PubMed Health⌕ Search

Biomedical subjects

D A Wharton

Publications and source records attributed to D A Wharton.

At least 37 records · Page 2Linked to original sources
PubMed1990

Capture and penetration processes of the free-living juveniles of Trichostrongylus colubriformis (Nematoda) by the nematophagous fungus, Arthrobotrys oligospora.

The nematode-trapping fungus Arthrobotrys oligospora traps and invades all the free-living juvenile stages of the trichostrongyle nematode Trichostrongylus colubriformis. The processes of capture and invasion of the 1st-stage juveniles are described using differential interference contrast optics and 3-D reconstruction techniques. The adhesive responsible for capture is well preserved using a freeze-substitution technique for scanning electron microscopy. The invasion process of the ensheathed 3rd-stage juvenile of T. colubriformis takes much longer than in the 1st- or 2nd-stage juvenile and involves the formation of secondary infection pegs between the sheath and the cuticle which appear to penetrate the cuticle by physical pressure.

Animals↗
PubMed1990

Carbohydrate/lectin interactions between the nematophagous fungus, Arthrobotrys oligospora, and the infective juveniles of Trichostrongylus colubriformis (Nematoda).

Removal of the sheath of the ensheathed infective juvenile of Trichostrongylus colubriformis prevents capture by the nematophagous fungus Arthrobotrys oligospora. Exposure of the trap hyphae to a variety of saccharides, which may block a recognition system based on lectin/carbohydrate binding, failed to prevent capture but some saccharides did inhibit penetration and invasion by the fungus. Capture and penetration thus appear to be two distinct processes with capture being less specific than penetration. Carbohydrate residues were absent from the outer surface of the cuticle and the sheath but were present on the inner surface of the sheath. The limited accessibility of these lectin-binding sites may explain the slow process of infection of the infective juvenile by the fungus. The sheath does not protect the infective juvenile against attack by this nematophagous fungus.

Animals↗
PubMed1990

Preparation of the nematode-trapping fungus, Arthrobotrys oligospora, for scanning electron microscopy by freeze substitution.

A freeze-substitution technique for preparing fungal specimens for scanning electron microscopy is described. This involves cryofixation in liquid nitrogen, freeze substitution in methanol at -20 degrees C and critical-point drying. The trapping complexes and conidiophores of the nematophagous fungus Arthrobotrys oligospora are well preserved and retain their normal three-dimensional arrangement.

Animals↗
PubMed1989

An approach to the teaching of host/parasite population modelling.

Computer-aided learning is considered to be useful in the teaching of host/parasite population modelling. The STELLA programme for the Apple Macintosh microcomputer allows dynamic models to be developed by drawing a diagram of the interactions and then defining the relationships between variables in terms of simple mathematics or as graphs. Our experience in using this programme for project-based teaching is described.

Animals↗
PubMed1989

Cold tolerance mechanisms of the free-living stages of Trichostrongylus colubriformis (Nematoda).

1. All free-living stages of the nematode parasite of sheep, Trichostrongylus colubriformis Giles, survived exposure to freezing temperatures in contact with water, with the exception of the first-stage juvenile (J1). The third-stage juvenile (J3) was the most resistant stage. The order of relative survival of the different stages was different from that of the lowest F50 (the temperature at which 50% froze), suggesting that an ability to supercool was not the only determinant of survival. 2. The F50 was shown to be a good measure of the degree of supercooling and to extend greatly the lower size limit of organisms that could be measured. 3. The J3 uses a freeze-avoiding strategy by supercooling when in air or covered by liquid paraffin. In water it uses a mixture of freeze-avoiding and freeze-tolerant strategies, with a proportion of the population surviving freezing caused by exogenous ice nucleation. 4. Removal of the J3 sheath results in a shift from freeze avoidance to freeze tolerance, with an overall reduction in survival. A major function of the sheath may be to reduce the probability of exogenous ice nucleation.

Animals↗
PubMed1988

The formation of the first-stage cuticle within the egg of Trichostrongylus colubriformis.

The ultrastructure of the formation of the first-stage cuticle within the egg of Trichostrongylus colubriformis is described. The first indication of cuticle formation was the formation of the epicuticle external to the epidermal plasma membrane. This becomes thickened to form the epicuticle of the fully formed cuticle. Microtubules and microfilaments associated with the epidermal plasma membrane were observed during the early phases of cuticle formation. The basal zone was secreted into the space between the epicuticle and the epidermal plasma membrane, followed by the formation of the cortical zone. The basal zone had a striated appearance during the earliest stages of its formation.

Animals↗
PubMed1986

The structure of the cuticle and sheath of the infective juvenile of Trichostrongylus colubriformis.

The infective third-stage juvenile of Trichostrongylus colubriformis is surrounded by its own cuticle as well as the incompletely moulted cuticle of the second-stage juvenile, which is referred to as the sheath. The sheath comprises an outer epicuticle, an amorphous cortical zone, a fibrous basal zone and an inner electron-dense layer. The basal zone of the sheath consists of three layers of fibres; the fibres are parallel within each layer, but the fibre direction of the middle layer is at an angle to that of the inner and outer layers. The cuticle comprises a complex outer epicuticle, an amorphous cortical zone and a striated basal zone. The lateral alae of the cuticle and the sheath are aligned and overlie the lateral hypodermal cords. The lateral alae of the sheath consist of two wing-like expansions of the cortical zone with associated specializations of the inner electron-dense layer which form a groove. The cuticular lateral alae consist of two tube-like expansions of the cortical zone. The lateral alar complex of the cuticle and the sheath may maximise locomotory efficiency and prevent rotation of the juvenile within the sheath.

