Comparative ultrastructural investigations of the pineal organ of the blind cave fish,Anoptichthys jordani,and its ancestor,the eyed river fish,Astyanax mexicanus

A comparative ultrastructural study has been made of the pineal organ in specimens of two closely related populations of the characid fish, Astyanaz mexicanus. The specimens of one population are living in the river, under natural light conditions. The specimens of the other population, originally described as Anoptichthys jordani, are living in a completely dark cave. In specimens of both populations the pineal organ consists of a spindle shaped end-vesicle, connected to the diencephalic roof by a slender stalk. The pineal tissue is compact and consists predominantly of glia-like supporting cells and sensory cells resembling the photoreceptor cells of the lateral vertebrate eye. Phagocytotic microglia-like cells can be found in close contact with the outer segments of the sensory cells. Nerve cells are located in the neighbourhood of neuropil formations, in which synaptic contacts are established between sensory cells and nerve cells. From these nerve cells fibers are emerging, forming the pineal tract that runs down the pineal stalk towards the diencephalon. On the basis of the ultrastructure described by other authors it is concluded that the pineal organ in specimens of the river population of Astyanax mexicanus resembles the pineal organ of other fish species. In specimens of the river population, reared under normal light-dark conditions for 3, 9 or 18 months, conspicuous morphological changes have not been detected in the presumably light-sensitive outer segments of the sensory cells or in other parts of the pineal tissue. In specimens of the cave populations, reared under identical conditions, an age-dependent, gradual regression of the regular outer segment organization of the pineal sensory cells takes place. In other parts of the pineal tissue, only small morphological changes can be observed. In specimens of the cave population, reared in constant darkness, the regression of the pineal outer segment organization begins earlier and is obvious. It is postulated that the gradual age-dependent regression of the regular organization of the outer segments in the pineal organ of cave specimens of Astyanax mexicanus is genetically determined and indicates a regressive evolution of the pineal light sensitivity. The expression of the regressive traits is dependent on the environmental light conditions.     

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     

Structure of the pineal organ of the sardine,Sardina pilchardus sardina (Risso), and some further remarks on the pineal organ of Mugil spp.

Summary The pineal organ of the sardine, Sardina pilchardus sardina, was investigated light and electron microscopically. The pineal parenchyma contains sensory cells, supporting cells, and ganglion cells, and the overlying tissues appear specialized for light penetration. The ganglion cells are arranged in 3 groups, their axons giving rise to the tractus epiphyseos. The sensory cell is of a photoreceptor type found in several other teleost species. No definitive evidence of a secretion was educed but some indications of an endocrine function are reported and discussed.The pineal receptor cell of neonates of Mugil spp. which have a pigment-free spot above the pineal organ, was investigated electron microscopically and found to have the same organization as that of adult Mugil auratus.Supported by grants from the Helge Ax:son Johnsons Stiftelse, Stockholm, and from the Kungliga Vetenskapsakademien, Stockholm, Sweden. This is gratefully acknowledged.The animal material has been provided by the Stazione Zoologica di Napoli.     

The structure of the pineal organ of the bluefin tuna, Thunnus thynnus

The pineal organ of the bluefin tuna was studied using light and electron microscopy. The pineal, an evagination in the diencephalic pallium, consists of a distal expanded end-organ and a stalk. A pineal nerve connects this end-organ to the habenular area of the diencephalon. The tissues above the pineal are modified for light transmission. Supporting cells are the most numerous type seen in the pineal tissue. The cytoplasm often contains glycogen, while the golgi apparatus is seldom seen. The plasma membrane is a complex of folded membranes interdigitating with adjacent cells. The sensory cells are characterized by a basal nucleus, a constricted neck region, and an ellipsoid region that is capped distally by lamellae. Sensory cells have the basic appearance of retinal photoreceptors. The lamellae are more disorganized than those of retinal rods, and some lamellae appear to be discharged from the cell. This study indicates that the pineal may function to deliver photoperiodic stimuli to the central nervous system, through the transmission of nerve impulses.     

