Quantitative fluorescence studies of the effects of catecholamines and hydrocortisone on endogenous amine levels in neurones and small intensely fluorescent cells of embryonic chick sympathetic ganglia in vivo and in vitro

Summary Chick embryo lumbar sympathetic ganglia (11 day) cultured for three days and uncultured (in vivo) ganglia of comparable age were freeze-dried and processed by the formaldehyde-induced fluorescence technique for the demonstration of biogenic monoamines. The catecholamine levels within principal neurone cell bodies and small intensely fluorescent (SIF) cells were then examined in plastic sections of the in vivo and in vitro ganglia by a quantitative fluorescence method under various experimental conditions. Culture of ganglia for three days in the presence of hydrocortisone acetate (10g/ml) resulted in an increased SIF cell fluorescence (P<0.001 compared to control) and a green to yellow colour shift in the fluorophore of SIF cells. No detectable alteration in the fluorescence level of neurones was observed. When neurones after three days in culture were incubated for 1 h in exogenous catecholamines, a significant increase in fluorescence levels (interpreted as an increase in catecholamine content) occurred with noradrenaline (2×10^–6 M; 2×10^–5 M). SIF cells in ganglia removed directly from 14-day old chicks similarly took up noradrenaline and dopamine, and also adrenaline (2×10^–5 M). Morphological results are presented which indicate that the cellular appearances and architecture of cultured ganglion explants are very similar to those in comparable ganglia in vivo.This work was supported by a grant from the Medical Research Council. We thank Mrs. G. O'Shea, Mr. T.T. Lee and Mr. P.F. Hire for their valuable technical assistance     

Mitochondrial degeneration after organic phosphate poisoning in prosimian primates

Summary The degenerative reaction of mitochondria to tricresylphosphate (TCP) poisoning in spinal ganglion cells of Slow Loris (Nycticebus coucang coucang) were studied with the electron microscope. In neurones of animals treated with TCP, mitochondria display various stages of alterations which confirm mitochondrial involvement in TCP poisoning. The role of degenerated mitochondria in the formation of neuronal lipofuscin is discussed. It is suggested that the lipofuscin granule is a metabolic product inherently related to mitochondrial degeneration, irrespective of the primary cause: ageing or intoxication.Fellow of the Deutscher Akademischer Austauschdienst on study leave from the Medical Faculty, University of SingaporeA grant from the Deutsche Forschungsgemeinschaft (Gl. 28/20) is gratefully acknowledgedThe skilfull technical assistance of Mr. Tajuddin b.M. Ali, Mr. P. Gopal, Mr. R. Dungan and Mrs. C. Weinrichter is gratefully acknowledged     

The fine structure of cephalopod blood vessels II. The vessels of the nervous system

Summary Blood vessels of the perioesophageal nerve ganglia (brain) of Octopus vulgaris and the stellate ganglia of Sepia officinalis are described. The vessels have an incomplete endothelium, a complete basement membrane and a complete investment of pericytes. The pericytes are joined by specialised membrane junctions but these are not tight junctions. The main type of neuron/vessel arrangement is one where there is a collagen-filled space between the pericytes and the surrounding glial cells. Axons or neurons are sometimes applied directly to the vessel pericytes and in the neuropil, pericytes contact glial cells that ensheath bundles of axons. Blood spaces between neurons are also present.We would like thank Professor J. Z. Young and Dr. E. G. Gray for encouragement and advice, Mrs. Jane Astafiev for drawing Fig. 11, Mr. S. Waterman for photographic assistance and Miss Cheryl Martin for secretarial and other assistance.     

Localization of melanin synthesis within the pigment cell: Determination by a combination of electron microscopic autoradiography and topographic planimetry

Summary Localization of melanin synthesis within the pigment cells of the Cloudman S-91 mouse melanoma was determined by means of a combination of high resolution autoradiography and topographic planimetry. Initial melanin biosynthesis occurred predominantly in the endoplasmic reticulum and associated ribonucleoprotein particles of the melanocytes. By measuring a number of cell organelles and employing the index of relative specific localization it could be shown that the nucleus and mitochondria are of little or no importance in the process of melanogenesis.This investigation has been supported by the following research grants: CA 06548 CB, NIH, PHS and an Institutional Grant from the American Cancer Society (to H. M. H.); CA-05887, NIH, PHS (to A. S. Z.); M-00388 and NB-00782, NIH, PHS (to J. F. H.). One of us (H. M. H.) holds a Research Cancer Development Award (5-K 3-GM-2634) of the National Institute of General Medical Sciences, Public Health Service.We are grateful to Mr. Ronald Abler for his help with the topographic measurements; to Dr. Erhard Haus for help and advice; to Mr. J. Thornby and Mr. A. P. Basu for assistance with the statistical aspects of this study; and to Mrs. Lenore Mottaz, Miss Bernice Uittenbogaard, and Mrs. Judith Strong for careful technical assistance.     

