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Biomedical subjects

A S Chan

Publications and source records attributed to A S Chan.

At least 91 records · Page 5Linked to original sources
PubMed1993

Dimensionality and clustering in the semantic network of patients with Alzheimer's disease.

The organization of semantic memory in patients with Alzheimer's disease (AD) was investigated with a triadic comparison task. A multidimensional scaling statistic was used to analyze proximity data and to generate 3-dimensional cognitive maps that were then compared by a discriminant function analysis. The results suggest that the structure of semantic memory in AD patients differs from that of elderly normal controls (NC) in 2 ways. First, AD patients are less consistent in using the attributes (predation, domesticity, and size) of concepts. Second, AD patients focus primarily on concrete perceptual information (size), whereas NC Ss stress abstract conceptual knowledge (domesticity). These results are consistent with the notion that AD is characterized by a breakdown in the structure of semantic knowledge.

Aged↗
PubMed1993

NADPH-diaphorase positive cells in the chick and rat thymus.

Nicotinamide-adenine dinucleotide phosphate-diaphorase (NADPH-d) positive cells were demonstrated in the medulla of the rat and chick thymus at light microscopic level. The NADPH-d positive cells were present as clusters and were predominantly localized near the corticomedullary boundary. These clusters were closely associated with the thymic cysts. Some cells were seen in close proximity to blood vessels. The presence of nitric oxide (NO) as indicated by the positive NADPH-d reaction in the cells forming the wall of the thymic cyst suggests a modulatory influence of NO over the activities of these cells. Other possible functions of NO are also discussed.

Amino Acid Oxidoreductases↗
PubMed1992

Association of nerve fibres with myoid cells in the chick thymus.

The presence of nerve fibres in the medulla of the chick thymus has been investigated ultrastructurally. Unmyelinated nerve fibres were found not only in the perivascular space but also among the myoid cells. The nerve fibres were situated in close proximity to the myoid cells. The unmyelinated axons, containing predominantly clear vesicles and some dense-core vesicles, were found in recesses of the plasmalemma of the Schwann cell. Contacts between nerve fibres and fibroblasts were also observed.

Animals↗
PubMed1991

Intraoperative radiotherapy with localized radioprotection: diminished duodenal toxicity with intraluminal WR2721.

The radiosensitive duodenum must be treated during IORT of human pancreatic head tumors, leading to an approximately 25% incidence of late bleeding. This study aimed to decrease the toxicity by administering WR2721 directly into the duodenal lumen. Duodenal toxicity in the canine was evaluated after intraoperative radiotherapy (IORT) with and without the intraluminal radioprotector WR2721. Eight adult dogs were divided into two groups. All underwent IORT using a 5.7 cm cone that covered the duodenum and pancreas. 30.0 Gy IORT was given with 6 MeV electrons. Cholecystojejunostomy and gastrojejunostomy were performed. Four dogs served as IORT only controls; one was unevaluable. Four dogs received WR2721, intraluminally at 720 mg/m2, in 16-18 ml Ringer's. Atraumatic clamps were placed on proximal and distal duodenum, without vascular compromise. WR2721 was injected into the duodenal lumen 30 minutes prior to IORT. Immediate postoperative recovery of the dogs receiving WR2721 was faster than controls. Necropsies were performed at 6 months. Grossly increased adhesions were noted in controls. Histopathologically, mucosal atrophy was greater in control dogs. Duodenal ulceration was noted in all controls, but in only one of four WR2721 dogs. Masson's trichrome and Verhoff Van Gieson stains demonstrated increased perivascular fibrosis, intimal proliferation, and fibrinoid medial necrosis of vessels in all controls, and one WR2721 dog. The other three WR2721 dogs had only mild perivascular fibrosis. Radioprotection, evaluated by the presence or absence of pancreatic atrophy, appeared to stop just beyond the bowel wall. In summary, WR2721 provided duodenal radioprotection in most dogs. The intraluminal administration of WR2721 allows decreased systemic side effects, and may eliminate tumor absorption. The study indicates that the intraluminal use of radioprotectors has broad potential application.

