Life Itself: Exploring the Realm of the Living Cell / Edition 1

by Boyce Rensberger

Overview

Hidden in a nondescript red-brick building in Rockville, Maryland, is the most unusual warehouse in the world, a bank of living cells called the American Type Culture Collection. Here, at 321 degrees below zero—a temperature at which life abandons its vital dance and enters limbo, but without dying—are some 30,000 vials holding 60 billion living forms in suspended animation, including mouse kidney cells, turkey blood cells, armadillo spleen cells, and some 40 billion human cells. These cultured cells are essential to modern biological research—in fact, cells today are the most intimately studied life forms in all of science, for both practical and philosophical reasons. For one, all disease—from cancer and the common cold, to arthritis and AIDS—stems from cells gone awry. And cell research not only promises a cure for a wide variety of disease—it also holds the key to the mystery of life itself.
In Life Itself, Boyce Rensberger, science writer for The Washington Post, takes readers to the frontlines of cell research with some of the brightest investigators in molecular, cellular, and developmental biology. Virtually all the hottest topics in biomedical research are covered here, such as how do cells and their minute components move? How do the body's cells heal wounds? What is cancer? Why do cells die? And what is the nature of life? Readers discover that—contrary to what we may have concluded from pictures in our high school textbooks—cells teem with activity and that, inside, they "are more crowded with components than the inside of a computer." We learn that scientists now know of at least ten molecular motors that move things about inside the cell—in most cells, this motion is short because the cell is tiny, but in the single-celled nerve fibers that run from the base of the spinal cord to the toes (measuring three or four feet in an adult human), molecular motors can take several days to make the trip. Rensberger describes the many fascinating kinds of cells found in the body, from "neural crest cells" (early in embryonic development, these cells crawl all over the embryo to the sites where they will pursue their fate—as nerve cells, or cartilage, or skin), to "dust cells" (nomadic cells in the lung that swallow and store indigestible particles, then migrate to the gullet where they themselves are swallowed and digested), to "natural killer cells" (millions of which roam the body looking for cancerous cells). We meet many of the scientists who have pioneered cell research, such as Rita Levi-Montalcini—an Italian who, shut out of her lab during World War II, continued to experiment in her bedroom at home, making the discovery ("nerve growth factor") for which she won the Nobel Prize—and American Leonard Hayflick, who proved that all human cells (except cancer cells) invariably die after about fifty divisions. Rensberger also provides an illuminating discussion of AIDS—revealing exactly why this virus is so difficult to defeat—and of cancer, explaining that before cancer can start, a whole series of rare events must occur, events so unlikely that it seems a wonder that anyone gets cancer at all.
The solutions to the most pressing challenges facing scientists today—from the efforts to conquer disease to the quest to understand life itself—will be found in the innermost workings of the cell. In Life Itself, Boyce Rensberger paints a colorful and fascinating portrait of modern research in this vital area, an account which will enthrall anyone interested in state-of-the-art science or the incredible workings of the human body.

Product Details

ISBN-13:	9780195125009
Publisher:	Oxford University Press
Publication date:	12/17/1998
Edition description:	Reprint
Pages:	304
Product dimensions:	6.18(w) x 9.24(h) x 0.87(d)
Lexile:	1330L (what's this?)

About the Author

Boyce Rensberger has been a science writer and science editor for more than 30 years, including long stints at the New York Times and the Washington Post. He now directs the Knight Science Journalism Fellowship Program at the Massachusetts Institute of Technology.

Read an Excerpt

Chapter One

A PARTICLE OF LIFE

Clouds of cold fog billow up through a circular hatch in the top of a stainless steel tank as Rob Hay pulls out the lid and its one-foot thickness of styrofoam insulation. As the fog rolls down to the floor, Hay peers into the dark tank, where the temperature is always 321 degrees below zero Fahrenheit.

It is kept so cold because that is a temperature at which life, normally warm and pulsing with activity, abandons its vital dance and enters limbo—but without dying.

