Pattern of synaptic connections in the pineal organ of the ayu,Plecoglossus altivelis (Teleostei)

Summary Synaptic connections were studied by means of electron microscopy in the sensory pineal organ of the ayu, Plecoglossus altivelis, a highly photosensitive teleost species. Three types of specific contacts were observed in the pineal end-vesicle: 1) symmetrically organized gap junctions between the basal processes of adjacent photoreceptor cells; 2) sensory synapses endowed with synaptic ribbons, formed by basal processes of photoreceptor cells and dendrites of pineal neurons; 3) conventional synapses between pineal neurons, containing both clear and dense-core vesicles at the presynaptic site. Based on these findings, the following interpretations are given: (i) The gap junctions may be involved in an enhancement of electric communication and signal encoding between pineal photoreceptor cells. (ii) The sensory synapses transmit photic signals from the photoreceptor cells to pineal nerve cells. (iii) The conventional synapses are assumed to be involved in a lateral interaction and/or summation of information in the sensory pineal organ. A concept of synaptic relationships among the sensory and neuronal elements in the pineal organ of the ayu is presented.Fellow of the Alexander von Humboldt Foundation, Federal Republic of Germany     

Ultrastructural study of the cellular types in the pineal organ of Gambusia affinis (teleost)

The pineal organ of Gambusia affinis was studied via light and electron microscopy. The cell types studied included photoreceptor cells, supporting cells, and a third cell type. The photoreceptor cells, which appear to form clusters, are divided into four regions: outer segment, inner segment, cell soma, and synaptic pedicle. Synaptic ribbons are commonly observed in the synaptic pedicle. The supporting cells separate the photoreceptor cells from the thick basal lamina that surrounds the entire pineal organ. The supporting cells show highly organized membrane formations, some lipid-like inclusions, and a diplosome. One of the centrioles gives rise to an invaginated cilium. The third cell type is observed infrequently and appears to be located mainly in the vicinity of the outer segments. The morphological characteristics of this cell type are similar to those of phagocytic cells. The ultrastructural features of the pineal organ of G. affinis are compared with those of other teleosts.     

Photic regulation of pineal function. Analogies between retinal and pineal photoreception

Light absorbed by a photopigment in a photoreceptor cell causes a photochemical reaction converting the 11-cis retinal chromophore into the all-trans configuration. These changes lead to a series of events that causes cGMP hydrolysis, a following decrease of cGMP in the cytoplasm of the photoreceptor outer segment and a closure of cGMP-gated cationic channels. As a consequence of these processes the membrane hyperpolarizes. In pineal photoreceptor cells of lower vertebrates these processes are only partly investigated. Molecules involved in the phototransduction process and the desensitization, like opsin, vitamin A, α-transducin and arrestin, have been immunocytochemically localized in pineal photoreceptors and also electrophysiological studies have shown that phototransduction mechanisms in pineal photoreceptors might be very similar to those found in retinal photoreceptors. This review will summarize some of the current knowledge on pineal photoreception and compare it with retinal processes.     

The pineal organ of Raja clavata: Opsin immunoreactivity and ultrastructure

Summary The pineal organ of Raja clavata was studied by light and electron microscopy, including the immunocytochemical antiopsin reaction. The pineal organ of the ray consists of three portions: (i) a large proximal pineal, (ii) a long tube-like connecting stalk, and (iii) a short distal terminal enlargement. This latter end-vesicle lies in the deep connective tissue layers of the braincase. All portions of the pineal are composed of pinealocytes, intrinsic neurons, ependymal/glial cells, and bundles of nerve fibers embedded in thin neuropil formations. The inner segments of the pinealocytes protrude into the lumen in all parts of the organ and usually contain basal bodies and numerous mitochondria. Often, two outer segments were found to arise from the basal bodies of a single inner segment. By means of light-microscopic immunocytochemistry the outer segments showed a strong antiopsin reaction.The axons of the pinealocytes form ribbon-containing synapses on dendritelike profiles, which appear to belong to the intrinsic pineal neurons. There are other axo-dendritic synapses established by presynaptic terminals lacking ribbons and containing granular and synaptic vesicles. Pineal neurons may contain granular vesicles approximately 60–100 nm in diameter; their processes contribute to the bundles of unmyelinated axons.The fine structural organization of the pineal organ and the opsin immunoreactivity of the outer segments of the pinealocytes indicate a photoreceptive capacity of the organ. The double outer segments represent a peculiar multiplication of the photoreceptor structures.This investigation was supported by grants from the Deutsche Forschungsgemeinschaft to A. Oksche (Ok 1/24; 1/25: Mechanismen biologischer Uhren)On leave from the 2nd Department of Anatomy, Semmelweis OTE, Budapest, Hungary     

Ultrastructure and serotonin immunocytochemistry of the parietal-pineal complex in the Japanese grass lizard, Takydromus tachydromoides.

