Ultrastructural immunocytochemical study of the massa caudalis of the subcommissural organ-Reissner's fiber complex in lamprey larvae (Geotria australis): Evidence for a terminal vascular route of secretory material

Summary The massa caudalis of the subcommissural organ-Reissner's fiber complex of lamprey larvae (Geotria australis) was studied immunocytochemically at the ultrastructural level by use of the immunoperoxidase-silver methenamine procedure. An antiserum raised against bovine Reissner's fiber was utilized as primary antibody.The caudalmost portion of the central canal and its ampulla caudalis communicate, via wide intercellular spaces in their dorsal wall, with large cavities or lacunae. In addition, distinct openings in the dorsal wall of the ampulla establish an open communication between the latter and the lacunae. The lacunae are lined by slender processes of cells of unknown nature. No junctional complexes can be observed between these cells, which lack a basal lamina. The lacunae communicate with structures resembling blood capillaries, however, they are devoid of a basal lamina. These peculiar vessels, in turn, are in direct communication with characteristic blood capillaries.Reissner's fiber (RF) and its massa caudalis are strongly immunoreactive with the antiserum used. The wide intercellular spaces in the dorsal wall of the central canal and the ampulla, as well as the lumina of the (i) lacunae, (ii) modified vessels and (iii) blood capillaries are filled with a flocculent, strongly immunoreactive material. No immunoreactive material was found outside these structures. Thus, the blood capillaries appear to represent the only final target of RF-material arriving at the ampulla caudalis.Supported by Grant I 38259 from the Stiftung Volkswagenwerk, Federal Republic of Germany, Grant S-85-39 from the Dirección de Investigaciones, Universidad Austral de Chile, and Grant 6027 from Fondo Nacional de Desarrollo Científico y Tecnológico, Chile. The authors express their gratitude to Mrs. Elizabeth Santibáñez and Mr. Julio Lamilla for providing the lamprey larvae and to Mr. Humberto Molina for preparing the three-dimensional drawing     

Reissner's fiber and the wall of the central canal in the lumbo-sacral region of the bovine spinal cord

Summary Reissner's fiber (RF) of the subcommissural organ (SCO), the central canal and its bordering structures, and the filum terminale were investigated in the bovine spinal cord by use of transmission electron microscopy, histochemical methods and light-microscopic immunocytochemistry. The primary antisera were raised against the bovine RF, or the SCO proper. Comparative immunocytochemical studies were also performed on the lumbo-sacral region of the rat, rabbit, dog and pig.At all levels of the bovine spinal cord, RF was strongly immunoreactive with both antisera. From cervical to upper sacral levels of the bovine spinal cord there was an increasing number of ependymal cells immunostainable with both antisera. The free surface of the central canal was covered by a layer of immunoreactive material. At sacral levels small subependymal immunoreactive cells were observed. From all these structures sharing the same immunoreactivity, only RF was stained by the paraldehyde-fuchsin and periodicacid-Schiff methods.At the ultrastructural level, ependymal cells with numerous protrusions extending into the central canal were seen in the lower lumbar segments, whereas cells displaying signs of secretory activity were principally found in the ependyma of the upper sacral levels. A few cerebrospinal fluid-contacting neurons were observed at all levels of the spinal cord; they were immunostained with an anti-tubulin serum.The lumbo-sacral segments of the dog, rat and rabbit, either fixed by vascular perfusion or in the same manner as the bovine material, did not show any immunoreactive structure other than RF.The possibilities that the immunoreactive ependymal cells might play a secretory or an absorptive role, or be the result of post-mortem events, are discussed.Supported by Grant I/38259 from the Stiftung Volkswagenwerk, Federal Republic of Germany, and Grant RS-82-18 from the Dirección de Investigaciones, Universidad Austral de ChileThe authors wish to thank Dr. Enrique Romeny from the Valdivia abattoir for kindly providing the bovine spinal cords     

Electron microscopic study of the origin and formation of Reissner's fiber in the subcommissural organ of Cebus apella (Primates,Platyrrhini)

Summary Two kinds of secretion are formed in cells of the subcommissural organ (SCO) of Cebus apella. The light secretion is found in saccules originating from the endoplasmic reticulum. This secretion is stored in the peripheral portion of the cells and is not involved in formation of Reissner's fiber (RF). In close association with the Golgi complex, electron-dense granules are developed, containing a finely granular substance. These granules accumulate beneath the apical plasmalemma of the cell. Their content is discharged into the third ventricle, where it occurs in the form of a thin layer of secretion. This material appears to constitute the RF at the level of the entrance to the mesencephalic aqueduct.     

