Annals of Biomedical Engineering,Vol.19,pp.529-545,1991 0090-6964/91$3.00+.00 Printed in the USA.All rights reserved. Copyright1991 Pergamon Press plc Cellular Engineering Robert M.Nerem The 1991 ALZA Distinguished Lecture Biomedical Engineering Society Annual Meeting Atlanta,GA (Received 4/22/91) Cellular engineering applies the principles and methods of engineering to the prob- lems of cell and molecular biology of both a basic and applied nature.As biomedi- cal engineering has shifted from the organ and tissue level to the cellular and sub-cellular level,cellular engineering has emerged as a new area.A cornerstone of much of this activity is cell culture technology,i.e.,the ability to grow living cells in the artificial environment of a laboratory.Cellular engineering includes the role of engineering in both basic cell biology research and in the making of products which use living cells,e.g.,tissue engineering and bioprocess engineering.The former in- volves the use of living cells in the development of biological substitutes for the res- toration or replacement of function,and the latter the use of living cells to manufacture a biochemical product,e.g.,through the use of recombinant DNA tech- nology.In fact,as biomedical engineering has expanded to include the cellular level, and bioprocess engineering has shifted in interest from microbial organisms to include mammalian cells,there are intellectual issues in which an interest is shared by these two formerly separate areas of engineering activity.Cellular engineering thus tran- scends the field of biomedical engineering. Keywords-Cellular engineering,Tissue engineering,Bioprocess engineering,Cell and molecular biology. INTRODUCTION Over the past twenty-five years the field of biomedical engineering has undergone significant development,evolving in many different ways.For example,the contri- butions by engineers to the archival literature for biomedical research has been steadily increasing.The medical device and product industry,including instrumen- Acknowledgments-This review was written based on the author's own research activities which are currently supported by National Science Foundation Grant ECS-8815656 and National Institutes of Health Grants HL-26890 and HL-41175.The author thanks P.Girard,T.Sambanis,T.Wick,and C.Zhu,his col- leagues at Georgia Tech who work in the area of cellular engineering.The author also thanks his research collaborators,R.W.Alexander,B.C.Berk,P.Delafontaine,M.J.Levesque,M.Sato,C.J.Schwartz,and E.A.Sprague,for their contributions to the work and ideas reflected in this paper.Finally,the author thanks his students who keep coming up with new ideas and who will be the cellular engineers of tomorrow *The lecture was delivered on April 22,1991 by R.M.Nerem,Parker H.Petit Professor for Engireering in Medicine,School of Mechanical Engineering,Georgia Institute of Technology,Atlanta,GA 30332-0405. 529
Annals ofBiomedicalEngineering, Vol. 19, pp. 529-545, 1991 0090-6964/91 $3.00 + .00 Printed in the USA. All rights reserved. Copyright 9 1991 Pergamon Press plc Cellular Engineering Robert M. Nerem The 1991 ALZA Distinguished Lecture* Biomedical Engineering Society Annual Meeting Atlanta, GA (Received 4/22/91) Cellular engineering applies the principles and methods of engineering to the problems of cell and molecular biology of both a basic and applied nature. As biomedical engineering has shifted from the organ and tissue level to the cellular and sub-cellular level, cellular engineering has emerged as a new area. A cornerstone of much of this activity is cell culture technology, i.e., the ability to grow living cells in the artificial environment of a laboratory. Cellular engineering includes the role of engineering in both basic cell biology research and in the making of products which use living cells, e.g., tissue engineering and bioprocess engineering. The former involves the use of living cells in the development of biological substitutes for the restoration or replacement of function, and the latter the use of living cells to manufacture a biochemical product, e.g., through the use of recombinant DNA technology. In fact, as biomedical engineering has expanded to include the cellular level, and bioprocess engineering has shifted in interest from microbial organisms to include mammalian cells, there are intellectual issues in which an interest is shared by these two formerly separate areas of engineering activity. Cellular engineering thus transcends the field of biomedical engineering. Keywords- Cellular engineering, Tissue engineering, Bioprocess engineering, Cell and molecular biology. INTRODUCTION Over the past twenty-five years the field of biomedical engineering has undergone significant development, evolving in many different ways. For example, the contributions by engineers to the archival literature for biomedical research has been steadily increasing. The medical device and product industry, including instrumenAcknowledgments-This review was written based on the author's own research activities which are currently supported by National Science Foundation Grant ECS-8815656 and National Institutes of Health Grants HL-26890 and HL-41175. The author thanks P. Girard, T. Sambanis, T. Wick, and C. Zhu, his colleagues at Georgia Tech who work in the area of cellular engineering. The author also thanks his research collaborators, R.W. Alexander, B.C. Berk, P. Delafontaine, M.J. Levesque, M. Sato, C.J. Schwartz, and E.A. Sprague, for their contributions to the work and ideas reflected in this paper. Finally, the author thanks his students who keep coming up with new ideas and who will be the cellular engineers of tomorrow. *The lecture was delivered on April 22, 1991 by R.M. Nerem, Parker H. Petit Professor for Engiv.eering in Medicine, School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405. 529
