Ultrastructural study of sensory cells of the proboscidial glandular epithelium of Riseriellus occultus (Nemertea,Heteronemertea)

Only one sensory cell type has been observed within the glandular epithelium of the proboscis in the heteronemertine Riseriellus occultus. These bipolar cells are abundant and scattered singly throughout the proboscis length. The apical surface of each dendrite bears a single cilium enclosed by a ring of six to eight prominent microvilli. The cilium has the typical 9×2 + 2 axoneme arrangement and is equipped with a cross-striated vertical rootlet extending from the basal body. No accessory centriole or horizontal rootlet was observed. Large, modified microvilli (stereovilli) surrounding the cilium are joined together by a system of fine filaments derived from the glycocalyx. Each microvillus contains a bundle of actin-like filaments which anchor on the indented inner surface of a dense, apical ring situated beneath the level of the ciliary basal body. The tip of the cilium is expanded and modified to form a bulb-like structure which lies above the level where the surrounding microvilli terminate. In the region where the cilium emerges from the microvillar cone, the membrane of the microvillar apices makes contact with a corresponding portion of the ciliary membrane. At this level microvilli and cilium are apparently firmly linked by junctional systems resembling adherens junctions. The results suggest that these sensory cells may be mechanoreceptors. © 1996 Wiley-Liss, Inc.     

Experimental uveitis induced by products of activated lymphocytes: Intraocular effects of rhodopsin-induced lymphokines

Immunization of strain 13 guinea pigs by footpad injection of bovine rhodopsin in complete Freund's adjuvant produces experimental autoimmune uveitis (retinopathy) with retinal pathology characterized by destruction of the retinal photoreceptor cell layer, as we reported previously. Since rhodopsin-induced EAU can be passively transferred by immune cells but not antirhodopsin antibody, the present studies were conducted to determine if soluble products of activated lymphocytes (PAL) could cause the retinal pathology seen in experimental uveitis. These studies, reported here, show that intravitreal injection of supernatant factors generated by guinea pig lymph node cells specifically activated in vitro with rhodopsin or purified protein derivatives of tuberculin or nonspecifically activated with the mitogen concanavalin A produce uveitis in the normal guinea pig eye. The PAL produced mononuclear cell infiltration in the vitreous and a retinopathy with loss of retinal photoreceptor cells. Inflammatory cell infiltrates were found in the retina but not found in the anterior segment of the eye. Control lymphocyte supernatants did not produce retinal pathology. In guinea pigs sensitized to rhodopsin-complete Freund's adjuvant, intravitreal injection of the PAL produced a mononuclear vitreal infiltrate, disruption of the photoreceptor cell layer, necrosis of the inner retina, and a granulomatous infiltrate in the choroid with damage to the retinal pigment epithelium. Injection of control supernatants into the rhodopsin-complete Freund's adjuvant sensitized guinea pig eye did not produce inflammation and the retinal photoreceptor cell layer remained intact. Results from our studies indicate a role for the PAL along with other, yet undefined, factors in the initiation of autoimmune uveitis and in the autoimmune pathology of the retina.     

Light and electron microscopic studies on the pineal tract of rainbow trout, Salmo gairdneri.

The pineal tract of rainbow trout from the pineal end vesicle to the posterior commissure was studied by light and electron microscopy. Five types of nerve fibres (photoreceptor basal process, ganglion cell dendrite, electron-lucent fibre and synaptic vesicles, myelinated and unmyelinated axons) and two modes of synapses (photoreceptor basal process ganglion cell dendrite and axon terminal with synaptic vesicles-photoreceptor basal process synapses) are distinguishable in the proximal region of end vesicle. The two distinct synaptic associations with the photoreceptor basal process suggest two different (excitatory and inhibitory) control of pineal sensory activity. At the distal portion of stalk about two thousand nerve fibres converge into dorsal and ventral bundles. Posterior to the habenular commissure several small branches run out laterally from the ventral bundles to the basal margin of the ependyma, but not into the habenular commissure. The dorsal bundle passes through the dorsal side of the subcommissural organ and runs ventral to the posterior commissure. The pineal tract is composed of unmyelinated axons, electron-lucent nerve fibres and myelinated axons. The number of fibres increases throughout the stalk and reaches the maximum number at the opening of pineal lumen to IIIrd ventricle, however, the number of fibres then decreases through the subcommissural organ and posterior commissure. This increase and decrease of nerve fibres suggest the continuous participation of axonal fibres of pineal nerve cells and the ramification or branching of pineal tract, respectively.     