Communicating (gap) junctions between chloride cells in the gill epithelium of the lamprey,Geotria australis

Summary Freeze-fracture replicas show that communicating (gap) junctions are present between chloride cells in the gill epithelium of young adults of the Southern Hemisphere lamprey, Geotria australis, acclimated to full-strength sea water. The junctions, which were already present when these lampreys were migrating downstream, may help coordinate the secretory activities of the chloride cells during the marine phase of the lamprey life cycle.     

Scanning electron-microscopic study of Reissner's fiber in the caudal spinal cord of the cat and rabbit

Summary The caudal portion of Reissner's fiber was examined by scanning electron microscopy (SEM) in the spinal cord of the cat and rabbit. In some preparations of both species the fiber displayed in the sinus terminalis of the central canal either stump-like terminations or structural modifications such as knot-like swellings and convolutions. In the same area homogeneous material could also be found, which obviously originated from the disintegrating fiber.     

Basal oxygen consumption,ventilatory frequency,and heart rate during the protracted spawning run of the Southern Hemisphere lamprey Geotria australis

Summary Basal oxygen consumption, ventilatory frequency, and heart rate were recorded at four different times during the unusually protracted 15–16-month spawning run of the Southern Hemisphere lamprey Geotria australis. At 15°C, the mean basal oxygen consumption of G. australis caught immediately after they had left the sea and embarked on the spawning run (45 l · g^-1 · h^-1) was less than in young adults about to commence their marine feeding phase (64 l · g^-1 · h^-1), but greater than in large ammocoetes (26.5 l · g^-1 · h^-1). Basal oxygen consumption fell progressively during the spawning-run of to 33 l · g^-1 · h^-1 after 5 months and 25 l · g^-1 · h^-1 after 10 months, before rising to 35 l · g^-1 · h^-1 after 15 months when the animals were approaching sexual maturity. The downwards trend in basal oxygen consumption contrasts with that recorded during the spawning run of Lampetra fluviatilis. Furthermore, these values for spawning-run of G. australis are far lower than those measured at any time during the upstream migration of L. fluviatilis or during the parasitic phase of landlocked Petromyzon marinus. A low and declining metabolic rate during much of the spawning run of G. australis would facilitate the conservation of energy reserves during this very long non-feeding period. Trends shown by ventilatory frequency and heart rate essentially parallel those of basal oxygen consumption. The Q₁₀s for basal oxygen consumption, ventilatory frequency and heart rate over the temperature range 5–25°C were 1.6, 1.6, and 1.7, respectively. The trends shown by basal oxygen consumption during metamorphosis and the upstream migration did not parallel those exhibited by circulating thyroid hormones.     

Intercellular junctions in the water-blood barrier of the gill lamella in the adult lamprey (Geotria australis,Lampetra fluviatilis)

Thin sections and freeze-fracture replicas of the water-blood barrier in the gill lamellae of adult lampreys (Geotria australis, Lampetra fluviatilis) demonstrate that the occluding junctions between epithelial pavement cells differ markedly from those between endothelial pillar cells in the structure and arrangement of their strands. The zonulae occludentes between pavement cells typically consist of complex networks of 4–6 strands, the mean number of which undergoes a small but significant decline when the animal is acclimated to seawater. In comparison, the occluding junctions between pillar cells are less elaborate and may represent maculae or fasciae, rather than zonulae occludentes. They do not apparently undergo a change when the animal enters saltwater. The results indicate that the main part of the paracellular diffusion barrier to proteins and ions is located in the epithelium rather than the endothelium. Communicating (gap) junctions are present between adjacent pavement cells, between pavement and basal cells and between pillar cells. These findings suggest that the epithelial cells and the pillar cells in the water-blood barrier of lampreys both form functional syncytia. The results are discussed in the context of ion-transporting epithelia in other aquatic vertebrates.This paper is dedicated to Professor H. Leonhardt on the occasion of his 75^th birthday     

The infundibular organ of the lancelet (Branchiostoma lanceolatum,Acrania): an immunocytochemical study

