Cellular Engineering 531 Although research designed to investigate the properties and behavior of biologi- cal cells goes back many centuries,it is in the last fifty years that the level of activ- ity in this field has,literally,exploded.An important part of this increase was due to the advent of cell culture,i.e.,the ability to grow living cells in the artificial en- vironment of a laboratory (19).In this out of body experience,cells not only survive, but can multiply and even express differentiated properties.Through the use of cell culture technology,i.e.,the ability to grow cells under controlled laboratory condi- tions,new opportunities for basic research have opened up.Furthermore,this tech- nology has led to commercial applications which will be discussed in the second half of this text. Modern cell culture dates back to the beginning of this century.A particularly im- portant contribution was that of Alexis Carrel,a French scientist working at the Rockefeller Research Institute in New York(27).On January 17,1912 he placed a tiny slice of heart muscle taken from a chick embryo in a culture medium.This culture continued for thirty-four years,until two years after Carrel's own death.Along the way,the heart muscle cells expired and what continued to propagate were fibroblasts. Still,Carrel's historic chick-cell culture flourished for more than thirty years.The fact that it was the fibroblasts,not the heart muscle cells,which flourished for thirty-four years,points out one fact about cell culture;some cells are not as easy to grow as oth- ers.Cells which are not easy to grow are called recalcitrant cells. At times,one is interested in using primary cells,i.e.,cells prepared directly from the tissues of an organism.In other cases,cells are passaged through subculturing, and in this way are maintained for extensive periods of time.Most cells in culture have a limited lifespan,i.e.,after a finite number of divisions in culture,they die. However,occasionally,cells become immortal and can be propagated indefinitely as a cell line.Such cells are often referred to as transformed.Whereas cells cultured from tissue are anchorage-dependent,i.e.,adherence to a surface is required for survival, transformed cells often grow in suspension.Whatever the case,a wide variety of cells is now available for basic research and for more applied studies. Because of the complex nutritional requirements of mammalian cells,the medium in which the cells are cultured is important if a specific cell type is to be successfully grown.Traditionally,what has been used in most cases is a basal medium supple- mented with serum;this has usually been fetal calf,newborn calf,horse,or human serum at concentrations from 2 percent to 20 percent or greater.For many types of cells,the addition of serum,which provides growth factors,hormones,transferrin (an iron-binding protein),selenium(a trace element necessary for the growth of hu- man cells),and other required nutrients,is essential if the cells are to grow However,serum is a complex fluid and can be variable;thus each lot must be tested prior to use.There are also certain drawbacks to serum,e.g.,in some cases this growth promoter can even be toxic to cells.Because of this,there have been a num- ber of attempts to develop serum substitute products which can replace all or part of the serum required in a medium.In general,transformed cell lines have simpler me- dia requirements than untransformed cell lines,and thus are more likely to grow in serum-free culture.However,even though there are increasing reports of success in using low-serum or serum-free media,there still are many problems to be solved.For example,different cell lines of the same cell type may have different media require- ments,particularly with serum-free media.The fact is that,in spite of the progress which has been made,serum has so far defied all attempts at simulationCellular Engineering 531 Although research designed to investigate the properties and behavior of biologi￾cal cells goes back many centuries, it is in the last fifty years that the level of activ￾ity in this field has, literally, exploded. An important part of this increase was due to the advent of cell culture, i.e., the ability to grow living cells in the artificial en￾vironment of a laboratory (19). In this out of body experience, cells not only survive, but can multiply and even express differentiated properties. Through the use of cell culture technology, i.e., the ability to grow cells under controlled laboratory condi￾tions, new opportunities for basic research have opened up. Furthermore, this tech￾nology has led to commercial applications which will be discussed in the second half of this text. Modern cell culture dates back to the beginning of this century. A particularly im￾portant contribution was that of Alexis Carrel, a French scientist working at the Rockefeller Research Institute in New York (27). On January 17, 1912 he placed a tiny slice of heart muscle taken from a chick embryo in a culture medium. This culture continued for thirty-four years, until two years after Carrel's own death. Along the way, the heart muscle cells expired and what continued to propagate were fibroblasts. Still, Carrel's historic chick-cell culture flourished for more than thirty years. The fact that it was the fibroblasts, not the heart muscle cells, which flourished for thirty-four years, points out one fact about cell culture; some cells are not as easy to grow as oth￾ers. Cells which are not easy to grow are called recalcitrant cells. At times, one is interested in using primary cells, i.e., cells prepared directly from the tissues of an organism. In other cases, cells are passaged through subculturing, and in this way are maintained for extensive periods of time. Most cells in culture have a limited lifespan, i.e., after a finite number of divisions in culture, they die. However, occasionally, cells become immortal and can be propagated indefinitely as a cell line. Such cells are often referred to as transformed. Whereas cells cultured from tissue are anchorage-dependent, i.e., adherence to a surface is required for survival, transformed cells often grow in suspension. Whatever the case, a wide variety of cells is now available for basic research and for more applied studies. Because of the complex nutritional requirements of mammalian cells, the medium in which the cells are cultured is important if a specific cell type is to be successfully grown. Traditionally, what has been used in most cases is a basal medium supple￾mented with serum; this has usually been fetal calf, newborn calf, horse, or human serum at concentrations from 2 percent to 20 percent or greater. For many types of cells, the addition of serum, which provides growth factors, hormones, transferrin (an iron-binding protein), selenium (a trace element necessary for the growth of hu￾man cells), and other required nutrients, is essential if the cells are to grow. However, serum is a complex fluid and can be variable; thus each lot must be tested prior to use. There are also certain drawbacks to serum, e.g., in some cases this growth promoter can even be toxic to cells. Because of this, there have been a num￾ber of attempts to develop serum substitute products which can replace all or part of the serum required in a medium. In general, transformed cell lines have simpler me￾dia requirements than untransformed cell lines, and thus are more likely to grow in serum-free culture. However, even though there are increasing reports of success in using low-serum or serum-free media, there still are many problems to be solved. For example, different cell lines of the same cell type may have different media require￾ments, particularly with serum-free media. The fact is that, in spite of the progress which has been made, serum has so far defied all attempts at simulation