The villous stroma of the human placenta

Summary In human placental villi the connective tissue is constructed by mesenchymal cells, small and large reticulum cells and fibroblasts. During early pregnancy mesenchymal cells dominate; starting with the third month of gestation the reticulum cells are in the majority within the terminal villi, the fibroblasts within the stem villi. Ultrastructurally intermediary types of cells can be differentiated. Together with reticular and collagenous fibres the reticulum cells form the basic architecture of the villous stroma during the first 2/3 of gestation: the reticular type of stroma. This consists of a network of cells and fibres with fetal vessels fitted in between. The remaining interspaces form a fluid system of compartments in which Hofbauer cells are suspended. They are called stromal channels. During the last trimester these channels and the Hofbauer cells as well are progressively replaced either by voluminous masses of fibres (fibrous type of stroma, mainly in the stem villi) or by sinusoidal enlargements of fetal capillaries (sinusoidal type of stroma, mainly in the terminal villi).Supported by grants from the Deutsche ForschungsgemeinschaftThe authors are indebted to Mrs. E. Böhm and Mrs. E. Schäfer for skilful technical assistance     

Herring bodies; an electron microscopic study of local degeneration and regeneration of neurosecretory axons

Summary The Herring bodies in the posterior lobe of the bovine hypophysis are very large (2–600 ) and can be classified into three types. The type I Herring body contains an accumulation of neurosecretory granules. These Herring bodies are very scarce and should not be confused with the numerous, but small, axonal swellings which also contain neurosecretory granules.The type II Herring body is characterized by the presence of a varying number of normal, moderately electron dense and empty vesicles, autophagic vacuoles, multilamellate bodies and occasional mitochondria. These Herring bodies are frequently observed.The type III Herring body is typified by the presence of dense vesicles connected to tubular formations which contain material of variable electron density, of filaments, and of long slender and very numerous mitochondria.The presence of multilamellate bodies and autophagic vacuoles suggests that the type II Herring body is in a degenerating phase. This concept is further substantiated by the similarity between this type of Herring body and transected neurosecretory axons in which degeneration is occurring.A similar comparison suggests that the type III Herring body is undergoing a regenerative process. Our current concept of the structure and function of Herring bodies is revised in the discussion.This work was supported by grants 5 RO1 NB 06641 NEUA and 5 R0107492 NEUA from the National Institutes of Health and the Space Sciences Research Center of the University of Missouri. The technical assistance of Mrs. G. Clark and Mr. R. Faup, and the clerical assistance of Mrs. S. Schmidt are gratefully acknowledged.Fellow of the Conséjo National de Investigaciones Científicas y Tecnicas de la República Argentína.     

L'ültrastructure du Foie Humain Lors D'ictères Idiopathiques Chroniques

Summary 15 liver biopsies from 13 patients suffering from non-hemolytic constitutional jaundice (6 cases of Gilbert's, 2 cases of Rotor's and 5 cases of Dubin-Johnson's disease) provided the basis of an ultrastructural study of pigment inclusions in liver cells following various techniques of preparation.The lipofuscin in Gilbert's syndrom and the pigment characteristic of Dubin-Johnson's syndrom contain two distinct components which are present in both types of inclusions but their proportions are different in these syndromes: a globular component, probably of lipidic nature, is the main constituent of the lipofuscin, a granulo-filamentous component the main constituent of the Dubin-Johnson's pigment. These two components react in both syndromes identically to various techniques of fixation, embedding and section-staining. In Rotor's syndrome other pigment inclusions are observed which are not detectable in ordinary histological preparations: they consist essentially of aggregates of dense, ferritin-like material. The presence, in the hepatocytes, of pigment inclusions which have some features in common in the 3 types of syndromes and other characteristics specific for each of them, confirms the close relation between these types of jaundice, but emphasizes also their differences; moreover, this ultrastructural study underlines the difficulty of an exact classification of these syndromes.

