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P Sheard

Publications and source records attributed to P Sheard.

At least 19 recordsLinked to original sources
PubMed2000

Examination of intrafascicular muscle fiber terminations: implications for tension delivery in series-fibered muscles.

Mammalian skeletal muscles with long fascicle lengths are predominantly composed of short muscle fibers that terminate midbelly with no direct connection to the muscle origin or insertion. The manner in which these short fibers terminate and transmit tension through the muscle to their tendons is poorly understood. We made an extensive morphological study of a series-fibered muscle, the guinea pig sternomastoid, in order to define the full range of structural specializations for tension transmission from short fibers within this muscle. Terminations were examined in single fibers, teased small bundles of fibers, and in sections at both the light and electron microscopic level. In many cases, sites of fiber termination were defined by reactivity for the enzyme acetylcholinesterase, which also marks myotendinous junctions. Additionally, transport of the lipophilic fluorescent dye, DiI, or injection of Lucifer Yellow were used to visualize undisturbed fiber terminations in whole muscles using confocal and fluorescence microscopy. At the light microscopic level, we find that intrafascicularly terminating fibers end about equally often in either a long progressive taper, or in a series of small or larger blunt steps. Combinations of these two morphologies are also seen. However, when analyzed at higher resolution with confocal or electron microscopy, the apparently smooth progressive tapers appear also to be predominantly composed of a series of fine stepped terminations. Stepwise terminations in most cases join face-to-face with complementary endings of neighboring muscle fibers, some via an extended collagenous bridge and others at close interdigitating myomyonal junctions. These muscle-to-muscle junctions show many of the features of myotendinous junctions, including dense subsarcolemmal plaques in regions of myofibrillar termination and we suggest that they serve to pass tension from fiber to fiber along the longitudinal axis of the muscle. In addition, we observe regions of apparent side-to-side adhesion between neighboring fibers at sites where there is no apparent fiber tapering or structural specialization typical of myofibril termination. These sites show acetylcholinesterase reactivity, and large numbers of collagen fibers passing laterally from fiber to fiber. These latter connections seem most likely to be involved in lateral transmission of tension, either from fiber to fiber, or from fiber to endomysium. Overall, our results suggest that tension from intrafascicularly terminating fibers is likely to be passed along the muscle to the tendon using both in-series and in-parallel arrangements. The results are discussed in light of current theories of tension delivery within the series-fibered muscles typical of large, nonprimate mammals.

Acetylcholinesterase↗
PubMed1999

Alpha motoneurons are present in normal numbers but with reduced soma size in neurotrophin-3 knockout mice.

Neurotrophin-3 (NT-3) is essential for survival of proprioceptive and mechanoreceptor neurons, but its role in motoneuron development in vivo has seemed slight. Recent evidence however has suggested that NT-3 may be involved in motoneuron maturation. Here, we quantitatively assess the number and state of development of motoneurons within the lumbar lateral motor column (LLMC) of newborn NT-3 (-/-), (+/-) and (+/+) mutant mice. We find that the number of alpha motoneurons in the LLMC is the same in all genotypes, but soma size is significantly reduced in (-/-) mutants. This suggests that NT-3 is not required for normal production and survival of alpha motoneurons, but is essential for their full maturation and/or maintenance. We also confirm a previous report that gamma motoneurons are absent in NT-3 (-/-) mice.

Animals↗
PubMed1990

Fate mapping the avian epiblast with focal injections of a fluorescent-histochemical marker: ectodermal derivatives.

A microinjection technique is described for fate mapping the epiblast of avian embryos. It consists of injecting the epiblast of cultured blastoderms with a fluorescent-histochemical marker, examining rhodamine fluorescence at the time of injection in living blastoderms, and assaying for horseradish peroxidase activity in histological sections obtained from the same embryos collected 24 h postinjection. Our results demonstrate that this procedure routinely marks cells, allowing their fates to be determined and prospective fate maps to be constructed. Two such maps are presented for ectodermal derivatives of the epiblast: one for late stages of Hensen's node progression (stages 3c through 4) and one for early stages of node regression (stages 4 + through 5). These new maps have six significant features. First, they show that regardless of whether the node is progressing or regressing, the flat neural plate extends at least 300 microns cranial to, 300 microns bilateral to and 1 mm caudal to the center of Hensen's node. Second, they confirm our previous fate mapping studies based on quail/chick chimeras. Namely, they show that the prenodal midline region of the epiblast forms the floor of the forebrain and the ventrolateral part of the optic vesicles as well as MHP cells (i.e., mainly wedge-shaped neurepithelial cells contained within the median hinge point of the bending neural plate); in contrast, paranodal and postnodal regions contribute L cells (i.e., mainly spindle-shaped neurepithelial cells constituting the lateral aspects of the neural plate). Third, they reveal a second source of MHP cells, Hensen's node, verifying previous studies of others based on tritiated thymidine labeling. Fourth, they demonstrate, in contrast to studies of other based on vital staining, carbon marking, and chorioallantoic grafting but in accordance with our previous studies based on quail/chick chimeras, that the cells contributing to the four craniocaudal subdivisions of the neural tube (i.e., forebrain, midbrain, hindbrain, and spinal cord) are not yet spatially segregated from one another at the flat neural plate stage, although more cranial neural plate cells tend to form more cranial subdivision and more caudal cells tend to form more caudal subdivisions. Thus, single injections routinely mark multiple neural tube subdivisions. Probable reasons for the discrepancy between our present results and the previous results of others is discussed. Fifth, they suggest that cells contributing to the surface ectoderm and neural plate are not yet completely spatially segregated from one another at the flat neural plate stage, particularly in caudal postnodal regions. Sixth, they delineate the locations of the otic placodes.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Analysis of frequency of intermingling between labeled clones in Xenopus embryos.