The fine structure and immunocytochemical localization of serotonin in the cells of the receptor line were studied in the parietal eye and pineal organ proper of the Japanese grass lizard, Takydromus tachydromoides. Typical photoreceptor cells (PC) were the predominant cell type in the receptor line of the parietal eye, the outer segments of which had regular stacks of numerous disks similar to those of cones. The pineal organ contained relatively few PCs, which showed less well-developed outer segments than those of the parietal eye. In contrast, secretory rudimentary photoreceptor cells (SRPC) accounted for the majority of receptor cells in the pineal organ. These cells were structurally characterized by whorl-like lamellar outer segments and numerous dense-cored vesicles (80-280 nm in diameter). A small number of SRPC were also found in the parietal retina, which were similar to those in the pineal organ. In the parietal-pineal complex, numerous mitochondria located in the PC were larger and rounder than those in the SRPC. In the PC, basal processes prossessed only synaptic ribbons, whereas in the SRPC some of these processes contained synaptic ribbons and others contained dense-cored vesicles, rarely having both. Serotonin-immunoreactive cells were found not only in the pineal organ but also in the parietal eye, which closely resembled the cells of the receptor line in their size and shape. Furthermore, on immunoelectron microscopy for serotonin using the protein A-gold technique, gold particles indicating serotonin-immunoreactive sites were restricted in the core of dense-cored vesicles in the SRPC of the pineal organ. Regional differences in the distributions of the PC, SRPC and serotonin-immunoreactivity were found in the parietal-pineal complex.     

Immunocytochemical localization of serotonin and photoreceptor-specific proteins (rod-opsin,S-antigen) in the pineal complex of the river lamprey,Lampetra japonica,with special reference to photoneuroendocrine cells

Summary The pineal complex of the river lamprey, Lampetra japonica, was examined by means of immunocytochemistry with antisera against serotonin, the precursor of melatonin, and two photoreceptor proteins, rod-opsin (the apoprotein of the photopigment rhodopsin) and S-antigen. Serotonin-immunoreactive cells were observed in both the pineal and the parapineal organ. The proximal portion of the pineal organ (atrium) comprised numerous serotonin-immunoreactive cells displaying spherical somata. In the distal end-vesicle of the pineal organ, the serotonin-immunoreactive elements resembled photoreceptors in their size and shape. These cells projecting into the pineal lumen and toward the basal lamina were especially conspicuous in the ventral portion of the end-vesicle. In addition, single serotonin-immunoreactive nerve cells were found in this location. Retinal photoreceptors were never seen to contain immunoreactive serotonin; amacrine cells were the only retinal elements exhibiting serotonin immunoreaction. Strong S-antigen immunoreactivity was found in numerous photoreceptors located in the pineal end-vesicle. In contrast, the S-antigen immunoreactivity was weak in the spherical cells of the atrium. Thus, the pattern of S-antigen immunoreactivity was roughly opposite to that of serotonin. Similar findings were obtained in the parapineal organ. The rod-opsin immunoreaction was restricted to the outer segments of photoreceptors in the pineal end-vesicle and parapineal organ. No rodopsin immunoreactive outer segments occurred in the proximal portion of the atrium. Double immunostaining was employed to investigate whether immunoreactive opsin and serotonin are colocalized in one and the same cell. This approach revealed that (i) most of the rodopsin-immunoreactive outer segments in the end-vesicle belonged to serotonin-immunonegative photoreceptors; (ii) nearly all serotonin-immunoreactive cells in the end-vesicle bore short rod-opsin-immunoreactive outer segments protruding into the pineal lumen; and (iii) the spherical serotonin-immunoreactive cells in the pineal stalk lacked rod-opsin immunoreaction and an outer segment. These results support the concept that multiple cell lines of the photoreceptor type exist in the pineal complex at an early evolutionary stage.     