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Travail subventionné par la Fondation Fritz Hoffmann-La Roche pour l'expansion en Suisse du travail scientifique exécuté par équipe et par le Fonds National Suisse de la Recherche Scientifique.

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Avec la collaboration technique de Mlle M.-M. Bertholet.     

Regeneration of the islets of Langerhans in the guinea pig

Summary The regeneration of the islets of Langerhans in the guinea pig was studied after intravenous injection of alloxan. A number of cells in the islets was destroyed within 24 h after alloxan, but after 48 h there was a rapid proliferation of the surviving cells of the islets. This depended on the dosage of the drug as well as the timing. Electron microscopy of the islet at 48 h showed that the dividing cells had small electron dense granules and resembled a subtype of normal A cells, whose function is not yet known. There were also many agranular cells in the islet. These two groups of cells seen in the regenerating islet could be precursor cells, which could differentiate into cells. There was no evidence for transformation of duct cells or acinar cells into islet cells. None of the guinea pigs became permanently diabetic. This was probably due to inadequate dosage which resulted in only partial degeneration of the cells followed by regeneration and recovery. There was also some regeneration of the liver, kidney and the adrenal cortex following alloxan.The author is grateful to Professor R. Barer for his guidance and for providing the facilities for this study. Thanks are also due to Mrs. D. Barraclough for technical assistance and to Mrs. M. Hollingsworth for assistance with the photographsThis work was financed by a grant from the Cystic Fibrosis Research Trust     

Association of glial cells with the terminal parts of neurite bundles extending from chick spinal cord in vitro

Summary The terminal parts of radially directed neurite bundles growing out from chick embryo spinal cord in vitro have been examined by phase and electron-microscopy,A type of ending is described in which the terminal parts of the neurites are associated with a glial cell. The latter sends a single major process proximally towards the explant. Distally it is attached to the substrate, and the neurite ends are related to its dorsal (nonsubstrate) aspect. Appearances suggesting a mechanism of adhesion of neurites to each other and to the gial cell are described.Growth vesicles were found in both neurites and glia.It is suggested that movements of terminal glial cells may affect the pattern of outgrowth of their attached neurite bundles.We are grateful to the Medical Research Council for financial assistance, to Mr. A. Aldrich and Mr. D. Gunn for photography, to Mr. P. Howell and Miss 0. Chmyliwsky for technical assistance, and to Mrs. B. Fisher for valued secretarial help.     

Control of the pars intermedia of the lizard,Anolis carolinensis

Summary The ultrastructure of the intermediate lobe of the hypophysis was studied in Anolis carolinensis with the use of a threefold aldehyde fixative. Lizards with a brown skin were selected. The possibility of two types of secretory cells is discussed; neither cell type is innervated. Type I cells are rarely found and contain dense granules approximately 0.3 m in diameter; Type II cells vary widely in secretory activity. Most of the Type II cells contain a large number of dense secretory granules (up to about 1.3 m in diameter) almost filling the cytoplasm. Rough endoplasmic reticulum (RER), Golgi apparatus and mitochondria are poorly developed. Only some of these cells show signs suggesting a high secretory activity, namely a well developed RER, Golgi apparatus and numerous mitochondria. In these cells the RER sometimes forms large intracisternal droplets (up to 7 m in diameter). Two of the animals exhibited a more uniform, high secretory activity. Large (about 2 m in diameter), pale vacuoles, probably of extracellular character, were found mostly in the vicinity of the perivascular septum. Their role in the release of MSH is discussed. The present data, which are discussed with reference to earlier findings (Forbes, 1972), form the morphological basis for an experimental study on regulation of MSH release (Larsson et al., 1979).Supported by grants from the Swedish Natural Science Research Council (to P. Meurling) and the Royal Physiographic Society of LundThe authors are indebted to Mrs. Ingrid Hallberg, Mrs. Kirsten Thörneby and Mrs. Lena Sandell for valuable technical assistance and to Miss Inger Norling for photographic aid     

On the histochemical demonstration of N-acetyl-β-galactosaminidase

Summary Aging neurons accumulate lipofuscin pigment granules which appear to be secondary lysosomes of the residual body variety. The biological significance of the residual bodies is debated. They were here studied with the aim of testing a hypothesis that the membranes surrounding these granules might be more vulnerable than the membranes around younger types of lysosomes.For this purpose large motor neurons of young and old rats were compared with respect to lysosomal membrane latency, using a modified Bitensky lysosomal lability test. Utilizing successively increasing incubation times, the lysosomes of old neurons, in particular the residual bodies in polar aggregates of old neurons—presumed to represent lipofuscin pigment granules—were found to have a clearly reduced latency in comparison with lysosomes of young neurons.These findings support the notion that the residual bodies are more fragile than younger lysosomes.Supported by the Swedish Medical Research Council (Project No. 12X-2037).     

