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DR. SEYMOUR GELFANT (Syracuse University): I would like to ask Dr. Wessells whether he has noticed GI and Gz cell populations in chick epidermis or pancreas similar to our observations in adult mouse epidermis (Gelfant, S., Exptl. Cell Research, 32: 521, ' 6 3 ) . If you were to observe unlabeled mitoses in the presence of tritiated thymidine in your experimental systems it would indicate the presence of G, population cells which are already different from other cells of the same type.
In the pancreas you could study this question of G2 population cells in another way by photometric measurements of DNA contents in cells that contain zymogen granules. 4C DNA contents would indicate G2 population cells.
DR. WESSELLS: No, in the case of pancreas. In experiments in which we counted the number of labeled mitoses, the level reached and remained at 100% over a period of several hours. One sees the typical cell cycle. Nevertheless, in relation to your hypothesis, there definitely are cells which incorporate tritiated thymidine and then, apparently, do not divide. Such cells retain high label levels, detected autoradiographically, for three or four days. They could well be in G-2 and not divide. I have seen the same thing in the chick skin.
QUESTION: Along the same line, have you looked at enough slides to be willing to say that you never see mitotic figures along with zymogen granules? DR. WESSELLS: No, definitely not. Both in the normal pancreas of the mouse embryo and in an occasional culture, one can find individual cells which contain a mitotic figure and small numbers of zymogen granules in the cytoplasm. One does not know, of course, when DNA was replicated in such cells.
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