The pineal gland in wild-type and two zebrafish mutants with retinal defects

Light and transmission electron microscopy were used to characterize the ultrastructural features of the pineal glands of wild-type and two mutant zebrafish strains that have retinal defects. Particular attention was given to the pineal photoreceptors. Photoreceptors in the pineal gland appear quite similar to retinal cone photoreceptors, having many of the same structural characteristics including outer segment disk membranes often confluent with the plasma membrane, calycal processes surrounding the outer segments, and classic connecting cilia. The pineal photoreceptor terminals differ from photoreceptor terminals in the retina in that they have short synaptic ribbons and make dyad synapses which may or may not be invaginated. Pineal photoreceptors in two zebrafish mutants with abnormal retinal photoreceptors were also studied. Pineal photoreceptors in the niezerka (nie) mutant degenerate, as they do in the retina, indicating that pineal and retinal photoreceptors share at least some genes. However, the synaptic terminals of no optokinetic response c (nrc) pineal photoreceptors are normal, suggesting that this mutation is specific to the retina.     

Immunocytochemical and electron-microscopic investigations of the pineal organ in adult agamid lizards,Uromastix hardwicki

Summary Lacertilian species display a remarkable diversity in the organization of the neural apparatus of their pineal organ (epiphysis cerebri). The occurrence of immunoreactive S-antigen and opsin was investigated in the retina and pineal organ of adult lizards, Uromastix hardwicki. In this species, numerous retinal photoreceptors displayed S-antigen-like immunoreactivity, whereas only very few pinealocytes were labeled. Immunoreactive opsin was found neither in retinal photoreceptors nor in pinealocytes. Electron microscopy showed that all pinealocytes of Uromastix hardwicki resemble modified pineal photoreceptors. A peculiar observation is the existence of a previously undescribed membrane system in the inner segments of these cells. It is evidently derived from the rough endoplasmic reticulum but consists of smooth membranes. The modified pineal photoreceptor cells of Uromastix hardwicki were never seen to establish synaptic contacts with somata or dendrites of intrapineal neurons, which are extremely rare. Vesiclecrowned ribbons are prominent in the basal processes of the receptor cells, facing the basal lamina or establishing receptor-receptor and receptor-interstitial type synaptoid contacts. Dense-core granules (60–250 nm in diameter) speak in favor of a secretory activity of the pinealocytes. Attention is drawn to the existence of receptor-receptor and receptor-interstitial cell contacts indicating intramural cellular relationships that deserve further study.Supported by the Deutsche Forschungsgemeinschaft (Ko 758/31) and the Deutscher Akademischer Austauschdienst (Senior DAAD Research Fellowship to M.A.H.)     

Photoreceptor cells and neural elements with long axonal processes in the pineal organ of the lamprey,Lampetra japonica,identified by use of the horseradish peroxidase method

Summary Horseradish peroxidase (HRP) was applied to the transected end of the pineal tract of the lamprey, Lampetra japonica. Distinct reaction products of HRP were observed in 2 types of cell other than ganglion cells. The first type of cell protrudes a knob-like process into the pineal lumen. This type of cell was clearly identified by electron microscopy as a photoreceptor cell; its outer segment was connected to the ellipsoid through a sensory cilium. The other type of cell was located among photoreceptor and supporting cells. The processes of these cells were thin and slender, and they obviously did not represent photoreceptor, supporting, or conventional ganglion cells. The present results indicate that, in the lamprey, some of the photoreceptor cells of the pineal organ project their axon-like processes toward the posterior commissure, but that there is also another type of cell displaying long axonal projections. HRP-containing cells were distributed randomly over the pineal organ and were occasionally also observed in the parapineal organ.     

Pineal organ-like organization of the retina in megachiroptean bats

Phylogenetically originated from photoreceptive structures, the pineal organ adapts the organism to circadian and circannual light periodicity of the environment, while the retina develops to a light-based locator. Bats have a nocturnal life and an echolocator orientation presumably modifying the task of photoreception. Looking for morphological basis of the special functions, in the present work we compared the fine structure and immunocytochemistry of the retina and pineal organ in micro- and megacrochiroptean bats. We found that there is a high similarity between the retina and pineal organ in megachiropterans when compared to other species investigated so far. Besides of photoreceptor derived pinealocytes, the pineal organ of both micro- and megachiropterans contain intrapineal neurons and/or ganglionic cells as well as glial cells. Like spherules and pedicles of retinal photoreceptors, axon-type processes of pinealocytes form synaptic ribbon containig terminals. Similar to retinal photoreceptors and neurons, pinealocytes and pineal neurons contain immunoreactive glutamate and aspartate. In addition, excitatory amino acids accumulate in the pineal neurohormonal endings and might have a role in the hormonal (serotonin?) release of the organ. Concerning the structure of the retina the highest similarity to the organization of the pineal organ was found in the megachiroptean fruit eating bats Cynopterus sphinx and Rusettus niloticus. The retina of these species forms folds and crypts in its photoreceptor layer. This organization is similar to the folds of the pineal wall successively developed during evolution. Since a folded photoreceptor layer is not viable for a photolocator screen in decoding two-dimensional images, we suppose that this peculiar organization of the megachiropteran retina is connected to a "pineal-like" photometer task of the eye needed by these species active at night.     