Morphological studies of possible neuroendocrine structures in Helisoma tenue (Gastropoda: Pulmonata)

Summary Studies of possible neuroendocrine structures in the pulmonate gastropod Helisoma tenue show that cerebral fuchsinophilic neurons with electron-dense granules (mean diameter 1,500 Å) apparently release their secretory content in an intercerebral commissure neurohemal area near the mediodorsal bodies (MDB) or in the median labial nerve neurohemal area.The MDB cells have axon-like processes which branch and end at the cerebral surface, separated by a thin capsule from the cerebral fuchsinophilic cells. The presence of granules (mean diameter 800 Å) in the terminals of the MDB cells suggests cell body origin, distal transport and release of the granular secretory material. The secretory product may have an influence on cerebral fuchsinophilic neurons.Electron microscopy reveals the presence of granules of different sizes and densities in expanded neurites at the periphery of the intestinal nerve of the visceral ganglion which may indicate the presence of a neurohemal area. However, the granules in the intestinal nerve neurites and in the visceral ganglion fuchsinophilic cells are similar to granules found in the heart which also suggests that the granules may carry a neurotransmitter.This study was supported by NTH traineship USPHS T1 CA 5045 and NSF grant GB-6424X. The author would like to thank Professor Howard A. Bern for guidance and support during the course of this work, Mr. John Underhill and Mr. Albert Blaker for photographic assistance, and Mrs. Emily Reid for preparation of the diagram in Fig. 16.     

Regional specialization of the sperm membrane in the tick Amblyomma hebraeum koch (Acari: Ixodidae)

Summary The spermatozoon of Amblyomma hebraeum is about 200 m long and comprises: (1) a thick, club-shaped anterior part, about 20 m long bearing at its apex a tactile hemisphere, and (2) an elongated tail-like part, about 180 m long. The surface of the tactile hemisphere is covered by numerous bulbous expansions, attached to it by short stalks. The base of the hemisphere is surrounded by a fringe of thin motile processes; the remaining surface of the spermatozoon is covered with long cellular processes which run more or less parallel to one another.The membrane-associated particles found on the membrane beneath the cellular processes are regularly arranged as groups of parallel strands. The external surface of the so-called peripheral granules, as revealed by freeze-etching, is smooth with a very small number of particles. Internally the particles exhibit a regular hexagonal pattern which has not been observed, so far, on any other membrane of these sperm cells.The regional specialization of the spermatozoon surface membrane in relation to sperm motility is discussed. The results obtained indicate that processes of three types: (1) bulbous expansions, (2) motile processes, and (3) cellular processes are regional specializations, all engaged in aspects of sperm motility.The technical assistance of Mr. R. Haemmerle of Balzers Research Laboratories, Mrs. F. Seif and Miss C. Pugin, is gratefully acknowledged     

Electron microscopy of two types of reflecting chromatophores (iridophores and leucophores) in the guppy,Lebistes reticulatus Peters

Summary Reflecting chromatophores in the integument of the guppy, Lebistes reticulatus Peters, are of two distinct types, iridophores and leucophores. The iridophores are smaller and fixed, producing a metallic iridescent color. The cytoplasmic organelles involved in the coloration of iridophores are the reflecting platelets, as in the iridophores of other fish and amphibian species on which earlier reports have been made. Spherical granules of pleiomorphic internal structure, quite variable in size but generally 0.2 m to 1.0 m in diameter, are also numerous in the iridophores. The nature of these granules remains unknown.The leucophores are larger, and highly dendritic; their pigment granules are migratory and they exhibit a dull whitish color. Pigment granules of the leucophores are spherical in form, varying from 0.5–0.8 m in diameter, with a double membrane enclosing the internal fibrous materials. Melamine-treatment of the fish caused degenerative changes in the pigment granules and also the other cytoplasmic organelles of the leucophores, whereas the other kinds of chromatophores, including the iridiophores, remained intact. Some problems in general characterization and classification between these two types of chromatophores were discussed.The author wishes to thank Mr. Yoshiro Yamazaki for his assistance in operating the electron microscope, and Dr. Takao Kajishima (Biological Institute, Nagoya University) for his encouragements     