Receptor cells in the pineal organ of the dogfish Scyliorhinus canicula linné

Summary The pineal parenchyma of the dogfish Scyliorhinus canicula contains sensory (receptor) cells and supporting cells. The ultrastructural characteristics of these cells are described. The sensory cell is a photoreceptor-like cell the outer segment of which is, however, often irregularly developed. Neuropil-like areas are present but no typical synapses have been observed. The classification of pineal receptor cells is discussed.Work done with the aid of a research scholarship from the Alexander von Humboldt Foundation, Bad Godesberg, Germany. — The electron microscope used in this study was placed at the disposal of Professor Oksche by the Deutsche Forschungsgemeinschaft. — The animal material was provided by the Stazione Zoologica di Napoli, Italy.     

Evolution of photosensory pineal organs in new light: the fate of neuroendocrine photoreceptors

Pineal evolution is envisaged as a gradual transformation of pinealocytes (a gradual regression of pinealocyte sensory capacity within a particular cell line), the so-called sensory cell line of the pineal organ. In most non-mammals the pineal organ is a directly photosensory organ, while the pineal organ of mammals (epiphysis cerebri) is a non-sensory neuroendocrine organ under photoperiod control. The phylogenetic transformation of the pineal organ is reflected in the morphology and physiology of the main parenchymal cell type, the pinealocyte. In anamniotes, pinealocytes with retinal cone photoreceptor-like characteristics predominate, whereas in sauropsids so-called rudimentary photoreceptors predominate. These have well-developed secretory characteristics, and have been interpreted as intermediaries between the anamniote pineal photoreceptors and the mammalian non-sensory pinealocytes. We have re-examined the original studies on which the gradual transformation hypothesis of pineal evolution is based, and found that the evidence for this model of pineal evolution is ambiguous. In the light of recent advances in the understanding of neural development mechanisms, we propose a new hypothesis of pineal evolution, in which the old notion 'gradual regression within the sensory cell line' should be replaced with 'changes in fate restriction within the neural lineage of the pineal field'.     

Coupling the microbial food web with fish: are bacteria attached to cyanobacteria an important food source for underyearling roach?

1. After observing that juvenile roach fed intensively on cyanobacteria and that cyanobacteria were densely colonized by heterotrophic bacteria, we tested whether the bacteria are used by underyearling roach and the extent to which they contribute to the energy requirements of the fish.
2. We radiolabelled attached bacteria in a natural cyanobacterial suspension, fed the fish with these particles, and estimated their assimilation by roach. Biomass of attached bacteria on cyanobacteria increased with the proportion of the cyanobacterium Microcystis in total cyanobacteria. Biomass-specific thymidine incorporation of attached bacteria was higher than that of free bacteria.
3. In feeding experiments, we detected assimilation of bacterial biomass into muscle tissue of underyearling roach. Fish consumed Microcystis to a lesser extent compared with Aphanizomenon but assimilation of attached bacteria was higher when roach fed on Microcystis because of the higher biomass of epibacteria on this cyanobacterium. However, biomass of attached bacteria was too low to be an important food source for underyearling roach.
4. We conclude that assimilation of epibacteria from cyanobacteria cannot explain the success of roach in eutrophic lakes.     

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.     

Acetylcholinesterase-containing nerve cells and their distribution in the pineal organ of the goldfish,Carassius auratus

Summary Histochemically, an intense acetylcholinesterase (AChE) reaction has been observed in the perikarya of the nerve cells and in the neuropil formations of the pineal organ in the goldfish, Carassius auratus. A group of AChE-rich nerve cells has also been observed between the caudal end of the pineal stalk and the habenular ganglion. No component of the complex revealed butyrylcholinesterase (BuChE) activity.Two different types of nerve cells were recognized on the basis of their size, AChE activity and distribution. Type I cells are characterized by large perikarya possessing a moderate AChE activity and by the presence of an extensive AChE-rich neuropil formation in their vicinity; they are restricted to the rostro-lateral regions of the pineal vesicle. Type II cells are situated in the medio-rostral area of the pineal vesicle and along the entire length of the stalk, and are smaller than Type I cells; they show an intense AChE activity in their perikarya.The neuropil formations in the medio-rostral area of the pineal vesicle are almost as large as those in the vicinity of the Type I cells; they exhibit a strong AChE activity. In the rostral half of the vesicle several sensory cells are associated with each nerve cell, while in the caudal portion only a few cells are apposed to each nerve cell. Thus, the ratio of the number of sensory cells to that of AChE-containing nerve cells in the anterior half of the pineal vesicle is high when compared with the remaining area. In the anterior half of the vesicle the outer segments of the sensory cells are more distinct and their inner segments possess a higher AChE activity than those in the posterior region and the stalk. A gradation in the degree of development of neuropil formations along the pineal axis is remarkable; their size and AChE activity gradually diminish in a caudal direction. In view of the structural specialization of the rostral region of the pineal organ, it has been argued that its terminal portion is more photosensitive.This work was supported by a fellowship from the Alexander von Humboldt Foundation, Federal Republic of Germany.     

