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Histological changes in the small intestine of the young pig and their relation to macromolecular uptake.

There is general agreement that in the suckling pig the transfer ofimmune globulins from the colostrum to the circulation takes place only during the first 24-48 h after birth and that during this period many other proteins can pass from the gut into the circulation (Norbring & Olsson, 1957, 1958; Olsson, 1959a, b; Lecce, Matrone & Morgan, 1961 a, b; Pierce & Smith, 1967 a; Hardy, 1969b). It has also been demonstrated that polyvinyl pyrrolidone (PVP) of mean mol.wt. 40000 (K. 30) can be absorbed into the circulation during this period (Lecce et al. 1961a, b). In addition, a quantitative study of factors which influence the absorption of the larger polymer PVP K. 60 (mean mol.wt. 160000) has been reported (Hardy, 1969a). The investigations referred to above have been mainly concerned with the passage of macromolecular solutes from the gut into the blood, so that, with the exception of qualitative observations of the presence of fluorescein-labelled proteins in the intestinal epithelial cells, there has been no systematic exploration of the initial uptake of macromolecules by the intestinal epithelial cells. A technique has been developed in the young rat which allows the quantitative determination of the uptake of [1251]PVP K. 60 by the small intestine as a whole, and also the identification of those cells which actually take up the polymer (Clarke & Hardy, 1969a). The first section of the results to be reported in this paper concerns the application of this technique to the young pig, and from this it will be shown that the cells of the terminal small intestine retain the ability to take up PVP for up to 12 days after the time at which its transfer into the blood ceases. The second part of the paper is devoted to an examination of the histological changes at different levels of the small intestines of pigs during the first 3 weeks after birth and to the relation between these changes and PVP uptake.