillae of the frog

The glossopharyngeal nerve of the frog is made up of afferent nerve fibers and efferent, parasympathetic and sympathetic nerve fibers. The precise origin and course of the parasympathetic efferent nerve fibers in the fungiform papillae of the frog's tongue were investigated. We found the ganglionic cells in the lingual branch of the frog glossopharyngeal nerve. The surface of the ganglionic cell bodies was partly covered by synaptic endings that impinged upon it. Synaptic endings contained clear synaptic vesicles and large dense-cored vesicles. After cutting of the glossopharyngeal nerve proximal to the jugular ganglion, synaptic endings were found to show definite signs of degeneration. These findings led us to the conclusion that the ganglionic cells in the lingual branch of the glossopharyngeal nerve are the parasympathetic postganglionic cells. After cutting of the glossopharyngeal nerve distal to the jugular ganglion, some unmyelinated nerve fibers in the fungiform papillae and postganglionic cells in the lingual branch remained intact. These results strongly suggest that the origin of some of the unmyelinated nerve fibers is the parasympathetic postganglionic cell in the lingual branch.     

Submicroscopic anatomy of photoreceptors in the female scale insect Eupulvinaria hydrangeae (Homopteres, Coccideae)

The submicroscopic anatomy of the eye in female Eupulvinaria is described. This scale insect has no compound eyes but simple pericerebral eyes composed of one pair of ocelli. The ocelli, located dorsolaterally to the brain, are of rhabdomeric type. They are situated in the subepidermal region underneath the basement membrane of the epidermis. Each ocellus consists of a cup made of a layer of numerous pigmented cells enclosing a few photoreceptive cells. The cuticular lens is facing the opening of the pigment-cup, while the optic nerve is emerging from the bottom of the cup. Pigmented cells show characteristic pigment granules. They are linked to each other by interdigitations or by desmosomes junctions. Both dendrites and cell body of the photoreceptor cell lie inside the eyecup, while the axon lies outside. The dendritic processes consist of large shaft-shaped rhabdomeres with microvilli extending perpendicular to the direction of the shaft. These microvilli are enveloped by digitiform cytoplasmic extensions of pigmented cells. These latter strands are free of pigment granules but contain microtubules orientated to the centre of the cup.     

Concentration of astrocytic filaments at the retinal optic nerve junction is coincident with the absence of intra-retinal myelination: comparative and developmental evidence

The structure of the lamina cribrosa (LC) and astrocytic density were examined in various species with and without intra-retinal myelination. Sections of optic nerve from various species were stained with Milligan's trichrome or antibodies to glial fibrillary acidic protein, myelin basic protein (MBP) and antibody O4. Marmoset, flying fox, cat, and sheep, which lack intraretinal myelination, were shown to possess a well-developed LC as well as a marked concentration of astrocytic filaments distal to the LC. Rat and mouse, which lack intraretinal myelination, lacked a well-developed LC but exhibited a marked concentration of astrocytic filaments in this region. Rabbit and chicken, which exhibit intraretinal myelination, lacked both a well-developed LC and a concentration of astrocytes at the retinal optic nerve junction (ROJ). A marked concentration of astrocytes at the ROJ of human fetuses was also apparent at 13 weeks of gestation, prior to myelination of the optic nerve; in contrast, the LC was not fully developed even at birth. This concentration of astrocytes was located distal to O4 and MBP immunoreactivity in human optic nerve, and coincided with the site of initial myelination of ganglion cell axons in marmoset and rat. Myelination proceeded from the chiasm towards the retinal end of the human optic nerve. Moreover, the outer limit of oligodendrocyte precursor cells (OPC) migration into the rabbit retina was restricted by the outer limit of astrocyte spread. These observations indicate that a concentration of astrocytic filaments at the ROJ is coincident with the absence of intraretinal myelination. Differential expression of tenascin-C by astrocytes at the ROJ appears to contribute to the molecular barrier to OPC migration (see Bartsch et al., 1994), while expression of the homedomain protein Vax 1 by glial cells at the optic nerve head appears to inhibit migration of retinal pigment epithelial cells into the optic nerve (see Bertuzzi et al., 1999). These observations combined with our present comparative and developmental data lead us to suggest that the astrocytes at the ROJ serve to regulate cellular traffic into and out of the retina.     