Vasoactive Intestinal Peptide Stimulates Chick Pineal Melatonin Production and Interacts with Other Stimulatory and Inhibitory Agents but Does Not Show α1-Adrenergic Potentiation

Vasoactive intestinal peptide (VIP) is known to mimic the effects of beta-adrenergic receptor stimulation in the rat pineal, including marked potentiation by alpha 1-adrenergic receptor stimulation, and to cause increased melatonin synthesis. In contrast, the chick pineal does not respond to beta-adrenergic stimulation, and melatonin synthesis is inhibited by norepinephrine via an alpha 2-adrenergic receptor. The present experiments show that chick pineal cells in primary culture do, however, respond to VIP with increased melatonin production. The effect of VIP was inhibited by addition of norepinephrine or of nitrendipine or by exposing the cells to "unexpected" white light. Stimulation by VIP was enhanced by addition of forskolin or Bay K 8644 but not by alpha 1-adrenergic receptor stimulations. Although stimulation by VIP appears similar in the chick pineal to that seen in the rat pineal and other systems, "dual-receptor regulation," at least with alpha 1-adrenergic receptors, appears to be absent.     

Comparative Ultrastructural Studies on the Pineal Organ of the Mexican Catfish Rhamdia laticauda Heckel, 1858 and One of its Cave-dwelling Derivates (Pimelodidae,Teleostei)

The pineal organ of the epigean catfish Rhamdia laticauda and of its cave-dwelling derivate from Cueva del Tunel in Zongolica, Mexico, was examined ultrastructurally. The pineal of the epigean form is relatively small but ultrastructurally well differentiated and characterized as a metabolically active, photoneuroendocrine organ. The outer segments of the sensory cells are composed of 32–55 very regularly arranged sacculi which are obviously functioning in photoreception. Furthermore, the presence of electron-dense vesicles, free ribosomes, conspicuously well-developed Golgi bodies, and large amounts of rough and smooth endoplasmic reticulum in the cytoplasm provide evidence for a secretory activity of the sensory cells. The only regressive traits in the pineal of the cave form regard the outer segments: there are few disintegrated, whorl-like outer segments and the number of sacculi exhibits an increased variability. Compared with other morphological structures dependent on light, for example the eyes, the degree of regression is very low. It is suggested that this is due to a remaining secretory role of the sensory cells, which prevents the functionless, photoreceptive outer segments from rapid degeneration by developmental constraints.     

The pineal region in the opossum,Didelphis virginiana

Summary In the pineal region of the opossum, Didelphis virginiana, two types of cells predominate: 1) pinealocytes, and 2) fibrous astrocytes. Pinealocytes are characterized by the presence of prominent Golgi bodies, numerous clear and dense-cored vesicles, sensory cilia (9+0), vesicle-crowned rods, and condensation of a material that was always associated with the rough endoplasmic reticulum. In addition, two other cell types are occasionally seen. These include 1) neuron-like cells, and 2) darker staining cells of unknown identity. The endoplasmic reticulum of the darker staining cells is typically expanded and filled with an amorphous substance. Although the pineal region is small in size, the present findings suggest that pinealocytes in this species are metabolically active cells displaying a secretory function. Moreover, the presence of sensory cilia (9+0) and vesicle-crowned rods indicates that pinealocytes of the opossum are phylogenetically related to the photoreceptor cells found in the pineal organ of lower vertebrates.     

Pineal photoreceptor cells are required for maintaining the circadian rhythms of behavioral visual sensitivity in zebrafish