Inside Rob Hay's stainless steel tank are about 60 billion life-forms gathered from all over the planet. They wait in chilly repose. They neither eat nor sleep. They do not breathe. Not even the simplest chemical reactions of metabolism take place inside their bodies. By any conventional definition of life, the creatures have surrendered their claim on it.

And yet by those definitions, anyone can work a miracle: Simply reach into the tank and take out one of the 30,000 hermetically sealed glass vials, or ampoules, each about an inch long and each holding about two million of the inanimate creatures in less than a teaspoonful of frozen fluid. Now let it warm to body temperature. From their icy limbo, the tiny creatures will undergo resurrection in minutes. They will, in the language of an age that could not have imagined what is routine in almost every biomedical laboratory today, "come back to life." Some of the little organisms begin moving around, crawling over the inside surface of their container. They will begin feeding on the nutrients dissolved in the now-melted liquid that surrounds them. And—thesurest proof that they are truly alive—the creatures will begin reproducing.

The organisms in the ampoules are cells—the fundamental units of life, the microscopic building blocks of which all living organisms are made. They have been removed from the bodies of thousands of different animals of nearly a hundred species. In some of the frozen ampoules are kidney cells from a mouse. In others are skin cells from a chimpanzee. There are turkey blood cells, armadillo spleen cells, iguana heart cells—dozens of different kinds of cells from scores of different species. And, of course, there are cells of human beings. More than 20,000 frozen ampoules hold about 40 billion human cells in suspended animation.

Inside ampoules marked "ATCC CCL 72," for example, are skin cells taken in 1962 from a nine-month-old baby girl. The child has long since died of a birth defect, but her cells survive, ready to "come back to life" any time anybody removes an ampoule and warms it up. In "ATCC HTB 138" are brain cells removed from a seventy-six-year-old man in 1976. He too is now dead, but not his brain cells. In ampoule "ATCC CCL 204" are lung cells from a thirty-five-year-old man who died in 1979 and whose body was maintained so that its organs could be transplanted. Before the life support system was switched off, researchers bestowed a kind of immortality on a tiny piece of his lung. They sliced out a chunk containing a few thousand lucky cells and granted them the opportunity to live on, growing and reproducing themselves independently for decades.

The most famous cultured cells, or cell line, in the annals of biomedical research are those in "ATCC CCL 2," known worldwide as HeLa cells because they came from Henrietta Lacks, a thirty-one-year-old Baltimore woman who died in 1951 of cervical cancer. The cells from her tumor have been multiplying in laboratories all over the world ever since. At one time HeLa cells were among the most popular with researchers, who, like gardeners sharing cuttings from a favored plant, freely passed a few cells from their own colony to other researchers. So many dishes and flasks of the HeLa cell line are now alive around the world that it is estimated they weigh far more than Henrietta Lacks ever did.

Some 1,400 other cell lines, representing more than 70 different types of tissue from hundreds of human beings, are also held frozen in sealed glass ampoules inside stainless steel tanks like the one Rob Hay is looking into. Hay is a biomedical scientist and director of cell biology at one of the most unusual organizations in the world, the only one of its kind in the United States. It is a bank of living, though mostly frozen, cells called the American Type Culture Collection, which occupies a nondescript, red brick building in Rockville, Maryland, a suburb of Washington, D.C. The ATCC, as it is known throughout the biomedical research community, every year ships more than 50,000 ampoules of frozen cells to scientists around the world. The ampoules are packed in dry ice chunks inside a thick styrofoam cooler.

"When they first started shipping cells years ago, it was considered a pretty weighty decision," recalls Robert E. Stevenson, a longtime ATCC director. "After all, they wondered, since it was human cells, would anybody be afraid that human life was being sent through the mail? They even checked with the Catholic Church. It turned out nobody really had a problem with it once they understood what was going on."

When researchers receive their cells, they perform routine resurrections, thawing the cells, transferring them to new bottles with fresh nutrient-rich broth, and rearing successive generations of multiplying cells.