Reissner's fibers are secretions produced by different ependymal areas of the chordate brain, viz., in adult vertebrates, by the dorsal subcommissural organ, and in all stages of cephalochordates (Branchiostoma lancelets), by the ventral infundibular organ. Fibers produced by these different organs are seemingly identical and the two fiber sources also share some immunocytochemical and lectin-binding properties. The secretions in these two glands are, however, not identical; the infundibular organ cells are strongly reactive with antibodies against vertebrate Reissner's fibers, but they do not react with antibodies raised against the source of the vertebrate fibers, viz., the subcommissural organ. The results support the possibility that, in adult vertebrates, the Reissner's fibers are composed of material not only from the subcommissural organ, but also from another, not yet identified, source that is identical or equivalent to the infundibular organ of the lancelet. There are indications that the infundibular organ is immunocytochemically closely akin to some secretory cells in the vertebrate embryonic brain and also to those that produce the juvenile vertebrate Reissner's fibers, viz., secretory cells in the flexural organ.     

Somatostatinergic nerves in the cervical spinal cord of the monkey

Somatostatinergic nerves in the spinal cord of the monkey were investigated utilizing immunohistochemistry with various antibodies against synthetic somatostatin. In contrast to earlier investigations, it is shown that somatostatinergic nerve endings occur in most of the areas of the grey matter of the spinal cord. The somatostatinergic axons are, however, characteristically distributed in three main regions: (1) Densely-packed endings are seen in lamina II of the substantia gelatinosa, forming a crescent-shaped pattern in the columna dorsalis. Somatostatin immunoreactivity is also seen in lamina I and in the Lissauer tract. (2) A fine network of fibers is observed around the central canal; the endings are concentrated on special cell bodies. Some single perikarya are also stained in this region. (3) A loose network of single fibers is found ending on perikarya of the columna lateralis or ventralis. The perikarya of the nerve axons, with the exception of those terminating in the columna dorsalis, have as yet not been identified. In order to better understand the somatostatinergic system of the spinal cord, these newly-detected somatostatinergic nerves must be studied and their exact pathways analyzed.     

The subcommissural organ of the adult and pouch-young brush-tailed possum (Trichosurus vulpecula)

Summary Scanning electron microscopy (SEM) was used to examine the surface features of the subcommissural organ (SCO) and the roof of the cerebral aqueduct of 35 adult and 33 pouch-young Trichosurus vulpecula. The animals were at various developmental stages and of both sexes. In the adult animals, the surfaces of the groove and the adjacent medial walls of the ridges of the SCO were characterized primarily by microvilli. Typically, Reissner's fibre was associated closely with the median groove of the SCO. The ridges and the paramedian grooves of the SCO were often heavily ciliated displaying many cerebrospinal fluid-contacting nerve processes. These processes were of varying lengths with terminal and preterminal varicosities. The observed morphology supports a hypothesis suggesting that under certain physiological and pathological conditions the flow of CSF may be directed away from the heavily ciliated ridges to the poorly ciliated groove containing the Reissner's fibre. In the youngest pouch-young animals there were no cilia, CSF-contacting nerve processes, nor supraependymal cells. Also the surface features of the SCO in the young assumed adult appearance before the adjacent roof of the cerebral aqueduct. These findings suggest the possibility that the SCO begins to function early in ontogenetic development. Acknowledgement. Technical assistance of Mrs. G. Hermanis is gratefully acknowledged.The author thanks the Director of Wild Life and National Parks, South Australia, for permission to use brush-tailed possums     

Focusing and accommodation in the brown kiwi (Apteryx australis)

Summary Brown kiwis are an endangered species of nocturnal, flightless birds which are native to New Zealand. The resting focus of two specimens has previously been studied by retinoscopy in a zoo while the birds were restrained by their keeper (Sivak and Howland 1987). Those birds appeared to be hyperopic (farsighted) by 2–7 D. In this study, examination with infrared photorefraction of the focusing of two unrestrained, feeding birds showed that they could focus objects at infinity and objects in their immediate environment and that they had modest powers of accommodation. Measurements on two 6 month old kiwi chicks showed their corneal radius of curvature to be between 2.90 and 3.00 mm (117 D and 101 D in power).     

