This monograph, Senescence; Dominant or Recessive In Somatic Cell Crosses? represents the second annual workshop to promote theory and concept development in aging research. These workshops are part of a resource to bank cultured cells of special interest to aging research that was established at the Institute for Medical Research in Camden. New Jersey. by the National Institute on Aging in 1974. The underlying theme of the workshops is the use of cultured cells in a variety of somatic cell genetic systems designed to define mechanisms of in vitra cellular scen escence and the possible insights that this may provide to the problems of in viva aging. The concept also includes bringing together workers from a variety of disciplines to stimulate new and innovative thoughts and work in the area. The current work shop focuses on the relative role of nucleus and cytoplasm on determining the in vitra lifespan of human diploid cells as well as the relative influence of old and young cells when combined within a single cell structure. The techniques and procedures discussed should make significant contributions to understanding in vitra senescence and may lead to the mapping of an area or areas of the genome linked to senescence as is being accomplished with viral transformation of normal cells. Warren W. Nichols Donald G. Murphy ~i Contents Theoretic Mechanisms of in vitpo Senescence 1 F. MaPott Sinex . . . . . . . . . . . . Senescence in Ce1l Cu1ture: An Accumu1ation of Errors or Terminal Differentiation? 13 Vincent J. GPistofaZo . . . . . . . . . . . . . .

In 1974 The National Institute on Aging established a somatic cell genetic resource for aging research at the Institute for Medical Research in Camden, New Jersey. Within this program there is a yearly workshop to promote theory and concept develop ment in aging research with the specific purpose of addressing the use of genetically marked cells for aging research and to stimulate interest in aging research by workers in a variety of disciplines. This monograph, The Regulation of Cell Proliferation and Differentiation, is the result of the first workshop held May 15-17, 1975. The concept of the workshop was to consider two main areas: First, a discussion of clinical syndromes expressing as a major manifestation excessive growth, deficient growth or failure to thrive; and second, to present work in cellular and molecular biology on a model system suitable for in vitro study of regulation of cell proliferation and diff2rentiation. The model selected for this was skeletal muscle. It has been widely accepted that normal somatic cells from individual human donors display limited replicative lifespans when cultivated in vitro (1,2). That such "clonal senescence" may be related to in vivo aging is suggested by observations relating the replicative lifespans of cultures to donor age (3-5,13) donor genotype (4-7) and donor's tissue of origin (5,8). A variety of theories have been developed to explain in vitro clonal senescence (9)."