The intestinal epithelium of the Coho salmon was used to study the effects of temperature and x-irradiation on the cellular dynamics in a poikilothermic organism. Histological studies showed an absence of villi, and crypts in this system as only primary mucosal folds were observed. Autoradiographic studies following3 H-thymidine (3 H- TdR) labeling indicated that intestinal epithelial cell replication occurs in basal proliferation zones of the intestinal folds and the mucosal cells migrate the entire length of the folds before being sloughed into the lumen. The renewal rates of the intestinal epithelium are temperature dependent and estimated to be 14 ± 1, 24 ± 1, and > 35 days for 18°, 10°, and 5°C, respectively, as evidenced by autoradiographic and biochemical studies of3 H- TdR incorporation and turnover of the nucleoprotein fraction. The uptake of3 H- TdR into the nucleoprotein fraction was temperature dependent. Inhibition of3 H- TdR incorporation into intestinal nucleoprotein by x-rays was time and dose dependent; histological expression of radiation insult (≥ 1000 R) required approximately 25 days at 10°C and was typical of irreversible gastrointestinal syndrome.

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