In non-mammalian vertebrates, the pineal gland functions as the central pacemaker that regulates the circadian rhythms of animal behavior and physiology. We generated a transgenic zebrafish line [Tg(Gnat2:gal4-VP16/UAS:nfsB-mCherry)] in which the E. coli nitroreductase is expressed in pineal photoreceptor cells. In developing embryos and young adults, the transgene is expressed in both retinal and pineal photoreceptor cells. During aging, the expression of the transgene in retinal photoreceptor cells gradually diminishes. By 8 months of age, the Gnat2 promoter-driven nitroreductase is no longer expressed in retinal photoreceptor cells, but its expression in pineal photoreceptor cells persists. This provides a tool for selective ablation of pineal photoreceptor cells, i.e., by treatments with metronidazole. In the absence of pineal photoreceptor cells, the behavioral visual sensitivity of the fish remains unchanged; however, the circadian rhythms of rod and cone sensitivity are diminished. Brief light exposures restore the circadian rhythms of behavioral visual sensitivity. Together, the data suggest that retinal photoreceptor cells respond to environmental cues and are capable of entraining the circadian rhythms of visual sensitivity; however, they are insufficient for maintaining the rhythms. Cellular signals from the pineal photoreceptor cells may be required for maintaining the circadian rhythms of visual sensitivity.     

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.     

The sensory innervation of the pineal organ in the lizard,Lacerta viridis,with remarks on its position in the trend of pineal phylogenetic structural and functional evolution

Summary The sensory innervation of the pineal organ of adult Lacerta viridis has been investigated. Some specimens of Lacerta muralis lillfordi were also used. In the pineal epithelium, a small number of nerve cell pericarya of a sensory type are present. They lie either solitary or in small clusters close to the basement membrane. The axons originating from the nerve cell bodies, i. e. the pineal sensory nerve fibers, first course in the intraepithelial nerve fiber layer which is only locally present and contains a restricted number of unmyelinated fibers. In Lacerta viridis, the pineal fibers generally leave the epithelium at the proximal part of the organ proper. They then form small bundles which run along the outer surface of the basement membrane in the leptomeningeal connective tissue covering. At the proximal end of the pineal stalk the single bundles assemble constituting the pineal nerve. In Lacerta muralis the fibers leave the pineal epithelium at the proximal end of the stalk running farther down within the epithelium. Many fibers become myelinated after leaving the pineal epithelium. The pineal nerve runs ventralward in the midplane just caudal to the habenular commissure to which no fibers are given off. Continuing their ventralward course between the habenular commissure and the rostral end of the posterior commissure which is traversed by some of them, the pineal fibers reach the dorsal border of the subcommissural organ. Small separate aberrant pineal bundles traverse the posterior commissure at various more caudal levels. Having reached the dorsal border of the subcommissural organ, part of the pineal fibers continue their ventralward course directly running along the lateral sides of this organ to reach the periventricular nerve fiber layer lateral and ventral to it. A restricted number of fibers first turns in a caudal direction running between the base of the posterior commissure and the base of the subcommissural organ before turning ventralward to reach the periventricular layer. Most probably, pineal fibers do neither join the posterior commissural system nor innervate the subcommissural organ. Once having reached the periventricular layer, some pineal fibers curve in a rostral direction while others, before doing so, send a collateral in a caudal direction. Both, the main fibers and the collaterals, contribute to the formation of the periventricular layer. The sites of termination of the pineal fibers could not be ascertained.From the presence of intraepithelial sensory nerve cell bodies and from literature data on the ultrastructure of pineal neurosensory cells it is concluded that the adult pineal organ of Lacerta has a, although rudimentary, (photo)sensory function. The demonstration by our guest-worker Dr. W. B. Quay, of the intraepithelial presence of a tryptamine compound, probably serotonin, points, moreover, to a secretory function of this organ.In adult Lacerta a well-developed parietal nerve connects the parietal eye with the left lateral habenular nucleus. It traverses the habenular commissure.In gratitude and with admiration this paper is dedicated to Prof. Berta Scharrer and to the memory of Prof. Ernst Scharrer.     

Ultrastructure of serotonin-containing cells in the pineal organ of Lampetra planeri (Petromyzontidae)