Innervation of the C cells of chicken ultimobranchial glands studied by immunohistochemistry,fluorescence microscopy,and electron microscopy

Innervation of the ultimobranchial glands in the chicken was investigated by immunohistochemistry, fluorescence microscopy and electron microscopy. The nerve fibers distributed in ultimobranchial glands were clearly visualized by immunoperoxidase staining with antiserum to neurofilament triplet proteins (200K-, 150K- and 68K-dalton) extracted from chicken peripheral nerves. The ultimobranchial glands received numerous nerve fibers originating from both the recurrent laryngeal nerves and direct vagal branches. The left and right sides of the ultimobranchial region were asymmetrical. The left ultimobranchial gland had intimate contact with the vagus nerve trunk, especially with the distal vagal ganglion, but was somewhat separated from the recurrent nerve. The right gland touched the recurrent nerve, the medial edge being frequently penetrated by the nerve, but the gland was separated from the vagal trunk. The left gland was innervated mainly by the branches from the distal vagal ganglion, whereas the right gland received mostly the branches from the recurrent nerve. The carotid body was located cranially near to the ultimobranchial gland. Large nerve bundles in the ultimobranchial gland ran toward and entered into the carotid body. By fluorescence microscopy, nerve fibers in ultimobranchial glands were observed associated with blood vessels. Only a few fluorescent nerve fibers were present in close proximity to C cell groups; the C cells of ultimobranchial glands may receive very few adrenergic sympathetic fibers. By electron microscopy, numerous axons ensheathed with Schwann cell cytoplasm were in close contact with the surfaces of C cells. In addition, naked axons regarded as axon terminals or “en passant” synapses came into direct contact with C cells. The morphology of these axon terminals and synaptic endings suggest that ultimobranchial C cells of chickens are supplied mainly with cholinergic efferent type fibers. In the region where large nerve bundles and complex ramifications of nerve fibers were present, Schwann cell perikarya investing the axons were closely juxtaposed with C cells; long cytoplasmic processes of Schwann cells encompassed large portions of the cell surface. All of these features suggest that C-cell activity, i.e., secretion of hormones and catecholamines, may be regulated by nerve stimu1i.     

Light and electron microscopical study of the relationships between the cerebellum and the vestibular organ of the frog

Summary The relationships between the frog vestibular afferents and the cerebellum as well as the efferent vestibular system, have been studied by electron microscopy and Nauta degeneration technique. The primary vestibular fibers were found to have synaptic boutons in both the granular and the molecular layers of the cerebellar marginal zone. In the granular layer synaptic contacts are made with the granule cell dendrites while the molecular layer projection is directed to the main dendrites of the Purkinje cells in a manner similar to that of the climbing fibers.As for the efferent system, the vestibular receptor cells of the macula saccularis are contacted by vesicle-filled boutons which terminate synaptically in relation to a submembranous sac within the cell. The efferent fibers contain neurofilaments and a few neurotubules. Following lesions at different sites, it was found that all the above fibers and boutons degenerated after a) vestibular nerve section, and that b) most of them were lost when the brain stem was hemisected above the vestibular nerve. On the other hand, brain stem sectioning above the Vth nerve produced degeneration of 35% of these boutons while cerebellar undercutting produced 20% degeneration. The Nauta technique shows that following cerebellar undercutting a small efferent bundle leaves the ventral caudal side of the nerve. These findings demonstrate the existence of a cerebello-vestibular efferent system originating most possibly from the auricular lobe Purkinje cells.     

Ultrastructure of the retina of Drosophila melanogaster: the mutant ora (outer rhabdomeres absent) and its inhibition of degeneration in rdgB (retinal degeneration-B)

The Drosophila mutant ora lacks rhodopsin in the R1-6 set of photoreceptors and has a diminution of the photopigment containing rhabdomeres of R1-6. Newly emerged flies have rhabdomeres, albeit small, which extend from the distal rhabdomere cap to the proximal basement membrane. As the fly ages, these are reduced until only distal remnants remain. Carotenoid deprivation does not protect ora flies from rhabdomere loss. When first characterized, ora was designated as a non-formation mutant rather than a degeneration mutant. The truth lies between, since rhabdomeres diminish with age but cells do not die. The plasmalemma of R1-6 have unusual dense striated areas like the "zippers" described earlier for the Drosophila mutant norpA. Similar membranes are also present within the receptor somata, especially in young flies. The latter probably become internalized from the plasmalemma. They are likely not related to the diminished rhabdomeres as claimed earlier. R7 and R8 have normal rhabdomeres, and in particular, they have normal coated vesicles and multivesicular bodies (MVB's), early steps in the degradation phase of normal rhabdomere maintenance. No MVB's are seen in the R1-6 somata, indicating that the routes for rhabdomere degradation differ from those of normal receptors. However, some MVB-like structures are seen in the intraommatidial cavity. The compound mutant rdgB;ora has a phenotype just like that of ora. This means that ora protects rdgB from the light induced degeneration of R1-6 which characterizes rdgB.     

