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PubMed1983

The system of cerebrospinal fluid-contacting neurons.

Cerebrospinal fluid (CSF)-contacting neurons are located periventricularly or inside the brain ventricles; they contact the CSF via their dendrites, perikarya or axons. Most of the CSF-contacting nerve cells send dendritic processes into the ventricular cavity where they form ciliated terminals. These ciliated dendritic endings resemble those of known sensory cells, yet their role is still unknown. There are two types of CSF-contacting dendritic terminals. One bears solitary 9 X 2 + 0 cilia; it is present in different hypothalamic regions such as the paraventricular organ and the vascular sac. The magnocellular neurosecretory nuclei also contain CSF-contacting neurons, which probably furnish information about the parameters of the CSF for the regulatory function of the hypothalamo-hypophyseal system. CSF-contacting nerve cells of the parvocellular hypothalamic nuclei are suspected to participate in hypothalamo-adenohypophyseal regulation. A second type of CSF-contacting dendritic terminal bears many stereocilia and is found in the central canal of the spinal cord. This type of terminal is also supplied with a 9 X 2 + 2 kinocilium that may contact Reissner's fiber, the secretory material of the subcommissural organ. Resembling mainly mechanoreceptors, these spinal CSF-contacting neurons appear to form axon terminals of the neurosecretory type at the external circumference of the spinal cord. Developing and/or regressing photoreceptor cells of the retina and pineal complex may display a similar dendritic structure characteristic of hypothalamic CSF-contacting neurons. Axons penetrating into the ventricles innervate the apical surface of the ependyma and/or the CSF-contacting dendritic terminals. Some bipolar neurons of the retina form so-called Landolt's clubs; these may be considered as the retinal component of the CSF-contacting neuronal system. Since in the lancelet nearly all nerve cells contact the CSF, the CSF-contacting neurons represent a specialized, but phylogenetically old cell type, a "protoneuron" in the vertebrate brain. They may be derived phylogenetically by inversion of the ciliated neurons found in the plate-like nervous system of more primitive deuterostomians.

Amphibians↗
PubMed2006

Amphioxus: a peaceful anchovy fillet to illuminate Chordate Evolution (I).

The cephalochordate amphioxus occupies a central place in evolutionary thoughts to the origin of Vertebrates. With a prototypical vertebrate-like body plan and a preduplicative genome, the friendly lancelet seems to be in morphological and genetic motionless since its separation from the major branch of evolution that eventually ended up in our corner in the Animal Kingdom. This makes it an ideal model system with which, with the current development of genomic and experimental tools, an Evo-Devo approach to the understanding of the origin of vertebrates looks proper, reliable, and excitingly promising.

Animals↗
PubMed2001

The effects of trawling on the benthic fauna of the Gulf of Nicoya, Costa Rica.

Four van Veen grab replicates where collected to sample macrofauna (organism retained on a 500 micron mesh sieve) at four stations in the Gulf of Nicoya, during October 24, 1997, January 16 and April 30, 1998. This information was used to search for any effects of trawling on the benthic fauna. Two stations where located in a trawled area, and two stations where in a protected area. Diversity (H') varied from 2.01 to 3.52 in the trawled area and from 2.13 to 2.78 in the protected area. Diversity was generally higher in the trawled area, and this was in contradiction to what we would have expected from other studies where the trend has been that trawling reduces diversity. Brittlestars and lancelets seemed to be the groups mostly harmed by the trawling, while amphipods where more abundant in trawled areas. The multivariate analyses did not reveal the patterns of faunal change as well as we hoped. This is surely because of our lack of more replicate samples. The multivariate analyses are easily confounded when few sites are analyzed. We have found differences in the type of fauna found in trawled and protected areas and, considering the differences in environmental variables in our stations and our lack of replication, this indicates that there are differences and a larger investigation is in order to reveal its magnitude.

Animals↗
PubMed1979

A comparison of epithalamic, hypothalamic and spinal neurosecretory terminals.

Nerve endings of epithalamic, hypothalamic and spinal neurosecretory areas were studied by light and electron microscopy in various vertebrates (from fishes up to mammals) including the lancelet. Areas investigated were the pineal organ, the pulvinar corporis pinealis, the neurohypophysis, the median eminence, the urophysis, the terminal filum and the medullo-spinal neurosecretory zones. We found that in all these areas the neurosecretory endings have common structures, which we call synaptic hemidesmosomes or neurohormonal terminals. These are characterized by accumulation of vesicles, and dense projections in a terminal on the basal lamina of the surface of the nervous tissue. A critical review of the literature suggests that a considerble neuroendocrine activity is associated with synaptic hemidesmosomes as special neurohormonal effector structures of the nerve cells. The cell-to-cell synapses formed by neurosecretory cells are discussed in connection with the dual capacity of these cells to function as both endocrine and "ordinary# neuronal elements. The importance of the external cerebrospinal fluid (CSF) space for the transport of materials released in the so-called neurohemal areas, is stressed.

Amphibians↗
PubMed1961

Structure of the longitudinal body muscles of amphioxus.

The structure of the longitudinal body muscles of Branchiostoma caribaeum has been studied by light and electron microscopy. These muscles are shown to be composed of fibers in the form of flat lamellae about 0.8micro in thickness, more than 100 micro wide, and reaching in length from one intermuscular septum to the next, a distance of about 0.6 mm. Each flat fiber is covered by a plasma membrane and contains a single myofibril consisting of myofilaments packed in the interdigitating hexagonal array characteristic of vertebrate striated muscle. Little or no sarcoplasmic reticulum is present. Mitochondria are found infrequently and have a tubular internal structure. These morphological observations are discussed in relation to a proposed hypothesis of excitation-contraction coupling. It is pointed out that the maximum distance from surface to myofilament in these muscles is about 0.5 micro and that diffusion of an "activating" substance over this distance would essentially be complete in less than 0.5 msec. after its release from the plasma membrane. It is concluded that the flat form of amphioxus muscle substitutes for the specialized mechanisms of excitation-contraction coupling thought possibly to involve the sarcoplasmic reticulum in higher vertebrate muscles.

Animals↗
PubMed2026

The highly heterozygous European amphioxus (Branchiostoma lanceolatum) at the edge of panmixia.

Amphioxus (Cephalochordata) are small marine chordates that have broad ecological ranges, yet as adults form local settlements and exhibit limited mobility. Genomic surveys of two amphioxus species have suggested that they rank among the most genetically diverse metazoans. Here, we present the first accurate assessment of genomic diversity in the European amphioxus (Branchiostoma lanceolatum) and investigate the processes underlying this diversity. We leverage whole-genome sequencing data from multiple individuals sampled at two geographically distant Atlantic and Mediterranean locations. Consistent with previous estimates in other amphioxus species, we measure exceptionally high genomic diversity, with an average heterozygosity of 2.73% in B. lanceolatum. Despite the large geographic separation between sampling sites, population differentiation is minimal, indicating extensive gene flow among distant adult settlements. Phylogenetic analyses combined with population genetic simulations confirm that this elevated genomic diversity is primarily driven by a large effective population size. Although adult amphioxus have limited mobility, our results indicate that long-distance larval dispersal mediated by ocean currents is sufficient to generate a near-panmictic population structure across their broad ecological range.

Animals↗