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M J Cavey

Publications and source records attributed to M J Cavey.

12 recordsLinked to original sources
PubMed2004

The steady-state force-Ca2+ relationship in intact lobster (Homarus americanus) cardiac muscle.

The heart of the decapod crustacean is activated by regular impulse bursts from the cardiac ganglion. The cardiac pump function depends on ganglionic burst frequency, burst duration, and burst impulse frequency. Here, we activated isolated lobster cardiac ostial muscle (Orbicularis ostii muscle, OOM) by stimulus trains in vitro in order to characterize the response of the contractile apparatus to [Ca2+]i. We employed stimulus trains that generate a steady state between the [Ca2+]i and force in order to estimate the Ca2+ sensitivity of myofilaments. Force and [Ca2+]i transients were simultaneously recorded using a silicon strain gauge and the fluorescence of iontophoretically microinjected fura-2 salt. We examined the effects of tetanus duration (TD), the interval between trains, and 6 microM cyclopiazonic acid, an inhibitor of the SR Ca2+ pump, on the steady-state force-[Ca2+]i relationship. The instantaneous force-[Ca2+]i relationships appeared sigmoidal (EC50 and Hill coefficient, 98.8+/-32.7 nM and 2.47+/-0.20, mean +/- SD, respectively), as did the curves superimposed after 500 ms following the start of stimulation, indicating that the force-[Ca2+]i relationship had reached a steady state at that time. Also, the maximum activated force (Fmax) was estimated using the steady-state force-[Ca2+]i relationship. Prolonged stimulus trains, decreasing the interval between recurrent trains from 5 to 2.5 s, and cyclopiazonic acid each increased the measured EC50 without changing Fmax. The EC50 correlated strongly with averaged [Ca2+]i over time. We conclude that the steady-state force-[Ca2+]i relationships in the OOM indicate cooperation between force generation and Ca2+ binding by the myofilaments. Our data also suggest the existence of a novel Ca2+-dependent mechanism which reduces Ca2+ sensitivity and accelerates relaxation of lobster cardiac muscle myofilaments.

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PubMed1993

Development of the liver in the chicken embryo. II. Erythropoietic and granulopoietic cells.

Hepatic hemopoiesis is apparent in the chicken embryo on day 7 of incubation (Hamburger and Hamilton Stage 30), and a peak in hemopoietic activity occurs on day 14 (Stage 40). During this period, the differentiation of hemopoietic cells was examined by light microscopy and by transmission and scanning electron microscopy. Glycol methacrylate sections were used in lieu of smears to study hemopoietic cells, thus minimizing the problems of cell shrinkage and rupture. The sections were superior to smears for close examination of nuclear and cytoplasmic morphologies and for precise localization of hemopoietic cells to intravascular and extravascular sites. The avian liver is involved directly with erythropoiesis and granulopoiesis only. Erythropoietic cells, occurring in intravascular and extravascular locations, appear throughout the time frame examined. Blood islands with granulopoietic cells were not observed until days 8-9 (Stage 35). Granulopoiesis in the liver produces only eosinophilic leukocytes. Individual granulopoietic cells appear first in the connective tissue sheaths of hepatic vessels, and these cells subsequently congregate into blood islands. Endothelial cells of the sinusoidal linings, through asymmetric divisions, frequently release daughter cells into the circulation, and Kupffer cells are actively engaged in phagocytosis of erythrocytes. From a comparative standpoint, the elements deemed critical to hemopoiesis in the mammalian liver--prehepatocyte population, hepatic vasculature, and compartments for stem cell differentiation--may not hold the same importance in the bird, owing to an inordinate reliance on intravascular hemopoiesis in this vertebrate class.

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PubMed1992

Development of the liver in the chicken embryo. I. Hepatic cords and sinusoids.

