Electrocyte Columns Research Articles

The diet of the coffin ray, Hypnos monopterygius (Shaw, 1795), and predation mode inferred from jaw, dentition and electric organ morphology

The diet of the coffin ray, Hypnos monopterygius (Family Hypnidae), an electric ray endemic to Australia, was described on the basis of stomach contents analysis. Benthos-associated teleost fishes and cephalopod molluscs, predominantly Octopus, dominated the diet. The large, paired electric organs of this species scaled close to isometrically in relation to body mass for electric-organ area and electric-organ mass. The number of electrocyte columns that comprise each electric organ (mean=435 columns) was independent of body mass. Jaws were long and slender, and supported small tricuspidate teeth only on the central 55% of their length. Diet, electric-organ size and jaw morphology in Hypnos and the torpedoes (Family Torpedinidae) are similar, but in marked contrast to members of the numbfishes (Family Narcinidae). The differences are interpreted in relation to the use of electric-organ discharge while foraging in the former families.

Ontogeny of the Electric Organs in the Electric Eel, Electrophorus electricus: Physiological, Histological, and Fine Structural Investigations

This study attempts to clarify the controversy regarding the ontogenetic origin of the main organ electrocytes in the electric eel, Electrophorus electricus. The dispute was between an earlier claimed origin from a skeletal muscle precursor [Fritsch, 1881], or from a distinct electrocyte-generating matrix, or germinative zone [Keynes, 1961]. We demonstrate electrocyte formation from a metamerically organized group of pre-electroblasts, splitting off the ventralmost tip of the embryonic trunk mesoderm at the moment of hatching from the egg. We show details of successive stages in the development of rows of electric plates, the electrocytes, by means of conventional histology and electron microscopy. The membrane-bound pre-electroblasts multiply rapidly and then undergo a specific mitosis where they lose their membranes and begin extensive cytoplasm production as electroblasts. Electrical activity, consisting of single and multiple pulses, was noticed in seven-day-old larvae that began to exhibit swimming movements. A separation of discharges into single pulses and trains of higher voltage pulses was seen first in 45-mm-long larvae. A lateralis imus muscle and anal fin ray muscles, implicated by earlier investigators in the formation of electrocytes, begin developing at a time in larval life when eight columns of electrocytes are already present. Axonal innervation is seen very early during electrocyte formation.

Anatomy and motor pathways of the electric organ of skates.

The electric organ of skates is a paired structure within the tail consisting of two longitudinal columns of electrocytes contained within the lateral musculature on each side of the vertebral column. The electrocytes develop from hypaxial skeletal muscle fibers, and, depending upon the species, are generally classified as either cup-shaped or disc-shaped. The disc-shaped electrocytes are considered to be the more derived type. Regardless of the morphology of the electrocyte, the electric organ discharge of all skates is characterized as a weak asynchronous and long-lasting signal. Although recent behavioral investigations have revealed a communicative function for the electric organ, details as to which specific behaviors utilize this motor system remain uncertain. The electric organ is innervated by segmental motor nerves that branch from the ventral root of caudal spinal nerves at all levels of the electric organ. The cells of origin of the electromotor nerves, or electromotoneurons (EMNs), are large multipolar neurons with extensive dendrites located within the ventral gray matter of the spinal cord. The EMNs are uniformly distributed among the somatic motoneurons at levels corresponding to the rostrocaudal extent of the electric organ, and therefore do not form a discrete nucleus. The medullary command nucleus is comprised of neurons located within the nucleus raphe magnus, and forms a descending spinal pathway to the EMNs.

High levels of phosphorylation in minor phospholipids of Discopyge tschudii electrocyte membranes

Phospholipid phosphate group metabolism has been studied in the electrocyte of Discopyge tschudii (Torpedinidae), following for several hours the incorporation and distribution of [(32)P] in living electrocyte stacks and acetylcholine receptor membranes prepared therefrom. Conditions are reported for the incubation of electrocyte columns in vitro with [(32)P]O(4)Na(2)H resulting in active and sustained incorporation of the precursor in minority phospholipids. Phosphatidic acid, phosphatidylinositol, phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-bisphosphate, representing 2.3, 2.1, 0.3 and 0.2% respectively of the total phospholipids were almost exclusively labelled. Relative specific activities of 13, 7, 56 and 140 respectively were attained. These values were much higher than those of major phospholipids like phosphatidylcholine (0.03) or phosphatidylethanolamine (0.01). Subcellular fractionation of the metabolically active electrocyte stacks yielded various membrane fractions differing in their degree of [(32)P]-labelling, specially in phosphatidic acid and polyphosphoinositides. The fraction richest in nicotinic acetylcholine receptor protein showed the highest levels of [(32)P] incorporation. The results indicate that phosphorylation reactions are actively operative in certain phospholipid classes of the electrocyte and in membranes obtained from these cells after in vitro labelling with [(32)P]. In particular, phosphomonoester groups in polyphosphoinositides and the phosphate group in phosphatidic acid display a high metabolic activity.