The frontal ganglion of Periplaneta americana L. (insecta)

Summary The frontal ganglion, part of the stomatogastric nervous system, contains about 60 to 80 neurons, 25 to 30 m in diameter. A well developed Golgi system, producing dense-core vesicles, lysosomes, multivesicular bodies and dense bodies are abundant. Glia elements are sparsely distributed. Many nerve fibres contain granules of different size and electron density. Five groups of fibres can be distinguished: Fibres with granules of about 200 nm (type A), fibres with granules of about 160 to 170 nm (type B), fibres with granules of about 80 to 100 nm (type C) and those with synaptic vesicles of 50 nm (type D) respectively. A fifth very small type contains neither vesicles nor granules. Special attention was paid to synaptic contacts. The divergent dyad seems to be the main type in the frontal ganglion. Frequently, neurosecretory endings are observed in presynaptic position. Immunocytochemical staining of neurosecretory material closely corresponds to the distribution of type A fibres, as observed electron microscopically. Immunoelectrophoresis of extracts from frontal ganglia with polyspecific anti-neurosecretion-serum reveals a single precipitation line, indicating that the immunocytochemical localization of neurosecretory material is due to reaction with a specific as well as a crossreagent antibody.Supported by the Ministerium für Wissenschaft und Technik der DDRThe authors wish to thank Mrs. B. Cosack and Mrs. A. Schmidt for excellent technical assistance     

Brain tissue transplanted to the anterior chamber of the eye

Summary Knowing the ontogenesis of the central monoamine neurons of the rat it is possible to obtain, by free-hand dissection from embryos and newly born animals, pieces containing dopamine (DA), noradrenaline (NA), and 5-hydroxytryptamine (5-HT) neurons that are small enough to permit homologous transplantation to the anterior chamber of the eye of adult animals. With this technique it was established that all three types of immature monoamine neurons are able to survive in the anterior chamber. Fluorescence histochemical analysis of whole mount preparations of the sympathetically denervated host irides revealed that both the catecholamine- and the 5-HT-neurons are able to partly reinnervate the irides, forming networks of varicose nerve terminals similar to the normally present sympathetic adrenergic ground plexus.Monoamine nerve cell bodies are attached to the irides but the majority of fluorescent nerve cell bodies is located within the transplants. Serial sectioning of these transplants showed rather well organized brain tissue, containing groups of fluorescent and non-fluorescent cell bodies, many areas being innervated by monoamine nerve terminals. When brain tissue was transplanted before the normal appearance of fluorescent neuroblasts (embryos with a crown-rump length less than 8 mm) monoamine neurons developed and matured within the eye.The amount of newly formed nerves of central origin recovered on the irides increased with time between the 2nd and 4th postoperative week and persisted after 2 months. The yield of new fibers was better using transplants from embryos with a crown-rump length between 15 and 30 mm than using transplants from larger embryos and newly born animals.If embryonic brain tissue known to be devoid of monoamine nerve cell bodies but containing monoamine nerve terminals in the adult state (cortex cerebri and cerebelli, spinal cord) was transplanted to sympathetically non-denervated eyes, the sympathetic adrenergic fibers seemed to be able to innervate the transplants.This work was supported by grants from the Swedish Medical Research Council (14×–3185), Karolinska Institutets fonder, and Magnus Bergvalls Stiftelse. We thank Miss Monica Eliasson, Mrs. Ulla Flyger, Mrs. Barbro Norstedt and Miss Ingrid Strömberg for skilful technical assistance. The generous gifts of Nialamide, Pfizer, and Pargyline, Abbott are gratefully acknowledged.     

Changes in the fine structure of venous endothelium in the rabbit following adrenaline infusion

Summary The fine structure of the endothelium lining the femoral and mesenteric veins in the rabbit was examined following the infusion of concentrations of adrenaline varying from 0.5 to 5.0 g/kg. Control animals were infused with saline.Vesiculation of the endothelial cells was observed following each infusion of adrenaline, the effect being most marked with the highest dose used. It is suggested that this vesiculation may be associated with the release of antihaemophilic globulin and/or plasminogen activator from the endothelial cells.The authors wish to express their gratitude to Mrs. M. Woodford, Miss. G. Moore and Mr. R. Ellis for their skilful technical assistance     

Juvenile hormone and the ultrastructural properties of the fat body of the adult Colorado beetle,Leptinotarsa decemlineata say

