United States government. Defense Nuclear firms, Hoffman-LaRoche and Ciba-Geigy, and by Agency, Armed Forces Radiobiology Research In- the American firm, Travenol. stitute. SIEGMUNDJ. BAUM Furthermore, we greatly appreciate the gener- G. DAVID LEDNEY ous financial support by the Swiss pharmaceutical vi Xlll List of Contributors Part I Regulation of Stem Cell Proliferation L. G. Lajtha 1 Regulation of Stem Cell Proliferation 3 L. G. Lajtha and E. G. Wright Introduction 3 Experimental Evidence 3 Summary 7 References 7 Contents 2 Surface Antigens of Hemopoietic Stem Cells: The Expression of BAS, Thy-1, and H-2 Antigens on CFU-s 9 Ger van den Engh, Jim Russell, and Diane de Cicco Introduction 9 Materials and Methods 10 Results 11 Discussion 13 Summary 14 Acknowledgments 14 References 15 3 The Regulation of Hemopoiesis: Effect of Thymosin or Thymocytes in a Diffusion Chamber 17 S. J. Sharkis, A. Ahmed, L. L. Sensenbrenner, W. W. Jedrzejczak, A. L. Goldstein, and K. W. Sell Introduction 17 Materials and Methods 17 Results 18 Discussion 20 Summary 21 Acknowledgments 21 References 21 4 Anti-CFU-s Activity of Rabbit Anti- mouse Brain Serum: Mechanism of Action 23 Solomon S. Adler, Richard D. Kuznetsky, and Frank E. Trobaugh, Jr. Introduction 23 Materials and Methods 23 Results 26 Discussion 29 Summary 31 Acknowledgments 31 References 31 vii Contents Part II Ralph van Furth, Theo J. L. M. Goud, and Physiology of Committed Dick van Waarde Stem Cells 33 Introduction 65 D.

It is perhaps not too much of an exaggeration to claim that experimental hematology as it flourishes today originated largely from the pioneering attempts to protect lethally radiated animals (1) by shielding of hemopoietic tissues by L. O. Jacobson (9), and (2) by treatment with bone marrow suspensions by E. Lorenz and his col- laborators (12). The site chosen for this annual meeting of the International Society for Experi- mental Hematology is given a special historic sig- nificance by the fact that it was 25 years ago that the first publication on this subject by Lorenz ap- peared from his laboratory at the National Insti- tutes of Health. Lorenz's discovery marked the beginning of a period which lasted until 1956, during which the protection afforded by hemopoietic cell suspensions was confirmed by many. This soon led to an intensive scientific de- bate on the mechanism of this protective effect: was it due to a humoral factor produced and pro- vided by the bone marrow-as Lorenz The Appearance of postulated-or to transplantation and subsequent proliferation of hemop- etic cells? This question was defini- 1 the Multipotential tively answered in 1956 by evidence from three different laboratories (7, 15, 26), which demonstrated the origin of the cells Hemopoietic in the repopulated tissues using a variety of cellu- lar and immunologic markers. By the same token, these contributions marked the birth of radiation Stem Cell chimeras.