A morphologic and histochemical study of the mesentery in the guinea pig

The guinea pig mesentery is a uniform, continuous, thin (18 micron) sheet of connective tissue covered by a single layer of flattened mesothelial cells on both surfaces. Tight and gap junctions provide for cell-to-cell adhesion among mesothelial cells. These cells possess numerous micropinocytotic vesicles; a conspicuous basal lamina separates the mesothelium from the underlying connective tissue. Most of the material found between the two serous coverings consisted of a three-dimensional meshwork of abundant collagenous fibers intermingled with a sparse net of very thin (0.4 micron) elastic fibers. Two distinct populations of collagen fibrils are segregated into different compartments of the mesentery. One population is formed of thick (56 nm) fibrils which associate to form closely packed fibers. The second population, composed of loosely arranged thin (38 nm) fibrils which do not become assembled into fibers, is found underlying the basal lamina that separates the mesothelium from the connective tissue. These observations strongly suggest that the mesentery contains both collagens type I and type III. The guinea pig mesentery contains 6.8 mg of sulfated glycosaminoglycans/g dry weight. Most of these glycosaminoglycans (78%) were identified as dermatan sulfate, whilst the rest (22%) corresponded to heparan sulfate.     

Morphological study of sarcoplasmic reticulum in the atrioventricular node and bundle cells in guinea pig hearts

The osmium-ferrocyanide method for staining of the sarcoplasmic reticulum (SR) was used for a morphological investigation of the various components of the SR in the atrioventricular node and bundle (AVNB) cells of guinea pig hearts. On the basis of light microscopic observations, the AVNB tissue in guinea pig hearts can be divided into five regions: atrionodal junction, midnode, proximal bundle, distal bundle, and bundle branches. Electron microscopic observations revealed two types of junctional SR (j-SR) saccules in the cells from all the regions of AVNB tissue. One is similar to that seen in the working cardiac cells, i.e., flattened saccules with junctional granules. The second type is dilated and contains electron-dense granular material throughout its lumen. The flattened type is seen more often than the dilated type in atrionodal junctional cells and midnode cells, whereas the dilated type occurs more often in distal bundle cells and bundle branch cells. In most cells from the atrionodal junction and midnode regions, the j-SR saccules are apposed more often to sarcolemmal areas associated with nonspecialized regions of intercellular junctions than to other sarcolemmal areas. This distribution was not found in the distal bundle and bundle branch cells. Free SR tubules around the myofilament bundles are poorly developed in the midnode cells, generally in accord with the extent of development of myofibrils. Z-tubules are found in cells from all regions but are poorly developed in midnode cells. Corbular SR vesicles are found in cells from all the regions of AVNB tissues but are rare in midnode cells. Thus, each of the regions in the AVNB tissue has a different, characteristic distribution of SR components. Because of their possible relationship to the regulation of the intracellular concentrations of calcium, these differences in SR morphology may contribute to the diverse physiological properties of the different regions of the AV node and bundle.     

The Ventral Photoreceptor Cells of Limulus : I. The microanatomy

The ventral photoreceptor cells of Limulus polyphemus resemble the retinular cells of the lateral eyes both in electrical behavior and in morphology. Because of the great size of the ventral photoreceptor cells they are easy to impale with glass capillary micropipettes. Their location along the length of the ventral eye nerve makes them easy to dissect out and fix for electron microscopy. Each cell has a large, ellipsoidal soma that tapers into an axon whose length depends upon the distance of the cell from the brain. The cell body contains a rich variety of cytoplasmic organelles with an especially abundant endoplasmic reticulum. The most prominent structural feature is the microvillous rhabdomere, a highly modified infolding of the plasmalemma. The microvilli are tightly packed together within the rhabdomere, and quintuple-layered junctions are encountered wherever microvillar membranes touch each other. Glial cells cover the surface of the photoreceptor cell and send long, sheet-like projections of their cytoplasm into the cell body of the photoreceptor cell. Some of these projections penetrate the rhabdomere deep within the cell and form quintuple-layered junctions with the microvilli. Junctions between glial cells and the photoreceptor cell and between adjacent glial cells are rarely encountered elsewhere, indicating that there is an open pathway between the intermicrovillous space and the extracellular medium. The axon has a normal morphology but it is electrically inexcitable.     