Hemopoiesis in the liver of the chicken embryo begins on day 7 of incubation (Hamburger and Hamilton Stage 30) and peaks on day 14 (Stage 40). During this time frame, the differentiation of hepatic cells was examined by light microscopy, transmission and scanning electron microscopy, and morphometry. The avian liver is a closely packed mass of dendriform cords and discontinuous sinusoids. Hepatocytes are pyramidal in shape, and they ring the bile canaliculi which run through the centers of the cords. Semithin sections, made possible by infiltration and embedding in glycol methacrylate, were stained with hematoxylin and eosin to assess the general architecture of the organ and the lipid content of the hepatocytes and by the periodic acid-Schiff reaction and hematoxylin to visualize the cytoplasmic stores of glycogen. The number of hepatocytes with demonstrable glycogen fluctuates erratically in early hemopoiesis, and the proportion of glycogen-containing cells progressively increases as hemopoiesis climbs to a peak. Most differentiating hepatocytes are devoid of lipid droplets until Stages 39 and 40. From Stage 30 to 35, hepatocyte volume falls to its lowest value. Subsequently (Stages 36 to 40), cell volume increases and hepatocytes achieve a relatively uniform size. Ultrastructural changes in the differentiating hepatocytes, including alterations to the mitochondria, endoplasmic reticulum, and Golgi apparatus, are documented. These morphological and morphometric findings on the prehepatocyte population and hepatic vasculature cover 2 of the 3 elements deemed critical to hepatic hemopoiesis in many vertebrates.

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PubMed1988

The effects of heat shock on the morphology and protein synthesis of the epidermis of Xenopus laevis larvae.

By scanning electron microscopy, we have observed that a 20-min heat shock at 37 degrees C, although not lethal, causes extensive damage to the epidermis of 30-h and 2-d (post-fertilization) Xenopus laevis larvae. The primary effects of heat shock are the apical swelling of the epidermal cells, giving the epidermis a "cobblestone" appearance, and the selective shedding of the ciliated cells. The shed cells may be cell fragments, however, because some of them are anucleate. Shed cells also exhibit the enriched synthesis of a group of heat shock proteins of 62,000 D molecular weight, suggesting that these proteins are specific to the shed cells. Prolonged heat shock of these larvae (i.e., 30 min at 37 degrees C) results in the complete disintegration of the epidermis, followed by larval death. At later stages of development (3-d and 4-d post-fertilization), the epidermis becomes more resistant to heat-induced damage inflicted by a 20-min heat shock. This increase in resistance coincides with the development of large secretory cells and the loss of ciliated cells in the epidermis and thus parallels a change in the state of histological differentiation.

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PubMed1983

Ultrastructure and differentiation of ascidian muscle. II. Differentiation of the caudal muscle cells in the larva of Diplosoma macdonaldi.

The larval muscle cells of Diplosoma macdonaldi contain subcortical and medullary myofibrils which are invested by fenestrated sheets of the sarcoplasmic reticulum. Cisternae of the sarcoplasmic reticulum are coupled with tubular invaginations of the sarcolemma. To appreciate better such uncommon features of cellular organization, six embryonic stages were selected for an ultrastructural study of myogenesis. The proliferative, synthetic, and elaborative phases of myogenesis were represented by embryos ranging from neurulae to prehatching larvae. The contractile apparatus originates during the synthetic phase of myogenesis, when thick and thin myofilaments appear in the cortical sarcoplasm at the epidermal and notochordal poles of the cell. The myofilaments promptly aggregate into unstriated fascicles, and the fascicles unite in series to establish the rudimentary myofibrils. All major sarcomeric bands, except the Z-lines, are evident along the myofibrils. Cisternae of the sarcoplasmic reticulum form peripheral couplings with the overlying sarcolemma, and they also form interior couplings with sarcolemmal invaginations from the ends of the cell. The interior couplings localize over the I-bands of the myofibrils. In the elaborative phase of myogenesis, mitochondria invade the cortical sarcoplasm, and the contractile apparatus passively shifts to the subcortex and medulla of the cell. Relocation of the myofibrils coincides with the disappearance of all peripheral couplings. Cisternae of the sarcoplasmic reticulum anastomose around the myofibrils, creating the fenestrated sheets that extend between sarcomeres. As Z-lines begin to bisect the I-bands, the perifibrillar cisternae become confluent with the cisternae in the precocious interior couplings.

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PubMed1982

Ascidian larval tunic: Extraembryonic structures influence morphogenesis.