Is propionylcholine present in or synthesized by electric organ?

When small blocks comprising four columns of electrocytes were excised from electric organs of Torpedo marmorata after stimulation in vivo via the electric lobe at 1 Hz for 1 h and allowed to recover at 20-22 degrees C for several hours in medium containing 100 microM d4 choline and 500 microM propionate, small quantities of propionylcholine amounting to no more than 1% of the endogenous acetylcholine of the tissue could be detected in tissue extracts by gas chromatography-mass spectrometry (GCMS). Kinetic studies demonstrated that there was no nonexchangeable propionylcholine in the tissue and in the absence of added propionate, propionylcholine levels were less than 0.2% of tissue acetylcholine. Vesicular propionylcholine amounted to less than 0.5% of vesicular acetylcholine and the distribution of d0 and d4 propionylcholine suggested that an appreciable proportion (up to one-third) of this could be an artifact of preparation for GCMS determinations. Propionylcholine formation during extraction and demethylation of an artificial mixture of acetylcholine, choline, and propionate was indeed detected. It is concluded that propionylcholine has no significance as an endogenous or as a false transmitter at this terminal, in conformity with the work of Sheridan et al. [Z. Zellforsch. 74, 281-307 (1966)] but in contrast to the report of O'Regan [J. Neurochem. 39, 764-772 (1982)].

Torpedo electromotor system development: a quantitative analysis of synaptogenesis.

Synaptogenesis in the electric organ of Torpedo marmorato has been studied quantitatively at the ultrastructural level of observation. In addition to establishing the normal developmental time course for this event we were interested in determining whether a gradient of synaptogenesis might be present because the electric organ produces several morphologically recognizable spatiotemporal gradients during its early ontogeny. These gradients genesis of electrocyte columns, both gradients of which are operative for periods of weeks. No gradient of synaptogenesis was found, indicating this to be a synchronous process. The idea is advanced that synaptogenesis in the electric organ is modulated by extrinsic influences, many of which may originate from the target electrocytes which, by this time, have become synchronized in their development.

Organotypic culture of embryonic electromotor system tissues from Torpedo marmorata

An explant culture system has been used to study the electric organ and electric lobe tissues of Torpedo marmorata at different stages during the development of the electromotor system. The myotubes in tissue expiants, taken from the electric organ primordia of 33–38 mm body-length embryos prior to electrocyte differentiation, contract spontaneously on explantation and have electrogenic membranes. The myotubes subsequently lose these properties in vitro and can differentiate in the absence of neural tissue into immature electrocytes which have morphologically characteristic postsynaptic membranes. Isolated expiants of differentiated electric organ tissue from 60–100 mm body-length embryos can be maintained for 3 to 4 weeks in vitro but cellular outgrowth is minimal. In contrast, a rapid, dense outgrowth of cells and a subsequent regeneration of myotubes occurs when differentiated electric organ explants are co-cultured with electric lobe tissue from embryos of the same stage. Cellular outgrowth from differentiated electric-organ tissue expiants can be stimulated by spinal cord, medulla, cerebellum and heart tissues but a subsequent regeneration of myotubes has not been observed. Myotube regeneration in the presence of electric lobe tissue is maximal with tissue from 60–80 mm body-length embryos. The myotubes that regenerate from differentiated electric organ expiants have not been observed to differentiate into electrocytes. Neuritic outgrowth in vitro occurs with electric lobe tissue taken at two different embryonic stages. The first stage corresponds to a period when most of the neuroepithelial cells in the lobe anlagen are withdrawing from the mitotic cycle and projecting axons into the branchial arches. The second, later stage is when the electromotorneurones are normally generating axon collaterals that are invading the interelectrocyte space of electrocyte columns. Maximum neuritic outgrowth at this second, later stage is obtained with tissue from 60–80 mm body-length embryos. Although neuritic invasion of electrocyte column expiants can be obtained in electric organ—electric lobe co-cultures at this later stage, synapses similar to those observed during the early stages of synaptogenesis in the electric organs in vivo have not been observed in vitro.

The developmental morphology of Torpedo marmorata: electric organ--myogenic phase.

The early development of the electric organ of Torpedo marmorata has been examined by light and electron microscopy to the 40-mm stage of embryo growth. The myogenic nature of this tissue is confirmed ultrastructurally by the presence of myoblasts and myotubes both containing myofibrils cross striated with Z,A and I bands. Fusion between these cells is also found taking place. A scheme is presented to explain the development of the overall structural plan of the organ and specifically the formation of the future electrocyte columns. AT 40 mm, a series of morphological transformations signals the onset of a divergent developmental pattern ultimately leading to the establishment of mature electrocyte columns. These features include rounding up of myotubes, dissolution of myofibrils and the appearance of intermediate size filaments (11 nm) and perhaps a non-muscular actin (5.5 nm). This early myogenic phase of development occurs in the absence of any specific nervous contact even though electromotor nerves are always in close proximity.