An ethanolic phosphotungstic acid (EPTA) analysis of photoreceptor and synaptic ultrastructure in the guinea pig retina

Ethanolic phosphotungstic acid (EPTA) has been used to elucidate the structure of certain organelles contained within retinal cells not clearly discernible using conventional preparations. Both synaptic and nonsynaptic components of the guinea pig neural retina have been analyzed. Within the photoreceptor (PR) cell EPTA-stained components include the connecting cilia, their basal bodies, and the root filament system. Cross-striated fibrillar organelles, similar in appearance to the root filaments, are also observed in the nuclear region, the synaptic terminal and other parts of the PR cell. The possible structural continuity and significance of these structures are discussed. Within retinal synapses of both the inner and outer plexiform layers, ribbons and associated paramembranous specializations are stained. The photoreceptor ribbons have a trialaminar structure with filamentous, tufted borders. Synaptic cleft material and postsynaptic densities are also stained. Bipolar cell synapses in the inner plexiform layer contain stained short ribbons as well as closely associated peg-like densities extending towards the presynaptic membrane.     

Asymmetric properties of the Chlamydomonas reinhardtii cytoskeleton direct rhodopsin photoreceptor localization

The eyespot of the unicellular green alga Chlamydomonas reinhardtii is a photoreceptive organelle required for phototaxis. Relative to the anterior flagella, the eyespot is asymmetrically positioned adjacent to the daughter four-membered rootlet (D4), a unique bundle of acetylated microtubules extending from the daughter basal body toward the posterior of the cell. Here, we detail the relationship between the rhodopsin eyespot photoreceptor Channelrhodopsin 1 (ChR1) and acetylated microtubules. In wild-type cells, ChR1 was observed in an equatorial patch adjacent to D4 near the end of the acetylated microtubules and along the D4 rootlet. In cells with cytoskeletal protein mutations, supernumerary ChR1 patches remained adjacent to acetylated microtubules. In mlt1 (multieyed) mutant cells, supernumerary photoreceptor patches were not restricted to the D4 rootlet, and more anterior eyespots correlated with shorter acetylated microtubule rootlets. The data suggest a model in which photoreceptor localization is dependent on microtubule-based trafficking selective for the D4 rootlet, which is perturbed in mlt1 mutant cells.     

Cytoskeleton Structure and Composition in Choanoflagellates

The structure and composition of the cytoskeleton has been studied in Monosiga ovata (Protozoa: Order Choanofiagellida Kent 1880) using a combination of methods in association with light and electron microscopy. Supplementary observations are included for Desmarella moniliformis. The basal body of the single anterior flagellum is subtended proximally and at right angles by a second, non-flagellar basal body. The edges of the two basal bodies are connected by a fibrillar bridge. A long, narrow, striated, fibrillar rootlet extends posteriorly from the lower edge of the non-flagellar basal body towards the Golgi apparatus. It is associated throughout most of its length with the surface of a flattened sac. Rootlet microtubules pass radially from a ring of electron dense material which encircles the distal end of the flagellar basal body. These microtubules extend outwards for about one-third of the length of the cell. Within each collar tentacle is a longitudinal bundle of microfilaments composed of actin as illustrated by rhodamine-phalloidin staining for fluorescence microscopy. The base of each microfilament bundle is associated with one or more rootlet microtubules by fine fibrillar bridges. The attachment between microtubules and tentacle microfilaments is further demonstrated by their coordinated displacement when the cytoskeleton becomes dislodged. The role of the cytoskeleton in maintaining the position of the collar tentacles during interphase and cell division is discussed.     

Ultrastructure of the posterior sense receptor of larval Austramphilina elongata (Amphilinidea)

Rohde K. and Garlick P. R. 1985. Ultrastructure of the posterior sense receptor of larval Austramphilina elongata (Amphilinidea). International Journal for Parasitology15: 399–402. Eight large papillae arranged in a circle at the posterior end of the body each contain one non-ciliate receptor. The receptor is the terminal swelling of a thin dendrite; it has many large mitochondria, a basal body from which cross-striated ciliary rootlets diverge, and a bundle of long non-striated filaments. The electrondense collar is formed by several thin rings, and some desmosomes are found between the receptor and the adjacent epidermis in addition to the apical septate desmosomes.     