Amifostine↗
PubMed1991

A comparison of two-lung high frequency positive pressure ventilation and one-lung ventilation plus 5 cm H2O non-ventilated lung CPAP, in patients undergoing anaesthesia for oesophagectomy.

A randomised prospective controlled study was conducted during a one-year period on patients scheduled for oesophagectomy via a right thoracotomy approach. Twenty-two patients received one-lung ventilation (OLV group) and twenty patients received high frequency positive pressure ventilation (HFPPV group). Episodic hypoxaemia (SaO2 less than 90% for greater than 30 seconds, FiO2 1.0) occurred in eleven patients in the OLV group and six patients in the HFPPV group. No patient in the HFPPV group had a severe desaturation episode (SaO2 less than 80%, FiO2 1.0) compared with nine patients in the OLV group (P less than 0.05). The mean peak inspiratory pressure and average mean airway pressure were significantly lower in the HFPPV group 28.8 (SD 7.7) and 7.2 (SD 2.4) cm H2O respectively, compared with the OLV group, 40.0 (SD 9.9) and 11.9 (SD 4.9) cm H2O (P less than 0.05). Two-lung high frequency positive pressure ventilation has some advantages over one-lung ventilation during the thoracotomy phase of oesophagectomy because it is easy to administer, does not significantly compromise the surgical exposure and is associated with fewer severe undesirable physiological disturbances.

Aged↗
PubMed1991

Postoperative analgesia for oesophageal surgery: a comparison of three analgesic regimens.

Sixty-four patients undergoing oesophageal surgery were randomly allocated to receive either a continuous lumbar epidural infusion of morphine or fentanyl, or, intramuscular morphine for postoperative analgesia. There was no statistical difference in analgesic requirements between the patients who underwent a thoracotomy for their procedure (n = 50) and those who did not (n = 14), as assessed by the total dose of opioid administered, visual analogue scale (VAS) and pain score (PS) comparison. However, by these criteria, epidural morphine infusion provided the most satisfactory analgesia (P less than 0.05). Despite the variable quality of analgesia achieved with the three regimens, the postoperative lung function tests were similar for all groups, and we conclude that routine lung function tests are not an appropriate method of comparing analgesic efficacy. Prophylactic administration of loratadine to 15% of our patients was not shown to be effective in diminishing the incidence of pruritus.

Adult↗
PubMed1991

Effects of hydrocortisone on the ultrastructure of the thymic cysts of the chick embryo.

Thymi from 80 19-day chick embryos that were injected with 2.5 mg of hydrocortisone on the 15th day of incubation were examined by light and electron microscopy. Six thymic tissue blocks from each embryo were evaluated. Hydrocortisone caused a marked depression of thymic volume due mainly to the depletion of lymphocytes. Cystic structures, consisting of intercellular and intracellular cysts and individual cystic cells, were more numerous per unit area of medullary tissue in treated embryos than in the controls. In the treated group, the shape of cystic cells varied from squamous to columnar, and hypertrophied columnar cells were also evident. The luminal surface possessed well-developed microvilli with membrane invaginations at their bases. The terminal web was prominent and the lateral plasma membranes were highly interdigitated. The rough and smooth-surface endoplasmic reticula and the Golgi complex of thymi from treated embryos were more developed than those in the controls. Mitochondria and vesicles were increased in number following hydrocortisone treatment. Cysts lined partially by cystic and endothelial-like and smooth muscle cells were encountered in the treated group. These structural modifications probably reflect responses to increased absorptive or transport function. The data suggest that hydrocortisone may alter the structure of cystic cells in chick embryos.

Animals↗
PubMed1991

Ultrastructural observations on cytodifferentiation of thymic cystic cells of chick embryo.

