The elastic properties of a polyurethane arterial prosthesis.

The relationship between the mechanical properties of a fibrous polyurethane arterial prosthesis and the graft manufacturing process variables was studied from uniaxial tensile tests. A non-linear model was used to characterize the cylindrical elastic properties. Experiments on cylindrical segments were carried out to determine the constitutive constants and to assess the applicability of the model to the polyurethane graft. The compliance of 4 mm internal diameter grafts with various wall-thicknesses was predicted. The results were used to produce grafts with compliance matched to that of the carotid and femoral arteries.

Binding of lithium and boron to human plasma proteins.

The binding of lithium and boron, at normal physiological levels, to plasma proteins has been investigated by the techniques of precipitation with ethyl alcohol and gel chromatography. Assays of lithium and boron were made by thermal neutron activation and mass spectrometric assay of 3He and 4He. Results of alcohol precipitation experiments for plasma from two apparently healthy donors showed that 13+/-4% and 16+/-3% of the lithium in plasma is protein bound, but essentially no boron is bound under the conditions used. We believe that because of denaturation of proteins which occurs during alcohol precipitation, these percentages represent lithium and boron tightly bound to protein molecules. The results of the gel-chromatography experiment, on the other hand, showed that lithium and boron are bound to a wide range of plasma proteins, from low (approximately 60,000 amu) to high (approximately 1,000,000 amu) molecular weights, and to very low- (approximately 6000 amu) molecular-weight ligands. Although a clear identification of the specific proteins which bind lithium and boron cannot be made at present, some possibilities can be suggested.

Diet, mucosal architecture and epithelial cell production in the small intestine of specified-pathogen-free and conventional rats.

Upper jejunum and terminal ileum were examined in specified-pathogen-free (SPF), conventional and conventional after SPF rearing (ex-SPF) rats. The effect of 2 differential diets on the last 2 groups was examined. Ex-SPF rats had taller villi and deeper crypts than SPF rats, but similar crypt to villus ratios and cell production rates. Ex-SPF rats had similar crypt depth and jejunal villus height to conventional rats on the same diet, but taller ileal villi and a lower cell production rate. Even after 6-8 weeks, in a conventional environment, ex-SPF rat intestine was still not identical with conventional rat intestine. Diet had a significant effect on mucosal architecture, and a smaller effect on cell production rate. It is concluded that diet, microbiological status of colony of origin, and environment after weaning, can all affect mucosal architecture and epithelial cell production, and should be properly controlled in experimental studies.

The effect of intestinal distension and of section thickness on counts of epithelial nuclei in the rat small intestine.

(1) A method is described for counting nuclei in wax-embedded histological sections of epithelium. The counts are independent of section thickness over a wide range (4-10 mum). (2) counts of nuclei were made in villi and crypts of rat small intestine fixed either collapsed or distended by a hydrostatic pressure equivalent to 350 mm of water. There was no significant difference between the counts of nuclei in collapsed or distended gut. (3) Distension reduced the absolute height of villi, and increased their absolute width at the base. (4) The implications of these findings for studies of the morphological basis of intestinal adaptation are discussed.

The time-course of changes in mucosal architecture and epithelial cell production and cell shedding in the small intestine of the rat fed after fasting.

Rats were starved for 3 days, then either allowed access to food or continued in starvation. The following measurements on the upper jejunum were made on groups of refed and starved rats at nine time intervals after refeeding: villus height, crypt depth, crypt/villus ration, rate of cell production per crypt, and number of epithelial cells shed per villus in 5 minutes. Villus height increased 2 hours after refeeding, while crypt depth changed less dramatically. Crypt/villus ratio was unchanged. The number of shed epithelial cells per villus was reduced below the starved level for 4 hours after refeeding, and did not rise significantly above the starved level until 9 hours; the rate of cell production was not significantly increased until 12 hours after refeeding. No evidence for a reserve of cells in G2 was found, and the 3 hour lag between the rise in cell shedding and the increase in cell production would probably not give enough time for cells in G1 to pass through S and G2. It is concluded that these observations do not support the hypothesis that increased cell shedding from the top of the villus stimulates increased cell production in the crypts of Lieberkuhn.

The effects of age on mucosal morphology and epithelial cell production in rat small intestine.

Six groups of male Wistar rats were used, with mean weights of 29, 63, 97, 161, 249 and 399 g. Pieces of small intestine from three sites were examined after staining in bulk with the Feulgen reaction. Crypt/villus ratio (the number of crypts per villus) rose with age at all three sites, Villus height and crypt depth were measured on microdissected specimens. Villi in the proximal intestine were always taller than those distally. Proximal villi increased in height in successively older rats, except in the oldest group. Villi at the two distal sites tended to be tall in the youngest group of rats, but suffered a temporary reduction in height in the next two age groups. Crypt depth increased markedly within the first three age groups, and more slowly thereafter. Colchicine-metaphase accumulation rate was estimated from counts on microdissected intact crypts. The rate was low in the youngest group (8 cells/crypt/hour) but about 30 cells/crypt/hour in all other groups. After the changes during the early phase of rapid growth, no marked changes were seen during later life. The significance of these findings is discussed.