Two type XII-like collagens localize to the surface of banded collagen fibrils

Two recently identified collagen molecules, termed twelve-like A and twelve-like B (TL-A and TL-B) have properties similar to type XII collagen. These molecules have been localized in human and calf tissues by immunoelectron microscopy. The observations strongly suggest that both molecules are located along the surface of banded collagen fibers. The epitopes recognized by the antibodies are contained in large, nontriple-helical domains at one end of the collagen helix. The epitopes are visualized at a distance from the surface of the banded fibers roughly equal to the length of the nonhelical domains, suggesting that the nonhelical domains extend from the fibril, while the triple-helical domains are likely to bind directly to the fibril surface. Occasionally, both TL-A and TL-B demonstrate periodic distribution along the fibril surface. The period corresponds to the primary interband distance of the banded fibrils. Not all fibrils in a fiber bundle are labeled, nor is the labeling continuous along the length of labeled fibrils. Simultaneous labeling of TL-A and type VI collagen only rarely shows colocalization, suggesting that TL-A and TL-B do not mediate interactions between the type VI collagen beaded filaments and banded collagen fibrils. Also, interfibrillar distances are approximately equivalent in the presence and absence of these type XII-like molecules. While the results do not directly indicate a specific function for these molecules, the localization at the fibril surface suggests that they mediate interactions between the fibrils and other matrix macromolecules or with cells.     

Larval development in <Emphasis Type="Italic">Guancha arnesenae</Emphasis> (Porifera,Calcispongiae, Calcinea)

Larval development and follicle structure of a representative of the Calcinea (Calcispongiae) Guancha arnesenae from the White Sea have been studied for the first time at the ultrastructural level. The follicle in G. arnesenae has an unusual structure: it consists of trapezoid cells rich in phagosomes and a surrounding dense collagen layer. Follicular cells differentiate from choanocytes. Cleavage results in formation of a hollow, equal, non-polarized coeloblastula. Larval morphogenesis occurs by means of direct hollow blastula formation without any individual cell or cell layer movements. The coeloblastula (calciblastula) larva of G. arnesenae is completely ciliated. The larva also contains rare non-ciliated cells: vacuolar cells, bottle-shaped cells and free cells in a central cavity. The basal ciliary apparatus of larval cells includes the basal body, an accessory centriole oriented perpendicularly to it, the basal foot, and two cross-striated rootlets. A bundle of microtubules emerges from the side of the basal body, opposite to the basal foot, running parallel to the outer surface. All bundles of cells are parallel to each other and oriented towards the posterior larval pole, forming a transverse cytoskeletal system. Specialized intercellular junctions in the apical regions of all ciliated cells are revealed for the first time in a Calcispongiae larva. The central larval cavity contains symbiotic bacteria, which are included inside the embryo at the blastula stage.     

ELECTRON MICROSCOPE OBSERVATIONS ON THE FLAGELLAR APPARATUS OF BRYOPSIS MAXIMA (CHLOROPHYCEAE)1

The flagellar apparatus in male gametes of the siphonaceous green alga, Bryopsis maxima Okamura, was studied and compared with that of other green biflagellate cells. The proximal portions of two basal bodies are connected by a single striated proximal band, unique among the biflagellate reproductive cells of green algae studied. Anterior to the flagellar bases is a pair of distal bands different from the single structure in other biflagellate cells. These bands which arise from the distal portion of each basal body, extend upward in the papilla and curve down toward the lower edges of the basal bodies. They seem to have no direct association with each other. Two pairs of distinct flagellar roots, one consisting of 3–5 microtubules and the other of a partially striated fiber of undetermined numbers of microtubules, diverge from the basal body region and extend towards the cell posterior. Their component microtubules are disorganized into single or smaller groups midway over the cell length. The uniqueness of the flagellar apparatus is briefly discussed.     

Special smooth muscle cells along the submucosal surface of the guinea pig colon with reference to its spontaneous contractions

Antiperistalses occur from the flexure region of the guinea pig colon. We previously demonstrated that the circular muscle at the mesenteric border of the flexure region produced spontaneous regular contractions and found special smooth muscle cells believed to be pacemakers along the submucosal surface of the circular muscle layer. In this study, we revealed bipolar- and multipolar-type special smooth muscle cells along the submucosal surface of the muscle layer. Their slender cell processes contacted each other and formed a cellular network. Caveolae, filament structures expressing smooth muscle actin, vimentin, some desmin, and basal lamina were prominent features. The special smooth muscle cells corresponded to c-Kit-immunopositive cells and so-called interstitial cells or interstitial cells of Cajal in other reports. Their population was larger in the flexure region and the proximal colon than in the distal colon. The circular muscle layer at the flexure region was thicker than in other regions. The contraction in the flexure region showed the highest frequency and regularity. The dense population of special smooth muscle cells at the flexure region and thicker muscle layer may make the mechanical contraction more regular. The antiperistalsis from the flexure region could be explained in relation to the highest frequency of the pulsating contraction.     

