Development of the axon cap neuropil of the Mauthner cell in the goldfish

Summary Development of the axon cap neuropil of the Mauthner neuron in post-hatching larval goldfish brains was observed electron-microscopically. The axonal initial segment of newly hatched (day-4) larvae is completely covered with synaptic terminals containing clear spherical synaptic vesicles. Profiles of thin terminal axons, the spiral fibers, containing similar synaptic vesicles, rapidly increase in number around the initial segment and form glomerular neuropil similar to the central core of the adult axon cap by day 7. Three types of synapses are formed in the core neuropil. Bouton-type synapses contacting the initial segment are most abundant in day-4 to-14 larvae; they decrease thereafter and are rare on the distal half of the initial segment of day-40 larvae. Asymmetric axo-axonic synapses are commonly observed between spiral fibers in the core neuropil of day-7 to -19 larvae, but become fewer by day 40. Unique symmetrical axo-axonic synapses showing accumulation of synaptic vesicles on either side of apposed membrane thickenings first appear in day-14 core neuropil, gradually increase in number, and become the predominant type in day-40 core neuropil. Thick myelinated axons, which lose their myelin sheaths in the glial cap cell layer, start to penetrate into the axon cap on day 10. They gradually increase in number and form the peripheral part of the axon cap together with the cap dendrites, which finally grow into the axon cap from the axon hillock region of the Mauthner cell by day 40.     

Nature of the air-breathing organs of the Indian fishes Channa, Amphipnous, Clarias and Saccobranchus as shown by electron microscopy

The structure of the air-breathing organs of the Indian fishes Channa punctatus, Channa Striatus, Amphipnous cuchia, Clarias batrachus and Saccobranchus fossilis has been investigated using electron microscopy. In all species the barrier separating the air from the blood consists of three main layers (epithelium, basal lamina and endothelium). The total thickness ranging from 0.78 μm in C. punctatus 1.6 μm in S. fossilis.
In Clarias and Saccobranchus the presence of pillar cells characteristic of gill secondary lamellae confirms evidence for the origin of these organs by modification of a typical gill structure.
In Amphipnous and two species of Channa , however, the evidence suggests that the accessory organs represent modified gills. The presence of valve-like structures between the afferent and efferent blood spaces of the vascular papillae gave the appearance of pillar cells under the light microscope.
The structure of these organs is correlated with physiological studies on the degree of their importance in the life of the animal and the degree of gill development     

Studies of herbicide toxicity in some freshwater fishes and ectoprocta

The results of studies on herbicide toxicity to fish and freshwater Ectoprocta are reported. The effects of herbicides, viz , Tafazine, Bladex, Aresin and Dalapon were studied on Sarotherodon mossambicus, Puntius ticto, Cirrhinus mrigala, Danio sps., Heteropneustes fossilis , and Channa punctatus. S. mossambicus was more susceptible to Tafazine toxicity. Danio sps., was more resistant to Tafazine than S. mossambicus , but P. ticto had the maximum tolerance. S. mossambicus had greater resistance to Bladex than C. mrigala. C. punctatus was resistant to Aresin toxicity. Herbicide toxicity to freshwater Ectoprocta was noted with reference to Planavin, Bladex, Tafazine, and 2,4-D butyl ester and mixtures of Planavin + Mercuric chloride and Planavin + Bladex. Herbicide toxicity effected the leptoblasts of Plumatella casmiana and spinoblasts of Lophopodella carteri .     

Neural Organization of the Median Ocellus of the Dragonfly : II. Synaptic structure

Two types of presumed synaptic contacts have been recognized by electron microscopy in the synaptic plexus of the median ocellus of the dragonfly. The first type is characterized by an electron-opaque, button-like organelle in the presynaptic cytoplasm, surrounded by a cluster of synaptic vesicles. Two postsynaptic elements are associated with these junctions, which we have termed button synapses. The second synaptic type is characterized by a dense cluster of synaptic vesicles adjacent to the presumed presynaptic membrane. One postsynaptic element is observed at these junctions. The overwhelming majority of synapses seen in the plexus are button synapses. They are found most commonly in the receptor cell axons where they synaptically contact ocellar nerve dendrites and adjacent receptor cell axons. Button synapses are also seen in the ocellar nerve dendrites where they appear to make synapses back onto receptor axon terminals as well as onto adjacent ocellar nerve dendrites. Reciprocal and serial synaptic arrangements between receptor cell axon terminals, and between receptor cell axon terminals and ocellar nerve dendrites are occasionally seen. It is suggested that the lateral and feedback synapses in the median ocellus of the dragonfly play a role in enhancing transients in the postsynaptic responses.     