"Luminal nutrition" versus "functional work-load" as controllers of mucosal morphology and epithelial replacement in the rat small intestine.

Isotonic solutions of substrate were infused into surgically prepared sacs of upper small intestine in conscious unrestrained rats which fed normally via the gut-in-continuity. Infusion of glucose, galactose or sodium chloride all caused a local increase in villus height, while alpha-methyl glucoside and mannose had no effect. Glucose, galactose and alpha-methyl glucoside all stimulated a similar increase in counts of vincristine metaphases in microdissected crypts. although the absolute level of cell production was lower throughout the gut in glucose-infused rats. Sodium chloride infusion also caused a small stimulation of cell production locally, but a considerable increase in the uninfused gut-in-continuity. Mannose infusion had a variable effect on cell production. These results do not support "luminal nutrition" as a controller of epithelial replacement in the rat small intestine, and suggest that a mechanism based on the work-load of the epithelial may be more appropriate.

A new method of measuring the rate of shedding of epithelial cells from the intestinal villus of the rat.

Epithelial cells shed from the intestinal villus in starved rats were trapped in mucus which the goblet cells were stimulated to secrete; the number of trapped cells per villus was counted in whole mounts of fixed Feulgen-stained specimens from three sites in the small intestine. Increase in the interval between stimulation and fixation caused an increase in the number of shed cells per villus; the rate of this increase was postulated to represent the rate of cell shedding. The rate of cell production per crypt was measured with Colcemid; this rate, multiplied by the number of crypts per villus, gave the rate of cell production per villus. There was good agreement between these estimates of cell production and cell shedding.

Replacement of class instruction in histology by audio-tape and booklet self-instruction sessions.

A system of self-instruction in histology is described, using audio-tape and booklet instruction to supplement the microscopical examination of tissue sections. A pilot test demonstrated a slight superiority in retention of knowledge among students using this system, compared with those in the conventional class.

Undergraduate education in the cost-effective use of laboratory and radiological investigations in clinical decision-making.

This paper describes a strategy for helping students to learn about laboratory and radiological investigations in diagnosis and management, and to make decisions about the use of investigations, taking into account such features as safety, patient comfort and cost. Learning takes place through the use of a programmed text, in which students commit decisions to paper, and then receive feedback on the results of their decisions. Acceptability to students and teaching staff has been evaluated, and future developments of the strategy are outlined.

The anatomy of the peritoneum.

The author describes two educational strategies for helping students to understand the complex three-dimensional structure of the peritoneal cavity and the developmental changes by which that structure arises. The first strategy consists of a series of conducted circular tours round the peritoneal lining of the postnatal abdomen. The second is a demonstration of the developmental history of the placing of the gut and its peritoneum using a model.

The cytological effects of infusion of luminal polyethylene glycol on the rat small intestinal mucosa.

An isolated sac of rat upper small intestine was prepared. Its lower end was drained to the skin. Solutions of known volume and composition were infused into the sac and the intestinal mucosa was examined by light and electron microscopy. Infusion of isosmotic (16.8 percent w/v) polyethylene glycol (PEG) resulted in prompt and progressively severe damage to enterocytes at the top of the villus, while goblet cells and basal-granulated cells were apparently unaffected. Thus, after 6-72 hrs a cap of goblet cells and basal-granulated cells accumulated at the tip of the villus, while the enterocytes underwent vacuolation and disintegration. Infusion of an isosmotic mixture of 1 percent PEG and 5.1 percent glucose caused no damage near the site of infusion, but damaged the intestinal sac further distally...

Structural changes in the small intestine associated with the uptake of polyvinyl pyrrolidone by the young ferret, rabbit, guinea-pig, cat and chicken.

