The Renewing Cell Populations of Ascidians

SYNOPSIS. Renewing cell populations are tissues or groups of cells which rapidly proliferate and whose cell division is balanced by cell loss. The rapid proliferation of cells can be determined in autoradiograms by the uptake of tritiated thymidine into DNA synthesizing cells, and the migration and loss of cells can be followed by taking samples of tissues at increasing time intervals after exposure to the radioisotope. In ascidians, renewing populations are the testis, ovary, blood cells, and epithelial lining of the digestive tract. They are made of subpopulations called compartments: 1) germinal compartments of relatively undifferentiated, dividing stem cells (these cells are labeled at short time intervals), 2) mature compartments of fully differentiated, nondividing cells (these become labeled with time by the migration of stem cells), and 3) transitional compartments of cells in intermediate stages. The stomach groove cell population is a model system for cell differentiation and renewal in the digestive tract. Germinal compartments can be distinguished from mature compartments by their morphology (pseudostratified vs. simple columnar epithelia), by the uptake of tritiated thymidine in autoradiograms, by histochemical staining, by their surface features in the scanning electron microscope, and by their intracellular organelles when viewed in the transmission electron microscope. Stem cells differentiate into absorptive cells and zymogen cells. In differentiating, stem cells increase in size and lose their abundant free ribosomes. Absorptive cells develop longer microvilli, a long cilium, smooth apical vesicles, and large supranuclear lysosomes. Zymogen cells produce abundant rough endoplasmic reticulum and numerous zymogen granules. The renewing cell populations of blood (lymph nodule) and digestive tract are important model systems for studying cell differentiation, morphogenesis, and the phylogeny of vertebrate hemopoietic, immune, and digestive systems.