Mauthner cell-initiated abrupt increase of the electric organ discharge in the weakly electric fishGymnotus carapo

Stimulation of the spinal cord of the electric fish Gymnotus carapo, evoked an abrupt increase in the discharge rate of the electric organ. At the maximum of this response, the rate increased an average of 26 ± 11.8%. The duration of the response was 4.9 ± 2.12 s; its latency was 10.4 ± 1.1 ms. Activation of the Mauthner axon played a decisive role in this phenomenon as indicated by the following: (1) recordings from the axon cap of the Mauthner cell demonstrated that the response was evoked if the Mauthner axon was antidromically activated and (2) a response that was similar to that produced by spinal cord stimulation, was elicited by intracellular stimulation of either Mauthner cell. Stimulation of the eighth nerve could also increase the discharge rate of the electric organ. The effect was greater if a Mauthner cell action potential was elicited. The findings described in the present report, indicate the existence of a functional connection between the Mauthner cell and the electromotor system in Gymnotus carapo. This connection may function to enhance the electrolocative sampling of the environment during Mauthner-cell mediated behaviors. This is a novel function for the Mauthner cell.Abbreviations EHP extrinsic hyperpolarizing potential - EOD electric organ discharge - M-AIR Mauthner initiated abrupt increase in rate - M-cell Mauthner cell - M-axon Mauthner axon - PM pacemaker nucleus - PM-cell pacemaker cell - PPn prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

Developmental interactions in the growth and branching of the lateral dendrite of Mauthner's cell (Ambystoma mexicanum)

The axolotl Mauthner (M) cell receives synapses from the vestibular and lateral line nerves on highly branched regions located ventrally and dorsally, respectively, of its lateral dendrite. One of the pair of M-cells was deprived of all ipsilateral vestibular supply and of some lateral line supply by unilateral ablation of the otic vesicle at stage 34, before nerve outgrowth and M-cell differentiation. Histological reconstruction of such deprived M-cells at stages 42 and 45, following M-cell differentiation, revealed that the deprived dendrite was poorly developed, being thinner and much less ramified than in controls. This effect was specific; no changes were consistently observed in the shape or size of the M-cell body, the medial dendrite, or the axon. Electron microscopic examination of deprived M-cells showed that morphologically normal synapses were present on the lateral dendrite; however, synaptic knobs identifiable as being of vestibular origin were absent. We suggest that patterned growth and branching of the M-lateral dendrite during differentiation is regulated through interactions with afferent axons.     

Structural correlates of recurrent collateral interneurons producing both electrical and chemical inhibitions of the Mauthner cell.

Intracellular injections of horseradish peroxidase provided a basis for morphological identification of inhibitory interneurons belonging to the recurrent collateral network of the Mauthner cell. Their axons dilate to form unusually large bulbs surrounding the axon cap. The morphological appearance of these bulbs as well as intracellular recordings at their level indicate that they behave as nodes and serve as a final source of current for electrical inhibition of the Mauthner cell. The axon of each interneuron gives rise to two different groups of fibres which are respectively fitted for the mediation of electrical and chemical inhibitions of their target cell.     

Histomorphological observations on the liver in some freshwater teleosts.