Decrease in brain POMC mRNA expression and onset of obesity in guinea pigs exposed to 2-chloroethyl ethyl sulfide, a mustard analogue

The full spectrum of physiological effects resulting from exposure to sulfur mustard and its analogs is currently unknown. In a guinea pig model, initially selected to study the role of an inflammatory cytokine cascade in mustard gas induced lung injury, we observed significant body weight gain in guinea pigs exposed to an intratracheally injected single dose of 2-chloroethyl ethyl sulfide, a mustard analog. The body weight gain was not associated with any apparent change in appetite. To further elucidate a molecular basis for the observed weight gain, we evaluated candidate genes for the obese phenotype by quantitative RT-PCR. We observed a time- and dose-dependent decrease in guinea pig pro-opiomelanocortin (POMC) message following treatment with mustard gas. This reduction in POMC message is consistent with the onset of obesity in the animals. We hypothesize that the POMC melanocortin pathway provides a mechanistic basis for the observed effects of sulfur mustard on body weight.     

The structure of the smooth muscle fibres in the body wall of the earth worm

1. The structure of the smooth muscle fibres in the longitudinal muscle coat of the body wall of Lumbricus terrestris has been investigated by phase contrast light microscopy and electron microscopy. 2. The muscle fibre is ribbon-shaped, and attached to each of its two surfaces is a set of myofibrils. These are also ribbon-shaped, and they lie with their surfaces perpendicular to the surfaces of the fibre, and their inner edges nearly meeting in the middle of the fibre. These fibrils are oriented at an angle to the fibre axis, and diminish greatly in width as they approach the edge of the fibre. The orientation of the set of fibrils belonging to one surface of the fibre is the mirror image of that of the set belonging to the other surface; thus, when both sets are in view in a fibre lying flat on one face, the fibre exhibits double oblique striation. A comparison of extended and contracted fibres indicates that as the fibre contracts, the angle made between fibre and fibril axes increases (e.g. from 5 to 30 degrees ) and so does the angle made between the two sets of fibrils (e.g. from 10 to 60 degrees ). 3. The myofibril, throughout its length, contains irregularly packed filaments, commonly 250 A in diameter, which are parallel to its long axis and remain straight in contracted muscles. Between them is material which probably consists of much finer filaments. Thus A and I bands are absent. 4. Bound to one face of each fibril, but not penetrating inside it, is a regularly spaced series of transverse stripes. They are of two kinds, alternating along the length of the fibril, and it is suggested that they are comparable to the Z and M lines of a cross-striated fibril. The spacing of these stripes is about 0.5 micro ("Z" to "Z") in extended muscles, and 0.25 micro in contracted muscles. A bridge extends from each stripe across to the stripeless surface of the next fibril.     

Biosynthesis of delta-7-cholesten-3-beta-ol, delta-5,7-cholestadien-3-beta-ol, and delta-5-cholesten-3-beta-ol by guinea pig intestinal mucosa in vitro

Methods were developed for the separation and determination of the various 27-carbon sterols of intestinal mucosa by means of thin-layer chromatography. Scrapings of the mucosa of the small intestine of guinea pig and rat were shown to incorporate isotope from (14)C-labeled acetate and mevalonate into sterols in vitro. For each substrate this activity was lowest in mucosa from the proximal third of the small intestine and greatest in mucosa from the more distal regions of the small intestine. The total 27-carbon sterol content of guinea pig mucosa varied only slightly along the length of the small intestine, but the concentration of cholesterol was highest distally. More than 95% of the radioactivity incorporated from acetate-2-(14)C into 27-carbon sterols by guinea pig mucosa in 4 hr was recovered as lathosterol and 7-dehydrocholesterol; less than 5% was in cholesterol. The specific activities of the 27-carbon sterols were consistent with the concept that synthesis proceeds from lathosterol to 7-dehydrocholesterol to cholesterol.     