These experiments were designed to provide data that would test alternative hypotheses of the basis of clonal intermingling during early development of Xenopus. Clones were initiated by injection of different fluorescent cell lineage tracers into two blastomeres in the same 512-cell embryo. In order to find out how frequently clonal intermingling or clonal separation developed between the progeny of two ancestral cells at different distances apart, the pair of injected cells was either adjacent, separated by one uninjected cell, or separated by two uninjected cells, chosen at random in the animal dorsal region of the embryo. The final positions of the labeled clones were analyzed at tailbud stages. The results closely match the probability of interclonal mingling predicted by a compartment model, in which two clones intermingle or remain separate as a function of the probability of a clonal restriction boundary developing between them, rather than as a continuous function of the distance between the two cells at the time of labeling.

Animals↗
PubMed1989

Shaping and bending of the avian neural plate as analysed with a fluorescent-histochemical marker.

Shaping and bending of the neural plate are cardinal events of neurulation. These processes are initiated in avian embryos shortly after the onset of gastrulation and concluded concomitantly with the completion of gastrulation. The epiblast undergoes extensive morphogenetic movements during gastrulation and neurulation, but the directions, distances, rates, mechanisms and roles of such rearrangements are largely unknown. To begin to understand these morphogenetic movements, we have mapped regional displacements of the epiblast by injecting a fluorescent-histochemical marker into selected prenodal, nodal and postnodal levels of the blastoderm. Lateral epiblast regions (600 microns lateral to the midline and consisting primarily of surface epithelium) are displaced craniomedially, medial regions (300 microns lateral to the midline and consisting of neural plate and preingressed mesoderm) predominantly medially, and midline regions (consisting of neural plate and primitive streak) predominantly caudally. Displacements within the avian neural plate parallel those previously described for the amphibian neural plate. Furthermore, similar tissue displacements occur within the prenodal and postnodal levels of the avian epiblast despite the fact that neurulation is occurring in the former and gastrulation in the latter. Finally, our results show that ectodermal rudiments contained within a single cross-sectional level of the embryo are a composite of cells derived from multiple craniocaudal and mediolateral levels. Thus, regional tissue displacements are important events to consider in the analysis of the early morphogenesis of axial and paraxial organ rudiments derived from the epiblast.

Animals↗
PubMed1987

Clonal restriction boundaries in Xenopus embryos shown with two intracellular lineage tracers.

It has been proposed that clonal restriction boundaries develop in Xenopus embryos between clones initiated at the 512-cell stage, and that these boundaries result in formation of morphological compartments, each populated by progeny of a group of ancestral cells. Although this hypothesis has gained some acceptance, it has also been criticized because the use of only one cell lineage tracer was not a conclusive test of the hypothesis. However, the critical experiment, an assessment of the extent of mingling between two labeled clones in the same embryo, has now been performed. A model of the proposed arrangement of the ancestral cell groups in the 512-cell embryo predicted that the two clones would remain separate in 49% of cases and intermingle in 51% of cases. In fact, there was a bimodal distribution, in which separation of the clones occurred in 46% of embryos and extensive interclonal mingling was observed in 54%. These results are not compatible with hypotheses in which a unimodal distribution of mingling would be predicted but are consistent with the compartment hypothesis.

Animals↗
PubMed1984

Critical periods in rat motoneuron development.