530 R.M.Nerem tation and imaging,has grown substantially,and although,in general,one would not characterize this as a high technology industry,there clearly are areas of commercial application where the limits of our knowledge are being pushed.In the last two de- cades the number of individuals who have identified themselves as biomedical engi- neers,or bioengineers,has dramatically increased,and the number of programs at academic institutions,and the number of students participating in these programs, continues to grow.Finally,both the engineer and the physical scientist are increas- ingly being recognized as important members of the interdisciplinary teams,and their approaches as necessary for research and development at the forefront of the biomed- ical field. With this evolution comes a different type of change-one in which the spectrum of activity has been extended from the organ and tissue level to include cellular phenomena.Just as medicine in general has moved to focus on cell and molecular biology,so have engineers participating in biomedical research and in health care tech- nology applications.This has given rise to what some might call a sub-specialty of bio medical engineering,i.e.,the field of cellular engineering.However,this emerging area of cellular engineering is not just a sub-specialty,because it transcends the nor- mal borders of biomedical engineering,reaching beyond to provide a bridge to the field of biochemical engineering or bioprocess engineering. Just what is cellular engineering?As defined here it is the application of the princi- ples and methods of engineering to problems in cell and molecular biology of both a basic and applied nature.To elaborate on this,let us examine the various elements of this definition.The application of the principles and methods of engineering means to include the ability to quantify information and to establish interrelationships,as well as to model biological,chemical,and physical phenomena.In applying this to problems in cell and molecular biology,it is both cellular and sub-cellular processes which are of interest.Finally,in including applications of both a basic and applied nature,the spectrum of activities ranges from very basic research,i.e.,investigations in cell biology conducted with an engineering perspective,to the development and manufacturing of products based on this science,i.e.,the commercialization of the technology arising out of the science of cell and molecular biology. The development of the technology necessary to grow cells in the laboratory has been critical in the advancement of cellular engineering.In fact,if there is a subtitle to the theme of this presentation,it might be "sex and the single cell."This is because the act of cell replication is an underlying topic,a thread,interwoven throughout the remainder of this text.In the next section we will briefly examine cell culture tech- nology,i.e.,the technology of growing cells in the laboratory. CELL CULTURE TECHNOLOGY The biological cell is the basic sub-unit of any living system,the simplest unit that can exist as an independent living system(2).An individual cell has the ability to rep- licate,to differentiate,to migrate,to communicate,and to perform a whole host of other functions.In effect,this biological cell is a very social animal.It also is an amazingly complex system,the understanding of which challenges our engineering abilities to the utmost.In doing research on cellular and sub-cellular processes,not only can engineers contribute to the basic understanding of the biology of cells,but our engineering skills can be expanded,perhaps far more than we suspect
530 R.M. Nerem tation and imaging, has grown substantially, and although, in general, one would not characterize this as a high technology industry, there clearly are areas of commercial application where the limits of our knowledge are being pushed. In the last two decades the number of individuals who have identified themselves as biomedical engineers, or bioengineers, has dramatically increased, and the number of programs at academic institutions, and the number of students participating in these programs, continues to grow. Finally, both the engineer and the physical scientist are increasingly being recognized as important members of the interdisciplinary teams, and their approaches as necessary for research and development at the forefront of the biomedical field. With this evolution comes a different type of change-one in which the spectrum of activity has been extended from the organ and tissue level to include cellular phenomena. Just as medicine in