The larval tunic of Corella inflata is composed of two cuticular layers, extracellular filaments and ground substance. It lies outside the epidermis and most of it is known to be produced by the epidermis. The dorsal, ventral and caudal fins are specialized parts of the tunic that are essential for larval locomotion. The following hypothesis was tested: Morphogenesis of the larval fins is dependent upon the presence of extraembryonic structures (test cells, chorion or follicle cells) before completion of the late tail bud stage of development. We tested this by dechorionating embryos of Corella inflata and Ascidia paratropa. The operation removes all extraembryonic structures. It was performed mainly on neurula, early tail-bud and late tail-bud stages. Fin formation is inhibited when neurulae are dechorionated but not when late tail-bud or older embryonic stages are dechorionated. Dechorionated neurulae produce all of the major components of the tunic (cuticular layers, filaments and ground substance) but they are unable to form functional fins. At the time of dechorionation, in all experiments, the embryos had no fins. Removal of the follicle cells does not inhibit fin formation. The test cells are known to secrete granular "ornaments" that attach to the surface of the tunic. The fibrous, acellular chorion may serve to contain the test cells and their products or products of the embryo that are not firmly attached. The test cells may induce or control the morphogenesis of the larval fins in ascidians before the late tail-bud stage of development. We suggest ways of testing this hypothesis and an alternative hypothesis.

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PubMed1981

Ultrastructure of the coelomic lining in the podium of the starfish Stylasterias forreri.

Ultrastructural examination of the podium of the asteroid echinoderm Stylasterias forreri reveals that cells of the coelomic epithelium and cells of the retractor muscle are, in fact, components of a single epithelium. The basal lamina of this unified epithelium adjoins the connective tissue layer of the podium. The principal epithelial cells in the coelomic lining are the flagellated adluminal cells and the myofilament-bearing retractor cells. Adluminal cells interdigitate extensively with each other and form zonular intermediate and septate junctions at their apicolateral surfaces. The adluminal cells emit processes which extend between the underlying retractor cells and terminate on the basal lamina of the epithelium. Retractor cells exhibit unregistered arrays of thick and thin myofilaments. The periphery of the retractor cell is characteristically thrown into keel-like folds which interdigitate with the processes of neighboring cells. Specialized intermediate junctions bind the retractor cells to each other and anchor the retractor cells to the basal lamina of the epithelium. The retractor cells are not surrounded by external laminae or connective tissue envelopes. It is concluded that the coelomic lining in the podium of S. forreri is a bipartite epithelium and that the retractor cells of the podium are myoepithelial in nature. There are no detectable communicating (gap) junctions between the epithelial cells of the coelomic lining.

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PubMed1981

Specializations for excitation-contraction coupling in the podial retractor cells of the starfish Stylasterias forreri.

Ultrastructural examination of the podium of the asteroid echinoderm Stylasterias forreri has revealed that cells of the coelomic epithelium and cells of the retractor muscle should be considered as components of a single epithelium. The podial retractor cells are, therefore, myoepithelial in nature. This report concentrates on those ultrastructural features of the retractor cells that are most likely involved with excitation-contraction coupling. The spatial arrangement of the sarcoplasmic reticulum, the couplings between the sarcoplasmic reticulum and sarcolemma, and an intramembranous specialization of the sarcolemma are documented and discussed. Current concepts regarding the innervation of the retractor cells of the podium and the protractor cells of the ampulla are reviewed, and specific proposals for further investigation of podial innervation are outlined.

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PubMed1976

Ultrastructure and differentiation of ascidian muscle. I. Caudal musculature of the larva of Diplosoma macdonaldi.

The larval caudal musculature of the compound ascidian Diplosoma macdonaldi consists of two longitudinal bands of somatic striated muscle. Approximately 800 mononucleate cells, lying in rows between the epidermis and the notochord, constitute each muscle band. Unlike the caudal muscle cells of most other ascidian larvae, the myofibrils and apposed sarcoplasmic reticulum occupy both the cortical and the medullary sarcoplasm. The cross-striated myofibrils converge near the tapered ends of the caudal muscle cell and integrate into a field of myofilaments. The field originates and terminates at intermediate junctions at the transverse cellular boundaries. Close junctions and longitudinal and transverse segments of nonjunctional sarcolemmata flank the intermediate junctions, creating a transverse myomuscular (TMM) complex which superficially resembles the intercalated disk of the vertebrate heart. A perforated sheet of sarcoplasmic reticulum (SR) invests each myofibril. The sheet of SR spans between sarcomeres and is locally undifferentiated in relation to the cross-striations. Two to four saccular cisternae of SR near each sarcomeric Z-line establish interior (dyadic) couplings with an axial analogue of the vertebrate transverse tubular system. The axial tubules are invaginations of the sarcolemma within and adjacent to the intermediate junctions of the TMM complex. The caudal muscle cells of larval ascidians and the somatic striated muscle fibers of lower vertebrates bear similar relationships to the skeletal organs and share similar locomotor functions. At the cellular level, however, the larval ascidian caudal musculature more closely resembles the vertebrate myocardium.

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