A comparison of cell proliferation patterns in the digestive tract of ascidians

Cell proliferation patterns in the postbranchial digestive tract of a variety of ascidian species were investigated with autoradiography and tritiated thymidine. Based upon the distribution of radioactive nuclei, gut cell populations were classified as either static, expanding, or renewing. In most ascidians, renewing populations occur in the esophagus and stomach, but expanding populations sometimes occur in all or part of the intestine. From species to species, the number of cell renewal units (pairs of germinal and mature zones) in each portion of the digestive tract increases with increasing organ size, and this multiplication of renewal units follows the folding patterns in the gut. Cell turnover is fastest and the size of a cell renewal unit smallest at the anterior end of the gut. The smallest colonial ascidians have a single renewal unit per cell population in the esophagus and stomach and an expanding population in the intestine. Epithelial folding and multiplication of cell renewal units occurs in solitary species of increasing body size. One cell population occurs on the stomach folds of more primitive solitary ascidians, two of those of advanced species. The digestive diverticulum, which only occurs in two families, is renewed much the same as the stomach, and probably evolved from that organ. In the intestine of primitive solitary ascidians, renewing populations only occur at the anterior end; with evolutionary advancement, renewing populations line the entire intestine.