general has moved to focus on cell and molecular biology, so have engineers participating in biomedical research and in health care technology applications. This has given rise to what some might call a sub-specialty of biomedical engineering, i.e., the field of cellular engineering. However, this emerging area of cellular engineering is not just a sub-specialty, because it transcends the normal borders of biomedical engineering, reaching beyond to provide a bridge to the field of biochemical engineering or bioprocess engineering. Just what is cellular engineering? As defined here it is the application of the principles and methods of engineering to problems in cell and molecular biology of both a basic and applied nature. To elaborate on this, let us examine the various elements of this definition. The application of the principles and methods of engineering means to include the ability to quantify information and to establish interrelationships, as well as to model biological, chemical, and physical phenomena. In applying this to problems in cell and molecular biology, it is both cellular and sub-cellular processes which are of interest. Finally, in including applications of both a basic and applied nature, the spectrum of activities ranges from very basic research, i.e., investigations in cell biology conducted with an engineering perspective, to the development and manufacturing of products based on this science, i.e., the commercialization of the technology arising out of the science of cell and molecular biology. The development of the technology necessary to grow cells in the laboratory has been critical in the advancement of cellular engineering. In fact, if there is a subtitle to the theme of this presentation, it might be "sex and the single cell." This is because the act of cell replication is an underlying topic, a thread, interwoven throughout the remainder of this text. In the next section we will briefly examine cell culture technology, i.e., the technology of growing cells in the laboratory. CELL CULTURE TECHNOLOGY The biological cell is the basic sub-unit of any living system, the simplest unit that can exist as an independent living system (2). An individual cell has the ability to replicate, to differentiate, to migrate, to communicate, and to perform a whole host of other functions. In effect, this biological cell is a very social animal. It also is an amazingly complex system, the understanding of which challenges our engineering abilities to the utmost. In doing research on cellular and sub-cellular processes, not only can engineers contribute to the basic understanding of the biology of cells, but our engineering skills can be expanded, perhaps far more than we suspect
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 simulation
Cellular Engineering 531 Although research designed to investigate the properties and behavior of biological cells goes back many centuries, it is in the last fifty years that the level of activity 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 environment 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 conditions, new opportunities for basic research have opened up. Furthermore, this technology 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 important 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 others. 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 supplemented 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 human 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 number 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 media 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 requirements, 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
532 R.M.Nerem For anchorage-dependent cells,another important factor in cell culture is the sur- face on which the cells are grown.The characteristics of this surface,e.g.,its micro- structure and surface chemistry,can induce changes in the cell.In fact,it is the interaction of a cell with its environment which may determine the structure and func- tion of a cell.This environment includes the medium,the surface to which the cells are adherent,and other factors,e.g.,the presence of flow. Cell culture is important because in many cases cellular engineering efforts are cen- tered around its use.At Georgia Tech this focus is the Bioengineering Center's Mam- malian Cell Culture Laboratory.Associated with this facility are 5 faculty and nearly 20 graduate students.The laboratory's research projects cover the spectrum from ba- sic to applied research,and the types of cells used in this laboratory can serve as an example of the wide range available for research today.Included are a variety of vas- cular endothelial cells (bovine aortic human dermal microvascular,and human um- bilical vein)and smooth muscle cells (bovine,and rat aortic)which are used in our studies of vascular biology and in research related to the development of tissue- engineered vascular prostheses.In addition,we use a number of different cell lines in our research.This includes 3T3 mouse fibroblasts,Bowes melanoma cells,BTC3 mouse pancreatic cells,and two lines of the AtT-20 mouse pituitary cell,one genet- ically engineered to secrete insulin,and the other human growth hormone.These are used in projects related to either tissue engineering or bioprocess engineering. Cell culture technology can thus provide the foundation for a wide array of activ- ities.As illustrated in Fig.1,this not only includes basic cell biology research,but also such applications as tissue engineering and bioprocess engineering.These will be dis- cussed later. Cell Culture Technology Cell Bioprocess Biology Engineering Tissue Other Engineering Applications FIGURE 1.The development of cell culture technology has not only resulted in an acceleration of basic research in cell and molecular biology,it also has led to the use of living cells in commercial product applications,e.g.,tissue engineering and bioprocess engineering