Microglia in tadpoles ofXenopus laevis: Normal distribution and the response to optic nerve injury

Summary We have studied the distribution of microglia in normalXenopus tadpoles and after an optic nerve lesion, using a monoclonal antibody (5F4) raised againstXenopus retinas of which the optic nerves had been cut 10 days previously. The antibody 5F4 selectively recognizes macrophages and microglia inXenopus. In normal animals microglia are sparsely but widely distributed throughout the retina, optic nerve, diencephalon and mesencephalon (other regions were not examined). After crush or cut of an optic nerve, or eye removal, there occurs an extensive microglial response along the affected optic pathway. Within 18 h an increase in the number of microglial cells in the optic tract and tectum can be detected. This response increases to peak at around 5 days after the lesion. At this time the nerve distal to the lesion contains many microglial cells; the entire optic tract is outlined by microglia, extended along the degenerating fibres; and the affected tectum shows a heavy concentration of microglia. This microglial response thereafter decreases and has mostly gone by 34 days. We conclude that the microglial response to optic nerve injury inXenopus tadpoles starts early, peaks just before the regenerating optic nerve axons enter the brain, and is much diminished by the time the retinotectal projection is re-established. The timing is such that the microglial response could play a major role in facilitating regeneration.     

Immunocytochemical localization of serotonergic fibers innervating the ocular circadian system of Aplysia

The isolated eye of the mollusc, Aplysia californica, contains a circadian pacemaker whose phase can be regulated by serotonin. The results of previous biochemical and physiological studies indicate that serotonin is used as a transmitter of circadian information in the eye. Although the effects of serotonin on various physiological processes in the Aplysia eye have been studied, very little is known about the anatomy of the serotonergic innervation. We have examined the innervation of the eye using immunocytochemical methods. Serotonin-immunoreactive processes were observed in the optic nerve, in the accessory optic nerves, in the connective tissue capsule surrounding the eye, and within the eye itself. There appeared to be two sources of serotonergic input to the eye of Aplysia. One set of immunoreactive fibers was contained in the optic nerve and entered the eye in the neuropil region before radiating outward towards the peripheral retina in the layer below the photoreceptor cell bodies. A second serotonin-immunoreactive input to the eye entered from the accessory optic nerves and these fibers formed a dense plexus of fibers in the connective tissue capsule surrounding the eye. Serotonin-immunoreactive fibers from the plexus penetrated the eye and appeared to terminate in the peripheral portion of the retina. No serotonin-immunoreactive cell bodies were observed in the eye, nerves, or connective tissue capsule. These results support the hypothesis that serotonergic fibers innervate the retina of Aplysia and that these fibers travel through two distinct anatomical pathways: the optic nerve and the accessory optic nerves.     

Fused rhabdomeres (fur) in Drosophila: an eye mutation that alters rhabdomere morphology and retinal function.

In the Drosophila compound eye, the photoreceptor cells are organized in highly precise units, the ommatidia. In each photoreceptor cell, the primary photopigment, opsin, is contained in the rhabdomere, an ordered array of densely packed microvilli. A genetic and phenotypic analysis of a new X-linked. P element-induced mutation, fur, (fused rhabdomeres) is presented. Light and electron microscope studies show that mutations at the fur locus result in the fusion of the adjacent rhabdomeres in the developing eye and the fusion takes place during the pupal stage of eye development. Electrophysiological experiments indicate that the fur mutant photoreceptors have reduced sensitivity to light and lack a PDA (prolonged depolarizing afterpotential), a response characteristic of normal photoreceptor cells. Recombination and deficiency mapping localize fur to the proximal region of the X chromosome. Reversion analysis indicates the fur mutant is the result of a P element insertion. These studies suggest that the fur locus encodes a gene that has specific roles in rhabdomere morphogenesis and retinal function.