The connection between spinal cord and notochord in Amphioxus (Branchiostoma lanceolatum)

Summary Silver impregnation of nerves, histochemical reactions for acetylcholinesterase, and electron microscopy reveal an efferent innervation of the notochord in amphioxus. Extensions of the notochordal lamellae end in groups (the notochordal horns) just below the ventro-lateral surface of the spinal cord where they are opposed to large nerve terminals originating as short collaterals of axons running longitudinally in the nerve cord. This neurochordal junction resembles an ordinary neuro-muscular junction in several respects and is interpreted as a part of the muscular system found in the notochord itself. Acknowledgement. The author is indebted to the staffs at the Marine laboratory in Plymouth and the Biological station at Helgoland for supply of material. The expert advices and criticism of Q. Bone, Ph. D., Plymouth, and Dr. U. Welsch, Kiel, are also greatly appreciated.     

Heterogeneity of astrocytic membranes in the optic nerve and spinal cord of the lizard,Anolis carolinensis

Summary The architecture of astrocytic membranes in the optic nerve and the spinal cord of the lizard, Anolis carolinensis, was investigated by use of the freeze-fracturing technique. Whereas astrocytes in mammals reveal so-called orthogonal arrays of particles (OAPs) in their membranes, astrocytes in lower vertebrates lack these structures. This study demonstrates for the first time OAPs in astrocytes from a submammalian species. They were found commonly in the optic nerve and less frequently in the spinal cord. However, the OAPs in astrocytes of spinal cord were confined to midtrunk levels; the astrocytes in the caudal spinal cord failed to reveal OAPs.Additionally, the ependymal cells around the central canal did not show any OAPs, either in the thoracic or in the caudal spinal cord. They were interconnected by gap and tight junctions, which were not intercalated with each other.The findings support our current working hypothesis according to which the presence and absence of OAPs in astrocytes may be correlated with regenerative incapability or capability of CNS-structures; i.e., whereas the thoracic spinal cord in Anolis carolinensis is known to be incapable of regeneration after injury, the caudal spinal cord is regenerative.     

Production and biomass of Tenagomysis tasmaniae Fenton, Anisomysis mixta australis (Zimmer) and Paramesopodopsis rufa Fenton from south-eastern Tasmania (Crustacea: Mysidacea)

Annual production estimates were calculated for Tenagomysis tasmaniae, Anisomysis mixta australis and Paramedopodopsis rufa using the size-frequency and Petrovich methods. Annual production for T. tasmaniae was more than twice that for P. rufa and A. mixta australis. The P:B ratio was however , higher for a. mixta australis than T. tasmaniae and P. rufa. The P:B values for the three species are among the highest ratios reported for mysids.     

Radial astrocytes and ependymocytes in the spinal cord of the adult toad (Bufo bufo L.)

Summary Immunohistochemical and ultrastructural techniques have been used to demonstrate glial fibrillary acidic protein (GFAP) immuno-positive cells in the adult toad spinal cord. Two types of GFAP-immunoreactive cells were observed: ependymocytes and radial astrocytes. GFAP-positive ependymocytes were scarce and contained the immunoreactive product in their processes. They showed intermediate filaments in the basal pole and in their processes when studied with the electron microscope. These immuno-positive ependymocytes represent the tanycytic form of ependymal cells because their processes ended at the subpial zone. The radial astrocytes showed a more intensive immunoreactive product in somata and processes when they were located far away from the ependymal layer. Cell bodies and processes were also associated with blood vessels, but most of the processes ended at the subpial zone forming a continuous subpial glia limitans. The GFAP-positive processes, which form this subpial glia limitans in the toad spinal cord, belong to both tanycytic ependymocytes and radial astrocytes, whose somata are located in the grey matter. These findings lead us to suggest that both types of GFAP-immunopositive cells might be the functional equivalents of mammalian astrocytes.     

Relationship between urotensin II- and somatostatin-immunoreactive spinal cord neurons of Catostomus commersoni and Oncorhynchus kisutch (Teleostei)

Summary This immunocytochemical study describes the presence of separate immunoreactive (IR)-urotensin II (UII) and IR-somatostatin (SOM) systems in the spinal cord of two species of teleost fish. Both systems are arranged in a close spatial interrelationship in which IR-SOM fibres apparently innervate cerebrospinal fluid (CSF)-contacting IR-UII neurons. Specimens of Oncorhynchus kisutch also display CSF-contacting IR-SOM neurons located in the lateral ependymal walls of the central canal, in addition to CSF-contacting IR-UII neurons located ventrally. It is suggested that, in this species, CSF-contacting IR-SOM and IR-UII neurons perceive different stimuli from the CSF and are integrated in such a way that one peptidergic system may modulate the function of the other.