DELAYED AND DIRECT LABELING AFTER A SYSTEMIC INJECTION OF THYMIDINE-3H

The variation in the radioautographic pattern of labeling was studied in uteri during decidual proliferation for some days after a systemic and after a local injection of thymidine- 3 H. In the systemically injected animals the decidual cells showed a delayed uptake of thymidine- 3 H from the circulation, which was responsible for up to 100% labeling of these cells, 48 hr after injection, against the 20% labeling of locally injected uteri, after an identical length of time. An analysis of the grain count distributions revealed that 10% of cells with high grain density was present in both preparations. It thus appeared that all the "high" cells derived their label by direct uptake. Because of the difference in concentration and duration of availability of tritiated thymidine, immediately after injection and during delayed release, criteria could also be established to differentiate, on the basis of different grain patterns, between directly and indirectly labeled nuclei among the population of "low" cells.