Growth and cell replacement in the new-born pig intestine

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Abstract
Both macroscopical and microscopical aspects of growth in the pig small intestine have been measured and related to the kinetics of cell turnover and changing physiological function in the period immediately following birth. The first 10 days of post-natal life was associated with an 80% increase in length and a 30% increase in diameter of the small intestine. The absorptive area doubled during this period of post-natal growth. Villi formed at birth were 29, 50 and 75% longer by day 10 (proximal, mid and distal regions respectively). Maximal elongation occurred on day 6 of post-natal life. Thymidine labelling of cells showed mitosis to be confined to the crypts. Newly formed cells migrated onto the villi. The mean rate of cell appearance in a section was 0.4 cell/h. This rate remained unchanged and independent of the area of the small intestine chosen for study during the first 4 days of post-natal life. Examination of villus sections at different times following injection of radioactive thymidine showed considerable cell mixing to occur. Labelled cells, which never formed protein-filled vacuoles, became mixed with vacuolated and non-vacuolated, unlabelled cells. There was also a redistribution of labelled cells upon the villus. The level of radioactivity within cells did not always correspond to that predicted from their respective positions on the villus. Two alternative theories are put forward to account for these findings. The time taken for complete cell replacement is approximately 19 days. The mucosal uptake of macromolecules finally ceases and the resistance of piglets to enteric viral infections increases at about this time. It is suggested that both events are dependent on the final disappearance of cells originally present in the pig intestinal mucosa at birth.