532 R.M. Nerem For anchorage-dependent cells, another important factor in cell culture is the surface on which the cells are grown. The characteristics of this surface, e.g., its microstructure and surface chemistry, can induce changes in the cell. In fact, it is the interaction of a cell with its environment which may determine the structure and function of a cell. This environment includes the medium, the surface to which the cells are adherent, and other factors, e.g., the presence of flow. Cell culture is important because in many cases cellular engineering efforts are centered around its use. At Georgia Tech this focus is the Bioengineering Center's Mammalian Cell Culture Laboratory. Associated with this facility are 5 faculty and nearly 20 graduate students. The laboratory's research projects cover the spectrum from basic to applied research, and the types of cells used in this laboratory can serve as an example of the wide range available for research today. Included are a variety of vascular endothelial cells (bovine aortic human dermal microvascular, and human umbilical vein) and smooth muscle cells (bovine, and rat aortic) which are used in our studies of vascular biology and in research related to the development of tissueengineered vascular prostheses. In addition, we use a number of different cell lines in our research. This includes 3T3 mouse fibroblasts, Bowes melanoma cells, ~3TC3 mouse pancreatic cells, and two lines of the ART-20 mouse pituitary cell, one genetically engineered to secrete insulin, and the other human growth hormone. These are used in projects related to either tissue engineering or bioprocess engineering. Cell culture technology can thus provide the foundation for a wide array of activities. As illustrated in Fig. 1, this not only includes basic cell biology research, but also such applications as tissue engineering and bioprocess engineering. These will be discussed later. Cell Culture Technology Cell Biology Bioprocess Engineering Tissue Engineering Other Applications FIGURE 1. The development of cell culture technology has not only resulted in an acceleration of basic research in cell and molecular biology, it also has led to the use of living cells in commercial product applications, e.g., tissue engineering and bioprocess engineering
Cellular Engineering 533 As described earlier,there are many factors which influence our ability to grow cells,however,to a large degree these are at best only partially understood.The re- sult is that,although "the cultural revolution has begun"as advertised a few years ago by Invitron,a St.Louis-based company,cell culture technology remains more an art than a science.Still,the technology used in culturing mammalian cells has become a cornerstone for the development of cell and molecular biology as a scientific dis- cipline and in the commercial applications arising out of this basic research. BASIC RESEARCH IN CELL BIOLOGY As noted in the previous section,the advent of cell culture technology has helped to dramatically accelerate advances in cell biology.This is true of the entire spectrum of research on cellular and sub-cellular phenomena.The engineer,through the vari- ety of ways in which the principles and methods of engineering can be applied,has been a participant in this area.Another key factor which links engineering to cell bi- ology is its relationship to the biophysics of a cell,i.e.,the role of physical mecha- nisms and the influence of physical factors on cellular behavior. Because most biomedical researchers,e.g.,MDs and life scientists,have a train- ing which,in general,is biochemistry based,and which does not emphasize physics, these researchers have tended to focus more on the biochemistry of a cell and not on the biophysics of a cell.However,physical factors have been demonstrated to be im- portant,and one example of this is in the influence of mechanical stresses and the re- sulting mechanics of deformation.Engineers involved in such studies of biomechanics have contributed and continue to contribute to our understanding of organ physiol- ogy and tissue behavior.They are now applying their knowledge to the investigation of the mechanical nature of much of cellular phenomena and the application of the principles of mechanics in order to understand the structure and function of cells(38). This type of cellular biomechanical phenomena can be illustrated by the process of cell division.In examining this process for an eukaryotic cell,one must consider the entire