| Research Abstract |
According to our suty on tridated thymidine cytocide, a colony forming unit in spleen (CFLJ-S) can. be divided into two major fractions ; i. e. stem cells possessing less pluripotency in differentiation with a Idnetics largely in S phase (the progenitor cells producing early appearing colonies), and the other ones possessing quiescent nature without cell cycle (the progenitor cells producing late appearing colonies). While former fraction reacts at larger extent to a variety of hemopoietic stimuli, latter does but relatively at a smaller extent. The heterogeneity of the CFU-S, as seen above, suggestes a heterogeneous radiation sensifivity in 6ach sub-fraction, however, none has been actually demonstrated pertaining to this heterogeneous radiation sensitivity. Bone marrow cells were irradiated with 400 cOy through 600 eGy, in vitro, and then been injected with lethally irradiated mice to form spleen colonies. There was a fraction found after these relatively larger dose of radiation at
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larger extent than numbers estimated by a survival curve fitting to a single-exponential model ; namely, the steep regular survival curve (D_0=90 cGy) changed into a flatter bne (D_0=270 cGy) after 300 through 400 cGy, probably because of a small radiosensitive subfraction, ca. 3-5%, mixed. with the regular CFLJ-S. This was confirmed by a variety of different murine stanns in our research project. Although irradiation was given at in vitro, oxygen effects must be minimum or none, and actually, no effects might have been involved in a mechanism of these flatter part of the survival curve, since total radiation time took for each fraction was ca. 4 min. and 14 sec. and or less. During these studies, we incidentally found another important finding in the Scid mice pertaining to the radioresistant fraction of the CFLJ-S. Originally, somatic cells in Scid mutaidon have been found to be very mioresistant because of a functional defect of repair for double strand breaks. We found, under the surveillance of D_0 in the Scid mice, again, a relatively radioresistant fraction sharply defined from the regular CFU-S. This phenomenon would deal with further study of this research program. Less
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