The presence of two populations of sensory-type cells in the pineal organ of the five-bearded rockling,Ciliata mustela L. (Teleostei)

The pineal organ of the five-bearded rockling, Ciliata mustela L., was examined by means of electron microscopy. Two categories of sensory cells are described: 1) Sensory cells 1 (or photoreceptor cells sensu stricto) showing the characteristic ultrastructure of photoreceptor cells with a well-developed receptor pole (outer segment) and a transmitter pole (ribbon-type synapse in the basal pedicle contacting dendritic processes), and a segmental organization of organelles. 2) Sensory cells 2 (or photoneuroendocrine cells) displaying no particular segmentation. The ultrastructure of the receptor pole (outer segment) is variable in shape (with either long or short disks) and in the number of disks; some outer segments are simple cilia of the 9 + 0 type. This second cell category is rich in smooth endoplasmic reticulum, beta-particles of glycogen, dense inclusions of variable size and content, and dense-core vesicles 130 nm in diameter. These cells have an extended contact area with the perivascular space. The functional significance of both cell categories is discussed in terms of the known physiological responses of the pineal organ. A possible confusion in identification of interstitial cells and neuroendocrine cells in some teleost species is discussed.     

Histological,histochemical and electron microscopical studies on the nervous apparatus of the pineal organ in the tiger salamander,Ambystoma tigrinum

Summary 150–190 photoreceptor cells form a basic structural component of the pineal organ of Ambystoma tigrinum. Most of the outer and inner segments of these cells project into the lumen horizontally. Only 10 percent of the total number of photoreceptor cells are located within the pineal roof which is composed of a single cell layer. The photoreceptor cells are connected with nerve cells by synapses displaying characteristic ribbons. Different types of synaptic contacts, i.e. simple, tangential, dyad, triad and invaginated, are found. They are embedded in extended neuropil zones. A particular type of synapse indicates the presence of interneurons. The basal processes of some photoreceptor cells leave the pineal organ and make synaptic contacts with nervous elements located within the area of the subcommissural organ. Employing the method of Karnovsky and Roots (1964) for histochemical demonstration of acetylcholinesterase (AChE) approximately 70 neurons (intrapineal neurons) can be discerned in the pineal organ of Ambystoma tigrinum. In analogy to the distribution of photoreceptor cells only few nerve cells are observed in the roof portion of the pineal organ. Evidently, two different types of AChE-positive intrapineal neurons are present. About 40–50 AChE-positive neurons (extrapineal neurons) are scattered in the area of the subcommissural organ. In this area two types of nerve cells can be distinguished: 1) neurons which send pinealofugal (afferent) axons toward the posterior commissure and 2) neurons which emit pinealopetal (efferent) axons into or toward the pineal organ.The nervous pathways connecting the pineal organ with the diencephalomesencephalic border area are represented by a distinct pineal pedicle and several accessory pineal tracts.Granular nerve fibers run within the posterior commissure and establish synaptic contacts in the commissural region adjacent to the pineal organ. Some of these granular elements enter the pineal organ.The morphology of the nervous apparatus of the pineal organ of Ambystoma tigrinum is discussed in context with evidence from physiological experiments.In partial fulfillment of the requirements for the degree of Dr. med., Faculty of Medicine, Justus Liebig University, GiessenThe author is indebted to Professors A. Oksche and M. Ueck for their interest in this study. Thanks are due to Professor Ch. Baumann, Giessen, and Professor H. Langer, Bochum, for stimulating discussions. The technical assistance of Miss R. Liesner is gratefully acknowledgedDedicated to Professor Berta Scharrer on the occasion of her 70th birthday. Supported by grants from the Deutsche Forschungsgemeinschaft to A.O. and M.U.     