Summary In the fat body of ovipositing female Colorado beetles, two types of lobes occur. The first type, the internal fat body, is highly specialised for protein synthesis. A lobe of the second type, the peripheral fat body, contains two types of cells, oenocytes and glycogen cells. Ovariectomy, performed at adult moult results in hypertrophy of the glycogen cells of the peripheral fat body. The lobes are characterized by the storage of lipid bodies and glycogen and by numerous mitochondria. Short-day conditions ab ovo, which induce diapause in adults, also result in hypertrophy of glycogen cells of the peripheral fat body. Furthermore, only few mitochondria occur but many proteinaceous bodies may be observed, which conditions are in contrast to the observed effects of castration. The fat body of allatectomized long-day females, has the same structure as that of short day beetles. Consequently a lack of juvenile hormone induces the proteinaceous bodies.Dr. A. De Loof gratefully acknowledges a scholarship as Aspirant of the National Foundation of Scientific Research in Belgium. We wish to thank Prof. Dr. h. C. J. de Wilde for his suggestions and helpfull criticism. We also thank Mr. W. Bohijn for his help in operating the EM and Mr. G. Maes for photography.     

Ultrastructural localization of some phosphatases in the prothoracic gland of the insect Leucophaea Maderae

Summary The cytochemical localization of acid phosphatase and thiamine pyrophosphatase activity was studied by light and electron microscopy in prothoracic gland cells of the cockroach Leucophaea moderae. Nymphal and young adult animals were used.Prominent sites of acid phosphatase activity included large membrane-bounded dense bodies or lysosomes, and certain cisternae of the Golgi apparatus. The results suggest a possible difference in the enzymatic activity toward glycerophosphate and aromatic phosphates as substrates.Thiamine pyrophosphatase activity was localized in elements of the Golgi apparatus and endoplasmic reticulum, and in lysosome-like dense bodies. This latter activity was abolished by sodium fluoride treatment, whereas the phosphatase activity in the Golgi apparatus and endoplasmic reticulum is unaffected by such inhibition.The cytochemical results confirm through direct evidence the suggestions of Scharrer (1964), that the large dense bodies present in the prothoracic gland cells are lysosomes, and that their activity may be related to stages in the life history of the glands. Furthermore, the lysosomes or their derivative structures may play an essential role in the autolysis of the prothoracic glands toward the end of their active period.The enzymatic activity of the endoplasmic reticulum may indicate the involvement of this organelle in the metabolism of steroid-like precursor materials necessary for the synthesis of ecdysone.This study was supported by U.S.P.H.S. grants 5 T1-MH-6418 and NB-05219, and grant RO 1-AM-3984 to Dr. Berta Scharrer. I would like to express my appreciation to Dr. Scharrer for her encouragement and assistance during this study. I also wish to thank Mrs. Sarah Wurzelmann for her competent technical aid.     

The lateral photoreceptor of the barnacle,Balanus eburneus

Summary The lateral eye of the barnacle, Balanus eburneus, fixed in highly concentrated osmium is a lens-shaped body of approximately 250 m in diameter and about 75 m thick. It contains three photoreceptor cells which occupy about 42% of its volume. The photoreceptor cells are irregularly shaped and extend countless dendritic processes which bear rhabdomeres at their ends. Individual rhabdomeres come into contact with rhabdomeres originating from dendrites of the same or of one of the other visual cells. Thirteen per cent of the volume of the photoreceptor cells is taken up by the rhabdomeres. The membranes of the rhabdomeric microvilli contain globular subunits which suggest a 70 Å spacing of rhodopsin molecules. There are two kinds of glial cells. One kind, type A glial cells, makes contact with the fibrous capsule of the photoreceptor. The other kind, type B glial cells, is associated with the photoreceptor cells and extends countless tiny cytoplasmic extensions which interdigitate with similar extensions of the receptor cells. There are approximately 95 type B glial cells and 130 type A glial cells in the receptor. The cytoplasm of the photoreceptor cells contains countless small Golgi fields, mitochondria, microtubules, multivesicular and multilamellar bodies. The extracellular space of the photoreceptor is less than 0.1% of its total volume.The authors wish to thank Mrs. G. Theisen and Miss D. Hupp for expert technical assistance and Drs. M. Behrens, C. Helrich, and C.C. Krischer for many inspiring discussions. This study was partly supported by the SFB 160 of the Deutsche Forschungsgemeinschaft