Summary The ultrastructure of the cells containing residual bodies (Collin, 1969) was investigated in the pineal organ of Lampetra planeri. These cells are characterized by their indoleamine metabolism (Meiniel, 1978; Meiniel and Hartwig, 1980). Morphologically, they belong mainly to two types: (1) a photoreceptor cell type, and (2) a pinealocyte cell type. The first type is present in the pineal sensory epithelium and in the atrium, while the second is observed in the deep part of the atrium. Intermediate cell types are rare. All these cells are characterized by the presence of voluminous dense bodies, the 5-HT-storing structures, in their cytoplasm.The elongated cone-type photoreceptor cells show a segmental organization and well-developed outer segments consisting of short disks (2–3 m), while their basal pedicles form synapses with the dendritic processes of neurons. The pinealocytes are spherical or oval in shape, their receptor poles being regressed to cilia of the 9+0 type. In these cells, no synaptic ribbons have to date been observed. In both cell types a Golgi apparatus is present producing dense granules 130 nm in diameter and a polymorphous dense material.The photoreceptor cells most probably respond to light and transmit a sensory (i.e., nervous) message. In addition, they produce and metabolize indoleamines, probably including, melatonin (Meiniel, 1978; Meiniel and Hartwig, 1980). The pinealocytes, in spite of their loss of direct photosensitivity, retain their capacity to metabolize indoleamines (Meiniel, 1978; Meiniel and Hartwig, 1980).The presence, in the same pineal organ, of another photoreceptor cell type (cf. Collin, 1969–1971) differing morphologically as well as biochemically (no detectable indoleamine metabolism) from the photoreceptor cell type described in the present investigation, points to the existence of two different sensory cell lines: (1) a pure photoreceptor line, and (2) a photoneuroendocrine line. The phylogenetic evolution of these two cell lines is discussed in terms of functional analogy.     

Vascular permeability (problem of the blood-brain barrier) in the pineal organ of the rainbow trout,Salmo gairdneri

Summary The problem of the blood-brain barrier in the pineal organ of the rainbow trout, Salmo gairdneri, was investigated following intraperitoneal or intracardial injections of several tracers and dyes with different molecular weights. As demonstrated at the light-microscopic level, repeated injections of trypan blue or horseradish peroxidase (HRP) resulted in an accumulation of these substances in the pineal epithelium (parenchyma). By use of the electron microscope, HRP was found in electron-dense bodies, probably lysosomes, in (i) the endothelial cells and perivascular macrophages 4 h after intraperitoneal injection, (ii) the supporting cells and intrapineal or luminal macrophages 8 h after injection, and (iii) the receptor cells 24 h after injection of the tracer. Ferritin particles penetrated the fenestrated endothelium of pineal capillaries. They were confined to vesicles, vacuoles and the smooth endoplasmic reticulum of the supporting cells as well as to the synaptic vesicles and the smooth endoplasmic reticulum of the pineal photoreceptors. The intercellular passage of tannic acid mixed with the fixative was blocked at the luminal junctional complex separating the pineal lumen from the basal portion of the pineal epithelium. The passive intercellular transport of substances with high molecular weight from the bloodstream to the cerebrospinal-fluid compartment is thus prevented. However, no blood-brain barrier exists for exogenously administered proteins, which are rapidly taken up by pineal cells and actively transported in a transcellular manner.The findings on the blood-brain barrier of the pineal organ of the rainbow trout are discussed with particular reference to the endocrine capacity of pineal sensory organs.Fellow of the Alexander von Humboldt Foundation, Federal Republic of Germany.     

Expression of IGF-I and -II mRNA in the brain and craniofacial region of the rat fetus

Insulin-like growth factors (IGF-I and -II) are present in the brain during development, with high levels of both being also found in the periphery particularly in the embryo. IGFs in the brain are believed to stimulate the proliferation of neuronal and glial precursors and their phenotypic differentiation. Using in situ hybridization, we have investigated the distribution of cells producing IGF-I and -II in the rat fetus during the second half of prenatal development with special emphasis on the peripheral and central nervous system. High levels of IGF-I mRNA were found in the olfactory bulb and in discrete neurons of the cranial sensory ganglia, notably in the trigeminal ganglion, as early as 13 days of gestation, in the pineal primordium of 18 day old fetuses, and in discrete groups of cells in the cochlear epithelium located laterally outside the forming spiral organ, in day 13 to 21 fetuses. High levels of IGF-II mRNA in the brain, besides the choroid plexus and the leptomeninges, were detected in hypothalamus, in the floor of the 3rd ventricle at all stages studied, in the pineal primordium at 18 days and in the pars intermedia of the pituitary or in the Rathke's pouch epithelium from which it is derived, with progressive fading towards the end of the gestation. In the peripheral nervous system the IGF-II mRNA was only found in association with the vascular endothelia of the ganglia. IGF-II mRNA in the nervous system was found in highly vascularized areas, meninges, blood vessels and choroid plexuses. It is thus associated with structures involved in the production of extracellular fluids and/or substrate transport and supply in the nervous tissues. A more specific role in the differentiation or fetal endocrine function should be considered for IGF-II in cells producing melatonin and melanocyte stimulating hormone (MSH) in the pineal and pituitary glands, respectively. The presence of IGF-I mRNA in the nervous system could be associated with fiber outgrowth and synaptogenesis in the cases of olfactory bulb and developing iris. The role of IGF-I in restricted populations of cells of the cochlear epithelium and in the pineal gland is unclear and requires further investigations including a search for IGF-I receptors in possible target cells. In the sensory ganglia, the presence of high levels of IGF-I mRNA eventually corresponds to the production, by post-translational processing, of the amino-terminal tripeptide of IGF-I, which might represent a neurotransmitter for these sensory neurons.     