Reinnervation of rat internal intercostal muscles was examined 2 weeks after intramuscular axotomy in the period embryonic Day 17 (E17) to postnatal Day 2 (PN2). The efficiency of reinnervation depended on the day of denervation. Responses to nerve stimulation were common only in muscles denervated in the interval E19-21. Functional reinnervation was seldom seen in muscles denervated earlier, and was absent in muscles denervated shortly after birth. We suggest that there are "critical periods" in the sequence of differentiation of motoneurons. At E17 these neurons are very susceptible to cell death and any that lose contact with their muscle will die. Motoneurons axotomized later, while motor unit size is still increasing, can regenerate some functional nerve-muscle junctions. Motor unit size reaches its maximum by E21 and if motoneurons are axotomized after that time, in neonates, they cannot form more synaptic terminals although they can still extend axons. During this early postnatal period motoneurons normally reduce their number of nerve-muscle contacts. Finally, at 3 weeks of age, motoneurons reach their adult state when they are capable of regeneration relatively unhindered by restrictions on the number and location of their peripheral connections.

Animals↗
PubMed1980

Response of human isolated bronchial and lung parenchymal strips to SRS-A and other mediators of asthmatic bronchospasm.

1 The responses of human isolated bronchial and lung parenchymal strips to cumulative doses of slow reacting substance of anaphylaxis (SRS-A), histamine, prostaglandin F2 alpha (PGF2 alpha) and acetylcholine have been examined, after storing the tissues overnight in Krebs solution at 4 degrees C. 2 Both tissues contracted to all four agonists. The order of potency (as determined by height of maximal contraction) was: bronchial strip: acetylcholine greater than histamine = PGF2 alpha greater than SRS-A, and parenchymal strip: PGF2 alpha much greater than histamine = SRS-A greater than acetylcholine. 3 Maximal contractions to SRS-A of both the human bronchial and parenchymal strips were approx. 30% of the maximal contractions produced by the most potent agonist on each tissue (PGF2 alpha on the parenchymal strip and acetylcholine on the bronchial strip). SRS-A, therefore, does not have a powerful direct contractile effect on either parenchymal or bronchial strip of human lung, and is approximately equipotent on both tissues. A part of the broncho-constrictor activity of SRS-A in vivo may be mediated via indirect pathways. 4 The selective SRS-A antagonist, FPL 55712, was approximately equipotent in antagonizing contractions induced by SRS-A on both human bronchial and parenchymal strips.

Acetylcholine↗
PubMed1977

New orally effective chromosome derivatives for the treatment of asthma.

Two new chromone derivatives have been identified which possess oral anti-allergic activity in the rat PCA model of immediate hypersensitivity. They may have a wider spectrum of anti-allergic activity than disodium cromoglycate (SCG) since they are effective in in vitro tests involving sensitized basophils, in which SCG is inactive. Both compounds, when given orally, provide relief from experimental and clinical asthma in man.

Animals↗
PubMed1977

Further studies on the SRS-A antagonist FPL 55712.

FPL 55712, previously reported to be a selective antagonist of SRS-A on the guinea-pig ileum, has now been shown to be similarly selective, though less potent, in its antagonism of contractions induced by SRS-A on in vitro preparations of respiratory smooth muscle (human bronchial strips and guinea-pig tracheal tubes). In vivo, bronchoconstriction induced in guinea-pigs by the endogenous release of SRS-A was partially inhibited by relatively large intravenous doses of FPL 55712 and by inhalation of aerosol compound. FPL 55712 was very rapidly cleared following intravenous administration.

Animals↗
PubMed1975

Mediators of passive lung anaphylaxis in the rat.

Passive lung anaphylaxis (PLA) was investigated in rats sensitized by the intravenous injection of high titre reaginic antiserum prepared in rats. 2 The effect of various pharmacological antagonists on anaphylactic bronchoconstriction in vivo were examined. An antihistamine (mepyramine), a kallikrein inactivator (aprotinin) or a prostaglandin synthesis inhibitor (aspirin) did not inhibit PLA, whereas an anti-5-hydroxytryptamine agent (methysergide) and an anti-slow reacting substance-A agent (FPL 55712) significantly reduced the response. 3 Isolated perfused lungs taken from sensitized rats released, on challenge with the sensitizing antigen, histamine, 5-hydroxytryptamine, slow reacting substance of anaphylaxis (SRS-A) and prostaglandins, but no rabbit aorta contracting substance (RCS). 4 Disodium cromoglycate inhibited both anaphylactic bronchoconstriction in vivo and the anaphylactic release of mediators in vitro. Inhibition in vivo was dose-related. 5 Mediators from the intestine, the primary shock organ of anaphylaxis in the rat, did not contribute to the lung response. 6 Vagal reflex pathways were found not to be important in PLA in vivo. 7 The relationship between the mediators released following antigen challenge of passively sensitized rat lung in vitro and passive lung anaphylaxis in vivo is discussed.

Anaphylaxis↗