Cell renewal in the gill of the freshwater mussel, Margaritifera margaritifera: an autoradiographic study using high specific activity tritiated thymidine.

High specific activity tritiated thymidine (50-3 Ci/mM and 56 Ci/mM) and autoradiographic techniques were used to study cell renewal in the gill epithelium of the freshwater mussel, Margaritifera margaritifera. The cell renewal system in the gill epithelium of M. margaritifera appears to consist of a stem-type population in the gill furrow and gill furrow edges which supplies, through division, cells for a maturing, dividing transient transitional population along the proximal gill ridge sides which, in turn, supplies cells to a simple transient, differentiated, functional population on the distal gill ridge sides and tip. Loss of cells from the cell renewal system appears to be through cell death and/or extrusion from the gill ridge tip. No emigration or immigration of labelled nuclei out of, or into, the gill epithelium was observed. The minimum transit time from the dividing transient population to the functional population in the gill ridge tip may be no more than 24 h. We were unable to detect any radiobiological effects or the presence of cytoplasmic labelling due to the use of high specific activities. However, such possibilities cannot be eliminated from consideration in further studies.