Tyndallblau-Struktur von Federn im Elektronenmikroskop

Summary The normal development and cytology of the pineal organ in the newt, Taricha torosa has been described in detail. Particular emphasis has been placed on the origin of the initial pineal bud in the embryonic diencephalic roof, and the manner in which new pineal cells are proliferated in a zone surrounding the orifice of the developing pineal vesicle. These new cells apparently migrate into the walls of the enlarging vesicle and a certain number undergo progressive differentiation to become photoreceptor-like pineal sensory cells; the highest degree of this differentiation being obtained by cells whose processes protrude into the anterior, posterior, and lateral margins of the vesicle lumen.The well-formed, wide-lumened vesicle typical of early larval stages has thusfar not demonstrated any detectable cytological alterations under the influence of light, dark, pressure, or chemical stimulating agents we have employed. Within a few weeks, this young larval vesicle becomes flattened to assume the appearance of a more glial vascularized organ. In adult pineal organs it has been possible to observe aldehyde fuchsin-positive accumulations in the processes of supportive cells terminating near capillary walls. Other aspects of adult pineal cytology and innervation have also been considered in this report.A series of implants of embryonic pineal primordia into older larval host eye chambers and tailfins has given information on the development of vesicles in these sites under the influence of varying amounts of diencephalic roof tissue included with the grafts. A tentative hypothesis has been formulated to account for the tendency of a single primordium to differentiate into a larger than normal pineal mass when implanted into the tail mesenchyme with a moderate amount of diencephalic roof tissue. This hypothesis brings into focus the normal growth characteristics of the young organ developing from a broad initial pineal field and their possible modification under the influence of surrounding tissues during normal ontogeny.Incidental to the main purposes of the study, observations have been made on the pigment behavior of larvae carrying supernumerary pineal implants. These observations are discussed in the light of recent proposals by other authors.With technical assistance of Mr. Charles Cintron, Mr. Gary Clark, Miss Jean Ewalt, and Mr. Paul Johnson. The author has also been fortunate to have the interest and suggestions of Dr. Stuart Smith. Since a portion of this study was accomplished at the Zoological Laboratory, Utrecht, Holland, special thanks are due the members of that organization for their hospitality and technical advice.Portions of this research were supported by a post-doctoral fellowship (BF7283-C) from the United States Public Health Service, a research grant (G-14423) from the National Science Foundation, and grants from the University of Colorado Council on Research and Creative Work.     

The pineal organ of teleost fishes

The pineal organ of teleost fish is a directly photosensory organ that contains photoreceptor cells similar to those of the retina. It conveys photoperiod information to the brain via neural pathways and by release of indoleamines, primarily melatonin, into the circulation. The photoreceptor cells respond to changes in ambient illumination with a gradual modulation of neurotransmission to second-order neurons that innervate various brain centres, and by modulation of indoleamine synthesis. Melatonin is produced rhythmically, and melatonin synthesis may be regulated either directly by ambient photoperiod, or by an endogenous circadian oscillator that is entrained by the photoperiod. During natural conditions, melatonin is produced at highest levels during the night. Although the pineal organ undoubtedly influences a variety of physiological parameters, as assessed by experimental removal of the pineal organ and/or administration of exogenous indoleamines, its role in any physiological situation is not clear cut. The effects of any interference with pineal functions appear to vary with the time of year and experimental photothermal regimes. There are strong indications that the pineal organ is one component in a central neural system that constitutes the photoperiod-responding system of the animal, i.e. the system that is responsible for correct timing of daily and seasonal physiological rhythms. It is important to envisage the pineal organ as a part of this system; it interacts with other photosensory structures (the retina, possibly extraretinal non-pineal photoreceptors) and circadian rhythm generators