SUMMARY1. Positional and temporal correlates for the development ofmicrovillus membranes and for two of the hydrolytic enzymes they contain have been determined and compared with the ability of enterocytes to transport valine during migration from crypt base to villus tip in jejunal tissue taken from rats maintained on diets containing different amounts of protein.2. Microvillus elongation and the appearance of both aminopeptidase N (APN) and isomaltase (IM) activities reached maximal rates of expression in enterocytes located 16 + 5,m from the crypt-villus junction. This close positional correlation was not found for the later development of the valine transport function.3. Feeding rats isoenergetic diets containing 20 % instead of 5 % protein caused significant increases in both villus height and crypt depth without changing the positional correlations described above.4. The maximal rates for microvillus elongation and APN and IM appearance were greater and occurred earlier in enterocytes taken from rats fed a high-protein diet. The time of onset and capacity to transport valine were found to be closely correlated for rats maintained on high-and low-protein diets.5. The ratio of APN to IM activity in fully differentiated enterocytes was either 0 7 or 1-2 depending on whether rats had been fed a low-or high-protein diet. The maximal length of microvillus membranes in fully differentiated enterocytes from rats on a low-protein diet was 1-4 times that found in rats maintained on a high-protein diet.6. Possible ways in which the microvillus membrane structure of enterocytes, enzyme activity and the ability to transport amino acids might be controlled are discussed. Relative estimates are also made of the probable effects that changes in diet will have on the capacity of the intestine to digest and absorb nutrients.

4. Sodium ions appear to play a major permissive role in the action of T3 on jejunal oxygen uptake. In the presence of sodium, T3 stimulated significantly the oxygen consumption by 23 % while in the absence of sodium the stimulation was only 10 % which was not significant. The sodium-sensitive oxygen uptake of the jejunum was expanded by 57 % after T3 treatment.

1. Newly weaned 21-d-old male rats were given isoenergetic diets containing 200, 100 and 50 g protein/kg for 7, 14,28 or 70 d. The mid-jejunum was removed from the rats and a micrometric analysis of the mucosa was made. The following measurements were made: number of villi/mme, villus dimensions, villus surface area, crypt depth, crypt: villus, the number of cells/crypt in metaphase arrest per h.2. Comparisons were made between animals of the same age but on different diets, and animals on the same diet but of different ages. The latter comparison gave information on the effect of protein deficiency on the pattern of maturation of each feature of the villus or crypt studied.3. The effect of protein deficiency was not consistent at each stage of maturation. For instance villus height was decreased when compared with the controls following 28 d on a protein-deficient diet but not after 7 or 70 d.4. The only measurement to be unaffected by protein deficiency was the number of villi per unit area. 5.In general the 50 g protein/kg diet had a more pronounced effect than the 100 g protein/kg diet. Protein 6. In rats given 50 g protein/kg diet, although the villus surface area did not increase as the rats matured there deficiency delayed maturation by either slowing or inhibiting changes seen in normal maturation.were increases in epithelial cell production rate and number of crypts per villus.

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PLATES CXXV-CXXIX WEANING in the young pig is often followed by a gastro-enteritis, which in turn is frequently associated with the isolation from the faeces of large numbers of organisms of certain serotypes of Escherichia coli. Kenworthy and Allen (1966) showed that in pigs that appeared clinically normal a malabsorption syndrome could occur, and suggested that E. coli was not the primary agent in post-weaning diarrhoea, even though the organism might be present in large numbers.Because enteritis associated with E. coli affects such a wide variety of animal species and its aetiology is not understood, we decided to study the malabsorption syndrome further. We examined the morphology, microflora and in-vitro absorptive efficiency of the small intestines of unweaned and weaned healthy pigs, and of weaned pigs with diarrhoea. The morphological studies included histological and histochemical studies with the light microscope, and examination of the ultrastructure of epithelial cells with the electron microscope. The electron-microscope studies, together with relevant data on the E. coli population in the intestine, form the subject matter of this paper.The strains of E. coli isolated from our animals are divided into two groups on the basis of haemolysin production. This is because most strains isolated from outbreaks of bowel oedema or gastro-enteritis in pigs produce haemolysins (Sojka, Lloyd and Sweeney, 1960). Haemolytic E. coli bacilli are commonly present in the faeces of apparently normal, healthy pigs, occasionally in large numbers Smith, 1963), but in the disease syndromes it is the profuseness and purity of growth on primary isolation that have attracted attention. MATERIALS AND METHODSThe pigs were 23 Large Whites from a closed herd. They sucked their dam until 3 wk old, when half of the litter was weaned. No bedding was available during the suckling period, and the pen was so arranged that the piglets did not have access to the sow's food. At weaning, half of the weaned pigs were bedded on sawdust litter, the other half being left without bedding of any kind. They were treated in this way since young pigs are prone to eat their bedding and this could be a complicating factor in the interpretation of results.