1. The entry of [(125)I]polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K. 60) into the epithelial cells of the small intestine has been measured in new-born animals of five species.2. The distribution along the intestine of cells capable of taking up [(125)I]PVP and the decrease and eventual cessation of uptake (closure) with increasing age have been investigated, and have been related to changes in the histological appearance of the small intestine.3. The small intestine of the ferret took up PVP readily until 33-34 days after birth. From 34 to 37 days of age PVP uptake declined sharply and disappeared completely by 40-45 days.4. In the ferret, unlike other species studied, some PVP was taken up by the duodenum. This continued for the first 4 weeks after birth. Thereafter PVP uptake gradually became confined to the terminal ileum.5. In the guinea-pig, PVP uptake was limited to the first 48 hr after birth. During this period the site of uptake was progressively restricted to the terminal ileum.6. In the rabbit, PVP could be taken up in the distal two-thirds of the small intestine for at least 20 days after birth. A decline in uptake occurred between 20 and 22 days after birth in most animals.7. Wide individual variations were seen in the kitten, but PVP uptake was seen in some animals up to 14 days after birth.8. Newly hatched chicks and chicks tested 48 hr after hatching did not take up PVP.9. Histological examination of the small intestine with the light microscope demonstrated that in all species PVP uptake was associated with the presence of vacuoles in the epithelial cells of the villus.10. In the young guinea-pig, large PAS-positive granules were seen in the macrophages of the lamina propria. These appeared to migrate through the epithelium into the intestinal lumen. The significance of this finding and its relation to macromolecular uptake remain unclear.

The use of [125-I] polyvinyl pyrrolidone K. 60 in the quantitative assessment of the uptake of macromolecular substances by the intestine of the young rat.

1. A method has been developed which allows the quantitative estimation of the uptake of labelled polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000 (K. 60) by the wall of the small intestine of young rats.2. Four hours after feeding a standard dose of [(125)I]PVP by stomach tube, the small intestine was thoroughly washed out, and the radioactivity of the intestinal wall measured. Under these conditions, the small intestine of animals less than 18 days old took up more than 50% of the radioactivity which had left the stomach. There was no increase in PVP uptake if the duration of absorption exceeded 4 hr. The PVP was taken up by the epithelial cells of the villus, and its intracellular localization has been demonstrated by fluorescence microscopy and can be related to vacuolation in the cells.3. In animals between 18 and 20 days old the uptake of PVP declined progressively, until, in animals more than 20 days old, less than 5% of the radioactivity was taken up by the intestinal wall.4. There is good agreement between the reported age of termination of antibody absorption in young rats and the age at which PVP uptake ceased in the present experiments. It is suggested that the loss of ability of the intestine to take up substances of high mol. wt. may be the factor which limits the duration of the period of antibody absorption in this species.

An analysis of the mechanism of cessation of uptake of macromolecular substances by the intestine of the young rat ('closure').

1. The longitudinal distribution of cells capable of absorbing macro-molecular material has been determined in the small intestine of the young rat by measurements of the uptake of [(125)I]polyvinyl pyrrolidone (PVP).2. PVP uptake was never seen in the duodenum and became restricted to the distal half of the small intestine by the 18th day of life. The subsequent decline to zero in PVP uptake was due to an almost uniform decrease in PVP uptake by the distal half of the small intestine.3. Histological studies indicate that this rapid decline in PVP uptake did not reflect a decrease in uptake by individual cells, but rather their replacement by new cells incapable of taking up PVP.4. Autoradiographic estimates of turnover time in the intestinal epithelium using tritiated thymidine reveal good agreement between the time needed to replace completely the epithelium of the ileum (62 hr) and the duration of the rapid decline in PVP uptake (approximately 3 days).5. It is concluded that on or about the 18th post-natal day there is an abrupt change in the functional characteristics of the apical plasma membrane of cells produced by the crypts of Lieberkühn in the distal half of the small intestine. The stimulus for this change is unknown, but cells produced subsequently cannot take up PVP.

Review of psychometric features of the Medical School Learning Environment Survey.

Growing concern over the stresses imposed on medical students by their undergraduate educational programme has led to a proliferation of surveys designed to measure students' global perceptions of the medical school as a learning environment. The present study aimed at a comprehensive analysis of the psychometric features of a recently-introduced questionnaire. Students at four Australian medical schools completed the Medical School Learning Environment Survey (Marshall, 1978). Data on the retest reliability, internal consistency and factorial validity of the MSLES are presented. Its potential usefulness for future evaluations within and between medical schools is discussed.

The approaches to learning of students in a traditional and in an innovative problem-based medical school.

The way in which students approach their learning is dependent on a variety of factors including the characteristics of the departments and teaching to which they are exposed. These factors appear to influence whether the students adopt a surface, deep or strategic approach. In order to explore further the relationship between educational context and approach to learning, a comparison was made between students attending a traditional medical school and those attending a problem-based medical school. The results showed marked differences, with the problem-based school being higher on deep approach and lower on surface approach than the traditional school. This study provides one of the first pieces of evidence of a difference between students in the two types of medical school which can be directly attributed to the educational environment. Students in the problem-based school appear to have an approach to learning which more closely approximates the aims of most medical schools. The results provide support for the philosophies and strategies of the problem-based schools.