Histomorphology of liver is studied in six species of teleost viz., Heteropneustes fossilis, Clarias batrachus, Puntius ticto, Danio malabaricus, Nandus nandus, Chana punctatus. The liver is a bilobed structure usually reddish brown in colour. There are accessory liver lobes in H. fossolis and C. batrachus. The histological examination shows compact mass of oval or polygonel hepatocytes, blood sinusoids and reticuloendothelial cells. In addition several patches of pigmented hepatocytes are also noticed. After partial hepatectomy the changes in liver structure are shrunken nature of hepatocytes, enlarged blood vessels and accumulation of lymphocytes near the cut portion. After biliary obstruction with the help of ligation of bile duct hepatocytes become turgid in appearance within seven days. There is fibrosis of blood vessels as well as bile spaces. The turgidity of hepatocytes increases fifteen days after the ligation so much so that some of them show necrotic appearance. There are numerous bile thrombi noticed after biliary obstruction.     

Histochemical studies on iron absorption in Ophiocephalus punctatus and Heteropneustes fossilis.

Iron absorption in both the teleost fishes Ophiocephalus punctatus and Heteropneustes fossilis, initially starts after 2h of feeding the iron diet. However, at this stage, there is no absorption of iron in the posterior intestine of both the fishes. Absorption initially starts along the brush border of enterocytes of the villi. Later, it gets accumulated in the supranuclear region of the epithelial cells and then through these cells, it is transported into the cores of the villi. The absorbed amount ultimately reaches along the bases of the villi. Through the blood capillaries, which are situated in the submucosa, absorbed iron is passed on to the blood stream. Although, the entire intestine of both the fishes is able to absorb iron but the regional variations have been noted. In contrast to the posterior intestine, the anterior and middle intestine of both the fishes show better iron absorption. The pyloric caeca of Ophiocephalus have comparatively less affinity for iron absorption. However, the intestine of Heteropneustes shows more affinity for iron absorption than the intestine of Ophiocephalus.     

Fine structure of the gills of some Indian air-breathing fishes

A detailed account is given of the structure of the gills of Clarias batrachus, Heteropneustes (= Saccobranchus) fossilis, Channa punctata, Monopterus (= Amphipnous) cuchia and Boleophthalmus boddaerti, based upon light and electron microscopy. In all five species the basic organization into primary and secondary lamellae is apparent but the latter are very much more modified in Monopterus. Three main layers separate the water and blood on the surface of the secondary lamellae. The outer epithelium is usually two layered but may be multilayered close to the origin of the secondary lamellae from the gill filament. The basement membrane is relatively thin and a middle dense layer containing collagen fibrils separates two clear layers. The pillar cells, so characteristic of secondary lamellae, are present in all except Monopterus and flanges from these cells surround the blood channels with the exception of the marginal channels. The latter are lined by endothelial cells which line all the blood channels of Monopterus. The overall thickness of the three layers comprising the water/blood barrier ranges from 1.5 to 13 microns. A number of modifications to this basic organization can be related to the degree of dependence of the different species on air-breathing. Boleophthalmus is the only species commonly found in brackish water and its secondary lamellae have well developed lymphoid spaces between two layers of the epithelium. Special densely-stained regions of the pillar cell flanges were also present in this fish and may have a supporting function.     

Effect of alloxan administration upon Channa punctatus (Bloch)

Channa punctatus, a fresh water teleost, was administered intramuscularly with alloxan in doses varying between 200 and 600 mg/kg body weight. The course of cytological events occurring in the islets of alloxanized fish was followed over a period of 120 hours post injection. Alloxan caused mild degranulation of some of the beta cells in the beginning but later nuclear pycnosis, vacuolization and necrosis of the beta cells was observed. Like other vertebrates, Channa punctatus is alloxan-sensitive and its islet beta cells are susceptible to the cytotoxic effects of alloxan.     