Ontogenetic development of the pineal organ,parapineal organ,and retina of the three-spined stickleback,Gasterosteus aculeatus L. (Teleostei)

The ontogenetic developments of the pineal organ, parapineal organ, and retina were studied by the use of light and electron microscopy in embryos and fry of the teleost, Gasterosteus aculeatus, from 60 to 168 h after fertilization. Sixty to 66 h after fertilization, the primordium of the pineal complex is discernible in the diencephalic roofplate; the parapineal anlage is located rostral to the pineal anlage. Photoreceptor cells endowed with outer segments are present in the embryonic pineal organ already after 72 h, whereas outer segments of retinal photoreceptors could not be demonstrated before 144 h (hatching occurs between 120-144 h). Furthermore, neuropil formations with synaptic specializations are present in the rostral part of the pineal organ 108 h after fertilization. At 72 h, the embryonic parapineal parenchyma is already differentiated into parapinealocytes, which give rise to the parapineal tract, and glia-resembling elements. Although parapinealocytes carry cilia (9 X 2 + 0), only a single outer segment of the photoreceptor type could be demonstrated in the parapineal organ of one adult stickleback. Photoreceptors present in the pineal organ of unhatched embryos are hardly involved in visual functions, but may already at this early developmental stage serve as photoneuroendocrine transducers.     

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.     

Responses of pineal photoreceptors in the brook and rainbow trout

Summary Electron microscopy of the pineal receptor cells in light- and dark-adapted brook trout, Salvelinus fontinalis and the rainbow trout, Salmo gairdneri, revealed no significant differences in the tubular and filamentous elements of the inner segment, neck and supranuclear regions. However, changes in synaptic relations between the photoreceptor and nerve cell were induced by light and darkness. In the light-adapted state, the synaptic relationship between axon terminals and photoreceptor basal processes predominates, while in darkness the synapses between photoreceptor basal processes and ganglion cell dendrites are more prominent. Further, in darkness, the photoreceptor basal processes show a number of synaptic vesicles and synaptic ribbons. These findings suggest that the sensory function of the fish pineal is enhanced during darkness but inhibited by light, and that the synaptic relationships are involved in the control of sensory activity in the pineal photoreceptor and ganglion cells. These results corroborate those of electrophysiological studies in that the maximal spontaneous discharge frequency of the ganglion cells occurs in the dark, and it also shows a burst when light is removed. The typical chemical synapse between the axon terminal and the photoreceptor basal process in light seems to function as an inhibitor.The authors thank Dr. Mary Ann Klyne for her assistance in several aspects of this work. Financial assistance was provided by the NSERC of Canada and the Ministry of Education of Québec     

Ultrastructural and biochemical responses of photoreceptor pinealocytes to light and dark in vivo and in vitro

The teleost pineal organ contains functional photoreceptors that synapse with pinealofugal neurons. This study examined the effects of environmental lighting on protein content and levels of putative amino acid transmitters, as well as structural components associated with protein synthesis and neurotransmission. Goldfish subjected to continual illumination for 3 days tended to have increased pineal levels of free amino acids and protein compared to dark adapted glands. Similar responses to environmental lighting occurred in cultured glands suggesting a functional relationship to photosensory mechanisms. Morphometric ultrastructural analyses of pineal photoreceptors showed an increased size of nucleoli (especially the fibrillar component), Golgi bodies, and synaptic ribbons when glands were subjected to continuous light both in vivo and in vitro. The good agreement between protein levels and nucleolar morphology indicates a general effect of environmental lighting on photoreceptor protein metabolism, which may be related to photoreceptor outer segment renewal. Parallel changes in levels of certain amino acids (e.g., glutamate) and size of synaptic ribbons is consistent with an hypothesized role of amino acids in photoreceptor neurotransmission.     

FINE STRUCTURE OF THE PINEAL ORGANS OF THE ADULT FROG, RANA PIPIENS

Frontal organs and epiphyses of the pineal system from the adult frog, Rana pipiens, were fixed in s-collidine-buffered osmium tetroxide, embedded in Epon 812, and examined by electron microscopy. Epiphyseal material was also fixed in a variety of ways and subjected to a series of cytochemical tests for light microscopy. An ultrastructure resembling that of lateral eye retina is confirmed in this species. Photoreceptor cells of the epiphysis and frontal organ display many cytological features similar to those of retinal rods and cones in the arrangement of their outer and inner segments and synaptic components. However, in these pineal organs the outer segments are disoriented relative to each other and may display a disarranged internal organization unlike normal retinal photoreceptors. Furthermore, other pineal outer segments often appear degenerate. Since immature stages in the development of new outer segments also appear to be present, adult pineal photoreceptors are probably engaged in a constant renewal of outer segment membranes. The evidence further suggests that macrophages are involved in phagocytosis of degenerated outer segments. Postulated photoreceptor activities and the possibility of secondary pineal functions, such as secretion, are discussed in view of current morphological and cytochemical findings.     