reproductive cycle of the cell(2).This cell cycle includes a number of sep- arate phases.The actual process of cell division is called M phase (M=mitosis),and the next cycle starts with the G phase(G gap)which is the period of time between the end of M phase and the beginning of DNA synthesis.The period of DNA syn- thesis is called S phase(S synthesis),and it ends when the DNA content of the nu- cleus has doubled and the chromosomes have replicated.The cell then enters the G2 phase,which may be viewed as preparatory for the actual process of cell division.The G2 phase is followed by M phase which in itself is composed of two specific events, mitosis and cytokinesis.Mitosis involves the splitting of the content of the nucleus, which causes a variety of intracellular movements,of mechanical nature,to take place during the different mitotic phases.Also,during cytokinesis,when the cell divides its cytoplasm,there is a very distinct mechanical event when the contractile ring,which has formed from cytoskeletal components,cleaves the cell into two daughter cells. These,of course,are not purely mechanical events.They are more accurately called mechano-chemical phenomena,and there is in fact an increasing recognition of the importance of such phenomena,and the strong coupling between structure and func- tion in an eukaryotic cell. The study of the influence of hemodynamics on vascular biology/pathobiology, and as a factor in the localization of atherosclerosis (45),is one area of biomedical
Cellular Engineering 533 As described earlier, there are many factors which influence our ability to grow cells, however, to a large degree these are at best only partially understood. The result is that, although "the cultural revolution has begun" as advertised a few years ago by Invitron, a St. Louis-based company, cell culture technology remains more an art than a science. Still, the technology used in culturing mammalian cells has become a cornerstone for the development of cell and molecular biology as a scientific discipline and in the commercial applications arising out of this basic research. BASIC RESEARCH IN CELL BIOLOGY As noted in the previous section, the advent of cell culture technology has helped to dramatically accelerate advances in cell biology. This is true of the entire spectrum of research on cellular and sub-cellular phenomena. The engineer, through the variety of ways in which the principles and methods of engineering can be applied, has been a participant in this area. Another key factor which links engineering to cell biology is its relationship to the biophysics of a cell, i.e., the role of physical mechanisms and the influence of physical factors on cellular behavior. Because most biomedical researchers, e.g., MDs and life scientists, have a training which, in general, is biochemistry based, and which does not emphasize physics, these researchers have tended to focus more on the biochemistry of a cell and not on the biophysics of a cell. However, physical factors have been demonstrated to be important, and one example of this is in the influence of mechanical stresses and the resulting mechanics of deformation. Engineers involved in such studies of biomechanics have contributed and continue to contribute to our understanding of organ physiology and tissue behavior. They are now applying their knowledge to the investigation of the mechanical nature of much of cellular phenomena and the application of the principles of mechanics in order to understand the structure and function of cells (38). This type of cellular biomechanical phenomena can be illustrated by the process of cell division. In examining this process for an eukaryotic cell, one must consider the entire reproductive cycle of the cell (2). This cell cycle includes a number of separate phases. The actual process of cell division is called M phase (M = mitosis), and the next cycle starts with the G1 phase (G = gap) which is the period of time between the end of M phase and the beginning of DNA synthesis. The period of DNA synthesis is called S phase (S = synthesis), and it ends when the DNA content of the nucleus has doubled and the chromosomes have replicated. The cell then enters the G 2 phase, which may be viewed as preparatory for the actual process of cell division. The G2 phase is followed by M phase which in itself is composed of two specific events, mitosis and cytokinesis. Mitosis involves the splitting of the content of the nucleus, which causes a variety of intracellular movements, of mechanical nature, to take place during the different mitotic phases. Also, during cytokinesis, when the cell divides its cytoplasm, there is a very distinct mechanical event when the contractile ring, which has formed from cytoskeletal components, cleaves the cell into two daughter cells. These, of course, are not purely mechanical events. They are more accurately called mechano-chemical phenomena, and there is in fact an increasing recognition of the importance of such phenomena, and the strong coupling between structure and function in an eukaryotic cell. The study of the influence of hemodynamics on vascular biology/pathobiology, and as a factor in the localization of atherosclerosis (45), is one area of biomedical