Ultrastructure of orexin-1 receptor immunoreactivities in the spinal cord dorsal horn

The ultrastructural properties of orexin 1-receptor-like immunoreactive (OX1R-LI) neurons in the dorsal horn of the rat spinal cord were examined using light and electron microscopy techniques. At the light microscopy level, the most heavily immunostained OX1R-LI neurons were found in the ventral horn of the spinal cord, while some immunostained profiles, including nerve fibers and small neurons, were also found in the dorsal horn. At the electron microscopy level, OX1R-LI perikarya were identified containing numerous dense-cored vesicles which were more heavily immunostained than any other organelles. Similar vesicles were also found within the axon terminals of the OX1R-LI neurons. The perikarya and dendrites of some of the OX1R-LI neurons could be seen receiving synapses from immunonegative axon terminals. These synapses were found mostly asymmetric in shape. Occasionally, some OX1R-LI axon terminals were found making synapses on dendrites that were OX1R-LI in some cases and immunonegative in others. The synapses made by OX1R-LI axon terminals were found both asymmetric and symmetric in appearance. The results provide solid morphological evidence that OX1R is transported in the dense-cored vesicles from the perikarya to axon terminals and that OX1R-LI neurons in the dorsal horn of the spinal cord have complex synaptic relationships both with other OX1R-LI neurons as well as other neuron types.     

Electron microscopic studies of the corpuscles of Stannius of an airbreathing teleost (Heteropneustes fossilis)

The ultrastructure of the corpuscles of Stannius (CS) ofHeteropneustes fossilis reveals a homogenous cellular composition characterized by only one cell type, with large secretory granules and abundant ribosomal endoplasmic reticulum. These cells are comparable to the type 1 cell described in the CS of other teleosts; type 2 cells, whose presence is ubiquitous in the CS of freshwater species are absent inH. fossilis. Our data on the CS ofH. fossilis demonstrate that not all freshwater species possess type 2 cells in their CS and these are not essential for life in freshwater.     

Motor Axon Synapses on Renshaw Cells Contain Higher Levels of Aspartate than Glutamate

Motoneuron synapses on spinal cord interneurons known as Renshaw cells activate nicotinic, AMPA and NMDA receptors consistent with co-release of acetylcholine and excitatory amino acids (EAA). However, whether these synapses express vesicular glutamate transporters (VGLUTs) capable of accumulating glutamate into synaptic vesicles is controversial. An alternative possibility is that these synapses release other EAAs, like aspartate, not dependent on VGLUTs. To clarify the exact EAA concentrated at motor axon synapses we performed a quantitative postembedding colloidal gold immunoelectron analysis for aspartate and glutamate on motor axon synapses (identified by immunoreactivity to the vesicular acetylcholine transporter; VAChT) contacting calbindin-immunoreactive (-IR) Renshaw cell dendrites. The results show that 71% to 80% of motor axon synaptic boutons on Renshaw cells contained aspartate immunolabeling two standard deviations above average neuropil labeling. Moreover, VAChT-IR synapses on Renshaw cells contained, on average, aspartate immunolabeling at 2.5 to 2.8 times above the average neuropil level. In contrast, glutamate enrichment was lower; 21% to 44% of VAChT-IR synapses showed glutamate-IR two standard deviations above average neuropil labeling and average glutamate immunogold density was 1.7 to 2.0 times the neuropil level. The results were not influenced by antibody affinities because glutamate antibodies detected glutamate-enriched brain homogenates more efficiently than aspartate antibodies detecting aspartate-enriched brain homogenates. Furthermore, synaptic boutons with ultrastructural features of Type I excitatory synapses were always labeled by glutamate antibodies at higher density than motor axon synapses. We conclude that motor axon synapses co-express aspartate and glutamate, but aspartate is concentrated at higher levels than glutamate.     

On the neurons and synapses of the lateral geniculate nucleus of the monkey,and the effects of eye enucleation