5′-Nucleotidase activity in the pineal organ of the pike

Summary To date, it is still unknown whether the metabolism of purine nucleotides and nucleosides plays an important role in the pineal organ of lower vertebrates. We have therefore investigated the sites of 5-nucleotidase activity in the pineal organ of the pike (Esox lucius L.). Various ultracytochemical procedures were used. An intense ecto-5-nucleotidase activity was characteristic of the entire plasma membrane of the phototransducers (cone-like and modified photoreceptor elements) and the interstitial cells, with exception of the portions facing the basal lamina of the pericapillary spaces. Additionally, intracellular sites of activity were also visualized in the inner segment and the pedicle of the phototransducers. Most of the intracellular deposits were apparently cytosolic and only few seemed to be associated with the membrane of the clear synaptic vesicles of the pedicle. Phagocytotic cells in the pineal lumen also showed a strong enzymatic activity on the outer surface of their plasmalemma (in ectoposition). This was apparently not the case for the cell types of the tissues surrounding the pineal vesicle. The present study emphasizes the importance of the occurrence and metabolism of purine nucleotides and nucleosides in a photoreceptive pineal organ.     

The parapineal and pineal organs of the elver (glass eel), Anguilla anguilla L.

Summary The parapineal organ of the glass eel (elver) consists of approximately 400 cells and is situated to the left of the connection of the pineal stalk to the third ventricle. A conspicuous nerve tract containing approximately 350 fibers arises from the parapineal organ and runs in spatial relationship to the habenular commissure toward the left habenular nucleus. The dominating cell type of the parapineal organ of the elver is a neuron (sensory neuron) of small diameter provided with atypical cilia (9×2+0, or rarely 8×2+0 types). Well-developed photoreceptor outer segments are lacking, and no interstitial cells of ependymal type have been observed with certainty in the parapineal organ. The axonal processes from the nerve cells form the tract leaving the parapineal organ.The pineal organ proper of the elver consists of photoreceptor cells with well-developed outer segments, interstitial cells of ependymal type, and ganglion cells. Axons from the latter form the pineal tract, which leaves the pineal organ and runs in close contact with the subcommissural organ toward the posterior commissure. The proximal part of the pineal stalk contains only a few photoreceptor cells the outer segments of which are less developed than those of the pineal body and the distal part of the pineal stalk.     

Immunocytochemical localization of Vitamin A in the retina and pineal organ of the frog,Rana esculenta

Summary Vitamin A immunoreactive sites were studied in the retina and pincal organ of the frog,Rana esculenta, by the peroxidase antiperoxidase, avidin-biotinperoxidase and immunogold methods. Indark-adapted material, strong immunoreaction was found in the outer and inner segments of the photoreceptor cells of both retina and pineal organ, as well as in the pigment epithelium, retinal Müller cells and pineal ependymal cells. Inlight-adapted retina, cones and green (blue-sensitive) rods were immunopositive.At the electron microscopic level, immunogold particles were found on the membranes of the photoreceptor outer segments as well as on the membranes of the endoplasmic reticulum and mitochondria. Individual retinal photoreceptor cells exhibited strong immunoreaction in the distal portion of the inner segment, the ciliary connecting piece and the electron-dense material covering the outer segment. In the pigment epithelium, the immunolabeling varied in intensity in the basal and apical cytoplasm and phagocytosed outer segments.The immunocytochemical results indicate that retinoids (retinal, retinol and possibly retinoic acid) are present not only in the photoreceptor cells of the retina but also in those of the pineal organ. The light-dependent differences in the immunoreactivity of vitamin A underlines its essential role in the visual cycle of the photopigments. Our results suggest that the pineal ependyma plays a role comparable to that of the Müller cells and pigment epithelium of the retina with regard to the transport and storage of vitamin A. The presence of a retinoid in nuclei, mitochondria and cytoplasmic membranes suggests an additional role of vitamin A in other metabolic processes.Dedicated to Professor Dr. T.H. Schiebler on the occasion of his 65th birthdaySupported by the Hungarian OTKA grant Nr. 1619 to B.V., and a grant from the Pardee Foundation to G.H.W.