Animals↗
PubMed1985

Ultrastructural changes during recovery from anabiosis in the plant parasitic nematode, Ditylenchus.

Ultrastructural changes after desiccation and rehydration of the anabiotic fourth-stage juveniles of the plant parasitic nematode Ditylenchus dipsaci (Kuhn) Filipjev are described and quantified. Anabiotic juveniles retain their structural integrity, although the cuticle decreases in thickness and the muscle cell sarcoplasm condenses. In contrast the structure of the non-anabiotic nematode Panagrellus silusae is completely disorganized by desiccation. Following rehydration of D. dipsaci there is a lag phase of 2-3 hr before the nematodes become active. During this period the juveniles undergo an ordered series of morphological changes. The lipid droplets within the intestinal cells coalesce and the cuticle increases in thickness. The muscle cell sarcoplasm expands, the spacing of the thick myofilaments increases and the mitochondria swell before recovering a more normal appearance. These morphological changes, together with earlier metabolic studies, indicate that repair occurs during the lag phase prior to recovery. This may involve membrane repair and the re-establishment of the ionic gradients essential for normal muscle and nerve function.

Adaptation, Physiological↗
PubMed1983

The production and functional morphology of helminth egg-shells.

The high energy costs of egg-shell production in many helminths suggests that this structure plays an important role in their biology. The mechanical and chemical resistance of the egg-shell and the barrier it provides to the entry and loss of material are important in the survival of the free-living stages within the egg. The presence of egg filaments may increase the availability of infective stages and the operculum may be important in the infective process.

Acanthocephala↗
PubMed1982

The survival of desiccation by the free-living stages of Trichostrongylus colubriformis (Nematoda: Trichostrongylidae).

The survival of the free-living stages of Trichostrongylus colubriformis under defined conditions of temperature and relative humidity was investigated. The survival of embryonated eggs was poor at 0, 33 and 54.5% relative humidity (rel. hum.) at 20 degrees C but hatching occurred from a proportion of eggs even after exposure for 104 days to 76 and 98% rel. hum. at 20 degrees C. Second-stage larvae were desiccation-susceptible and were killed within 6 h even at 98% rel. hum. and 20 degrees C. Infective larvae, dried separately or in clumps, survived prolonged exposure to desiccation at 33-98% rel. hum. and 20 degrees C with 50% survival times of 58-164 days. Clump formation did not enhance survival in this range. Infective larvae also survived exposure to vacuum desiccation with 50% survival times of 8.8 h in clumps and 4.5 h when dried separately. The infective larva thus readily survives desiccation and may prove a useful model for the study of anhydrobiosis.

Animals↗
PubMed1981

The effect of temperature on the behaviour of the infective larvae of Trichostrongylus colubriformis.

The effect of temperature on the behaviour of the infective larvae of Trichostrongylus colubriformis has been investigated. Larvae acclimatized at 3 degrees C or 20 degrees C respond to sudden or gradual increases in temperature by coiling. Increased temperature may indicate a risk of exposure to desiccation and elicits a behavioural response which optimizes survival. Larvae are straight when inactive; coiling is an active response to specific stimuli. Larval activity is initiated by exogenous stimuli. Mechanical disturbance is a major stimulus, the response being affected by changes in temperature. Larvae acclimatized at 3 degrees C or 20 degrees C show a peak of activity at temperatures between 15 and 25 degrees C.

Larva↗
PubMed1979

Oogenesis and egg-shell formation in Aspiculuris tetraptera Schulz (Nematoda: Oxyuroidea).

The ovary of Aspiculuris tetraptera has a prominent terminal cap cell. This is considered to be part of the ovarian epithelium. Oogonia detach from the short rachis and increase in size from 6 to 60 microns; accumulating hyaline granules, shell granules and glycogen. The hyaline granules persist in the eff cytoplasm after shell formation has been completed and are considered to be lipoprotein yolk. The shell granules contribute to the non-chitin fraction of the chitinous layer. A classification of the cytoplasmic inclusions of the nematode oocyte is proposed. Upon fertilization a vitelline membrane is formed which constitutes the vitelline layer of the egg-shell. The chitinous layer is secreted in the perivitelline space, between the vitelline layer and the egg oolemma. Upon completion of chitinous layer synthesis, the egg cytoplasm contracts away from its inner surface. The material of the lipid layer is secreted at the surface of the egg cytoplasm and adheres to the inner surface of the chitinous layer. During secretion of the chitinous and lipid layers by the egg cytoplasm, the uterine cells secrete the unit membrane-like external uterine layer and the crystalline internal uterine layer. A complex system of interconnecting spaces develops in the internal uterine layer. This system is open to the exterior via breaks in the external uterine layer. There is no direct involvement of the uterine cells in the formation of this structure.

Animals↗
PubMed1979

The structure of the egg-shell of Aspiculuris tetraptera Schulz (Nematoda: Oxyuroidea).

The egg of Aspiculuris tetraptera is an ellipsoid measuring 93 x 40 microns. The shell consists of 5 layers: the external uterine layer, internal uterine layer, vitelline layer, chitinous layer and the lipid layer. This nomenclature is based upon the formation and histochemistry of the shell layers. The internal uterine layer contains a system of interconnecting spaces, partly filled by uterine secretion, which open to the exterior of the egg via breaks in the external uterine layer. The surface of the egg is covered by a system of interconnecting grooves. Freeze-etching reveals that the internal uterine layer is open to the exterior via pores, which open into the grooves. Rod-shaped particles are also revealed in the external uterine layer. The operculum of the egg consists of a modification of the uterine and chitinous layers of the shell.

Animals↗