Summary Two neuron types are distinguished by electron microscopy in the lateral geniculate nucleus (LGN) of the monkey-a large cell (P cell) interpreted as a geniculostriate relay cell, and a small cell (I cell) interpreted as an inhibitory interneuron. The I cell, distinguished by its small size, infolded nucleus, small mitochondria, cilium and small granular bodies, forms about 10% of the total neuron population. It could not be determined whether this cell has an axon, but its dendrites, which contain aggregates of flattened vesicles, are thought to form a proportion of the F processes, profiles which are post-synaptic to the retinal (RLP) axons and presynaptic to the dendrites of the P cells. The small dark (RSD) axon terminals of unknown origin contact the dendrites of both cell types.After eye enucleation the P cells of the affected laminae of the LGN shrink and partially withdraw their dendrites from the neuropil. By 29 months' survival, they have only a narrow cytoplasmic rim around the nucleus. A necrotic process also occurs, affecting fine dendrites by 22 days and large profiles by 45 days, but it is not clear whether whole cells are destroyed by this process. At 45 days the I cells are commonly seen to form somatodendritic synapses. The appearance of these synapses is interpreted as the result of a withdrawal to the soma of the presynaptic dendrites.It is concluded that the I cells are probably inhibitory interneurons subject to excitation and presynaptic inhibition by the RLP and RSD axons, and a diagram is presented to demonstrate the possible significance of these connections for the transmission of information through the LGN.The author wishes to thank Dr. J. Campos-Ortega for much practical advice.     

Studies on the competitive intestinal absorption of amino acids, and sugars in two teleost fishes.

In both fishes Ophiocephalus punctatus and Heteropneustes fossilis, the compound of the same group inhibited the transport rate of another compound such as, both glucose and fructose inhibited the uptake of xylose. Glucose was comparatively more inhibitory in nature than fructose. Transport of one amino acid was also inhibited in the presence of another amino acid. In glycine transport, leucine was stronger inhibitor than tyrosine. Almost similar results have been observed in both fishes irrespective of different feeding habits.     

Synaptic organization of the nucleus rotundus in some teleosts

Synaptic organization of the nucleus rotundus was studied with the electron microscope in three teleost species belonging to the same order. In spite of the different histological organization (non-laminated, incompletely laminated, and laminated), the same kinds of axon terminals (S and F) are observed in all species. A fibrous layer which is clearly formed only in the laminated nucleus is composed of F1 terminals and dendrites from a layer of small cells. The same kind of synapses formed between F1 terminals and dendrites of small cells are also found among glomeruli in the non-laminated and incompletely laminated nuclei. The main constituents of glomeruli are S and F2 terminals and dendrites of large cells in the non-laminated and incompletely laminated nuclei, and are S terminals and star-like structures which correspond to the tips of the dendrites of large cells in the laminated nucleus. The star-like structure contains numerous mitochondria and clusters of small polymorphic vesicles. Some of the vesicles aggregate at thickened cell membranes of the structure as in presynaptic dendrites.     

Specific reduction of development of the Mauthner neuron lateral dendrite after otic capsule ablation in Brachydanio rerio

During development, afferent fibers may stimulate development of postsynaptic target neurons. By surgically ablating an otic vesicle in zebrafish embryos 30 hr after fertilization we deprived the developing Mauthner (M) neuron of vestibular axonal input to its lateral dendrite. After 8 days, 14 M cells were examined by light microscopy, and in each case the size and branching of the lateral dendrite was reduced. No consistent changes were observed in shape and size of other regions of the deprived cells or in the contralateral control cells. Synapses onto five of these pairs of cells were examined by electron microscopy. Except for missing vestibular terminals on the deprived dendrites, the synaptic input to the dendrites and to other regions of the M cell was normal in distribution and pattern. These data suggest that growth-promoting or trophic effects of vestibular axons upon the M cell are localized to its lateral dendrite.     

Long-term effects of alloxan-induced diabetes on the nucleus ventralis posterolateralis in the thalamus of the rat.

The present paper describes the long-term ultrastructural changes in the nucleus ventralis posterolateralis of the thalamus of male Wistar rats after alloxan-induced diabetes. Degenerating dendrites were characterized by an electron-dense cytoplasm with scattered endoplasmic reticulum and ribosomes. Degenerating axon terminals were characterized by an electron-dense cytoplasm and clustering of small spherical agranular vesicles. Degenerating axon terminals formed axosomatic synapses with seemingly normal cell bodies and axodendritic synapses with normal as well as degenerating dendrites. Degenerating axons (both myelinated and unmyelinated) were readily encountered in the neuropil. Activated microglial and astrocytic cells in the neuropil were in the process of phagocytosis or had residua in their cytoplasm.