Thymuses from chick embryos from the 13th-19th day of incubation were examined by light and electron microscopy. At the 13th day of incubation, the primitive medulla consists of undifferentiated, cystic, reticular and lymphoid cells interspersed with developing cysts. Undifferentiated cells contain indented nuclei, poorly developed endoplasmic reticulum and numerous free ribosomes. Synthesis of cytoskeletal material and assembly of microvilli denote their differentiation to cystic cells. Cystic cells possess rough- and smooth-surfaced endoplasmic reticulum, a Golgi complex, mitochondria and microvilli. They are organized individually or in clusters to form intra- and intercellular cysts respectively. By the 15th day of incubation, cystic structures are more apparent. Their lumina contain a low density flocculent or moderately dense granular material. At the 19th day, there is a further increase in number and complexity of the cysts. The results demonstrate the existence of undifferentiated cells and their differentiation to cystic cells. Morphological evidence suggests secretory and/or absorptive activities of the cystic cells in the prehatching period.

Animals↗
PubMed1990

Modified jet ventilation during total laryngectomy: a prospective study using pulse oximetry and a pressure regulator.

A method of jet ventilation during total laryngectomy is described. During the construction of the terminal tracheostomy, a small metal tube is used, instead of the traditional tracheostomy tube, to provide intermittent jet ventilation down the distal trachea. A pressure regulator is also employed to choose a driving pressure best suited to the chest and lung compliance of each patient. Excellent surgical access for tracheo-cutaneous anastomosis is achieved. Satisfactory ventilation during the jet period is also confirmed by unaltered PaCO2 and increased PaO2 levels. The use of pulse oximetry as a non-invasive and continuous monitor of arterial oxygenation is a simple alternative to arterial blood sampling.

Adult↗
PubMed1989

A comparison of anaesthetic breathing systems during spontaneous ventilation. An in-vitro study using a lung model.

Five anaesthetic breathing systems (Magill, Lack, Humphrey ADE, enclosed Magill and Bain) were compared using spontaneous ventilation in a simple lung model. The fresh gas flow at which rebreathing occurred was determined for each system by the application of four modified definitions of rebreathing. Two were based on the measurement of minimum inspired and two on end-expired carbon dioxide. The four A systems performed similarly with each individual definition. The rebreathing points found for each individual breathing system differed markedly between definitions, with those determined by the minimum inspired CO2 occurring at low, and probably misleading, FGF/VE ratio. The Bain system demonstrated rebreathing at considerably higher fresh gas flows whichever definition was used.

Anesthesiology↗
PubMed1988

Radiation dose response of normal brain.

Dose response relationships were determined after hemibrain x-irradiation of normal beagle dogs. Radiation doses of 11.5, 13.5, 14.3, and 17 Gy were delivered in a single dose and results were compared to previous studies using doses of 15 and 30 Gy. Brain injury was quantified using computed tomography (CT), with serial studies obtained monthly up to 1 year following irradiation. Quantitative endpoints included low density volume and contrast enhancement. Doses above 14.3 Gy resulted in high lethality 5-8 months following irradiation, and an LD50 of 14.9 Gy was calculated. At these lethal doses, low density volume representing edema, demyelination, and necrosis had a similar response with an ED50 of 14.6 Gy. Radiation-induced decreases in white matter density appeared 5-6 months after sublethal doses (less than or equal to 14.3 Gy) and the volume of tissue characterized by this low density increased with time and dose. This sublethal low density change had an ED50 of 12.8 Gy, and may reflect a loss or generalized atrophy of glial cells and/or myelin. These results show that: (a) the dose response curves obtained after hemibrain x-irradiation are extremely steep; and (b) at least two processes may be involved in the development of late radiation damage, one that is rapid upon onset (a "delayed acute" reaction) and the other which is a slower and more degenerative process.

Animals↗
PubMed1987

Effects of hydrocortisone on the ultrastructure of the thymic cysts of chicks.