Change of intracellular fluidity during keratinocyte differentiation measured by fluorescence polarization

Summary The fluorescence polarization method was applied to measure the intracellular fluidity of fractionated guinea pig keratinocytes. Guinea pig epidermal cell suspension was obtained by treatment with EDTA and trypsin, and was separated into high, intermediate, and low density fractions using Percoll density gradient centrifugation. Morphological observation and cytofluorometric analysis of DNA content in the fractionated epidermal cells showed that the high, intermediate, and low density fractions were basal, spinous, and granular cell-rich fractions, respectively. Intracellular fluorescence polarization of each fraction was determined by a polarization spectrofluorometer (Hitachi MPF-4, prototype) with fluorescein diacetate. The P-values were calculated for high, intermediate, and low density fractions as 0.192 ± 0.021, 0.172 ± 0.019, and 0.147 ± 0.012, respectively. Since low P-values indicate a high degree of fluidity, the results indicate that intracellular fluidity of keratinocytes is lower in basal cells and higher in granular cells. Dye-binding experiments showed that fluorescein-binding proteins were not detected in the soluble fraction of the epidermal cells. The present findings suggest that intracellular fluidity of the guinea pig keratinocyte increases during the process of its differentiation.     

des-Met carboxyl-terminally modified analogues of bombesin function as potent bombesin receptor antagonists, partial agonists, or agonists.

In the present study we examined the effect of carboxyl-terminal modifications of des-Met14-bombesin (Bn) on Bn receptor affinity in murine 3T3 cells, rat and guinea pig pancreatic acini, and the ability to initiate biologic responses by synthesizing 18 des-Met14-Bn(6-13) analogues. With guinea pig acini and 3T3 cells, affinity was affected by the chain length of the alkyl moiety (R) added to [D-Phe6]Bn(6-13)NH2R with relative potencies: propyl greater than ethyl greater than butyl = hexyl greater than heptyl greater than free amide, whereas in rat acini affinity was not increased by the chain length. In each cell system the affinity of the alkylamide was not increased by insertion of a phenyl group in the alkyl side chain, by making the analogue more neuromedin B-like or by addition of a reduced peptide bond. The affinity in each cell system was increased by additions of other electron releasing groups to the COOH-terminal carboxyl group such as [D-Phe6]Bn(6-13)ethyl or methyl ester, or hydrazide. In guinea pig pancreas and 3T3 cells, 12 analogues were antagonists, 1 a full and 5 partial agonists. In rat pancreas, 8 were antagonists, 5 full agonists, and 5 partial agonists. Potent antagonists in each cell system were the methyl and ethyl ester, hydrazide, and ethylamide analogues. In 3T3 cells or guinea pig pancreas, agonist activity of the alkylamide was critically dependent on the chain length, whereas with rat pancreatic Bn receptors any alkylamide longer than the ethylamide had agonist activity. In all three cell systems any alteration that made the alkylamide more neuromedin B-like caused agonist activity. These results demonstrate that the nature of the substitution on the carboxyl terminus of des-Met14-Bn analogues is critically important, not only for determining Bn receptor affinity, but also for determining the ability to initiate a biologic response. In contrast to previous studies, the present results demonstrate that the presence of the COOH-terminal amino acid in position 14 of Bn is not essential for initiating a biologic response. Several des-Met14-Bn analogues were potent partial agonists, whereas others such as the hydrazide or ethyl ester are very potent antagonists.     

Migration and function of glia in the developing Drosophila eye.

Although glial cells have been implicated widely in the formation of axon tracts in both insects and vertebrates, their specific function appears to be context-dependent, ranging from providing essential guidance cues to playing a merely facilitory role. Here we examine the role of the retinal basal glia (RBG) in photoreceptor axon guidance in Drosophila. The RBG originate in the optic stalk and have been thought to migrate into the eye disc along photoreceptor axons, thus precluding any role in axon guidance. Here we show the following. (1) The RBG can, in fact, migrate into the eye disc even in the absence of photoreceptor axons in the optic stalk; they also migrate to ectopic patches of differentiating photoreceptors without axons providing a continuous physical substratum. This suggests that glial cells are attracted into the eye disc not through haptotaxis along established axons, but through another mechanism, possibly chemotaxis. (2) If no glial cells are present in the eye disc, photoreceptor axons are able to grow and direct their growth posteriorly as in wild type, but are unable to enter the optic stalk. This indicates that the RBG have a crucial role in axon guidance, but not in axonal outgrowth per se. (3) A few glia close to the entry of the optic stalk suffice to guide the axons into the stalk, suggesting that glia instruct axons by local interaction.