Thymi from 10-day old male chicks injected subcutaneously with 5 mg of hydrocortisone for 2 consecutive days and sacrificed on the 4th day were examined by electron microscopy. With depletion of the lymphoid cells, cystic structures, consisting of inter- and intracellular cysts are more apparent in the hydrocortisone-treated chicks. The intercellular cysts vary in size and shape and are lined by cystic, endocrine-like and lymphoid cells. Luminal surfaces of the cystic cells contain microvilli. The lateral plasma membranes show complex interdigitations. The prominent terminal web contains vesicles and numerous core rootlets. Frequent membrane invaginations are apparent at the bases of the microvilli. The endoplasmic reticulum and the Golgi complex are moderately developed. Mitochondria are variable in number but are numerous in some cells. The cytoplasm also contains dense bodies, vacuoles and myelin figures. Endocrine-like cells, containing numerous secretory granules and bordering directly on the follicular lumen suggest that their secretion may be discharged into the lumen. Cystic cells containing intracellular cysts are observed. Presence of cleft-like spaces, microvilli closely opposed to adjacent cytoplasmic processes or lateral microvilli may reflect responses to increase in absorptive or transport function. The present data suggest that hydrocortisone may modulate the structure and function of the cystic structures in the chick thymus.

Animals↗
PubMed1986

Single dose ofloxacin in treatment of uncomplicated gonorrhoea.

78 patients (66 male and 12 female) suffering from uncomplicated gonorrhoea were randomly selected to receive a single oral dose of either 400 mg or 600 mg ofloxacin. Of the 59 patients completely followed up penicillinase-producing Neisseria gonorrhoeae strains were cultured in 15 (25.4%) and the overall cure rate was 100%. Both groups showed excellent clinical effectiveness in over 80% and adverse reactions were uncommon and transient. We conclude that a single oral dose of ofloxacin in the studied dosage regimens was highly effective and safe in the treatment of uncomplicated gonorrhoea.

Adult↗
PubMed1986

Ultrastructure of epithelial thymic cysts of the chicks.

The medulla of the chick thymus contains variable numbers of intercellular and intracellular cysts. The epithelium varies from squamous to columnar or stratified and contains cystic, endocrine-like, and lymphoid cells. The lumen-lining cystic cells contains microvilli, moderately developed rough- and smooth-surfaced endoplasmic reticulum, and Golgi complex. Mitochondria, free ribosomes, and varying number of microfilaments are observed. Endocrine-like cells are separated from the lumen by the cystic cells, which in places are extremely attenuated. The cytoplasm is characterized by the presence of membrane-limited secretory granules. Cleft-like spaces, partially lined by microvilli, are also encountered. Numerous vesicles, some in tight association with the core rootlets, are evident. Cystic cells with single or multiple cysts, lined with microvilli, are also observed. The lumina of these cystic elements contain flocculent or dense granular material, amorphous masses, cellular elements, or debris. Morphological evidence suggests synthetic and secretory activities of the cystic cells in the young chicks.

Animals↗
PubMed1983

Ultrastructural observations on the formation of follicles in the human fetal thyroid.

Thyroid glands from 40 human fetuses ranging from 30 to 175 mm in length were examined by electron microscopy. At the 30 mm stage, the cells are arranged in cords or clusters and contain numerous free ribosomes. The rough-surfaced endoplasmic reticulum (RER) is poorly developed and the Golgi complex consists of saccules, smooth and coated vesicles. Mitochondria, microfilaments, glycogen particles, occasional lysosome-like structures, and single cilia are observed. At sites of incipient follicle formation, intercellular clefts containing interdigitating microvilli, delimited by junctional complexes, are evident. In addition, the apical region of some cells contains vacuoles. At the 50-mm stage, intercellular clefts are more apparent, some of them organized into small follicles through the enlargement of the lumen, the incorporation of additional cells, and the formation of a basal lamina. With increasing gestational age, more follicles are established. The lumina vary from round to clover-leaf shaped and have an electron-dense content. Cisternae of RER are dilated and contain flocculent material. The Golgi complex is more extensive, and lysosome-like structures are more numerous. By the 100-mm stage, the follicular cells appear morphologically well differentiated. The results demonstrate an extracellular origin of the follicular lumen in the human fetal thyroid.

Cell Differentiation↗