On the cutting edge of modern medical research, cultured cells are the most intimately studied life-forms. Biologists still use mice, white rats, guinea pigs, and fruit flies. But more and more it has become evident that many of the most challenging questions—from the practical desire to conquer disease to the purely intellectual quest to understand how life works—can only be answered with a detailed knowledge of the innermost workings of individual cells.

Human life, biologists now know, is really the sum of the lives of many individual cells organized in specific ways. And all disease is the result of processes that go awry within cells or among cells. Cancer and the common cold, heart disease and hay fever, even AIDS and arthritis, all afflict people because of failed mechanisms within cells. This is why biologists say basic research—aimed at understanding how cells work rather than at conquering any specific disease—promises knowledge that can be used to attack many, and perhaps all, diseases.

Sickle-cell anemia, for example, is the result of the slightly flawed shape of one kind of molecule in a person's blood cells. The errant molecule warps the red blood cell's normal doughnut shape into a crescent, or sickle, shape. Some forms of diabetes are caused by faulty molecules called receptors that are embedded in the membranes that enclose cells. These molecules are specialized gatekeepers, intended to recognize only insulin molecules and, when one comes along and binds to it, they will relay the appropriate signal to the cell's interior. With faulty shapes, the molecules fail to recognize insulin and the body becomes as sick as if it had no insulin at all.

As scientists probe the innermost workings of these microscopic components of the human body, they are beginning to understand one of the most profound mysteries ever contemplated—the nature of life and how it works.

Until the mid-nineteenth century, scientists were ignorant of the existence of cells. Life seemed to come only in units of the whole organism—a cat, a bird, a human being. Many held that the fundamental components of organisms were structural members—fibers and vessels, specifically; these strings and tubes, they believed, somehow grew like crystals, enlarging and changing shape until they attained adult proportions. Skin was seen as a modified tube, closed at the ends. Muscle was obviously bundles of fibers. The various internal organs were simply vessels of different shapes, no more amazing than a pipe or a pot.

The orderly predictableness with which the body grew from conception—always forming a variety of specialized organs structured in the same way—was attributed to a theoretical property called the "vital force." It was often thought of as the breath of life that God bestowed on Adam after fashioning his body from clay Philosophers, arguing on behalf of a theory called "vitalism," said that scientists could probe only so far in seeking to understand how life works. Then they would run up against something that was inherently unknowable, something beyond the realm of science. The presence of this entity in a human body was said to be that of the soul—a phenomenon that mortals could not hope to study nor understand. In the early eighteenth century George E. Stahl, the first of a long line of German scholars to dominate early cell biology, championed an explicit version of vitalism called "animism." He taught that all the physical parts of the body were passive and that motion, or animation, came from the soul. Although it is tempting nowadays to ridicule such views, in the days before there was any significant knowledge about cells, the theory of vitalism was mainstream science. In that age when modern science was just beginning, scientists were not at all uncomfortable mixing naturalistic explanations with supernatural ones.

Death in those days was thought of as the departure of the vital force, the exiting of the soul, the giving up of the ghost, or spirit. All the structural forms of the body might seem to remain intact in the corpse, but once the vital force had gone, the body was no longer animated but now inert and subject to decay and corruption.

Over the ensuing centuries, however, biologists would probe deeper into the living things they studied and gradually find that most of the phenomena they attributed to mystical forces could, in fact, be explained by entirely natural processes. As scientists gradually demanded more and more evidence for their theories, the concept of vitalism slowly faded. In 1838 and 1839, for example, two Germans, Matthias Schleiden, a botanist, and Theodor Schwann, a zoologist, independently advanced the radical "cell theory." Although they did not originate the idea (the French biologist Rene J. H. Dutrochet and the German naturalist and philosopher Lorenz Oken first asserted that cells were the fundamental structural units of life), Schleiden and Schwann elevated it to one of the most important theories in science. The fundamental units of life, they said, were not fibers and vessels or other gross anatomical structures. They were microscopic cells, each of which was a living entity of its own. The term "cell" had arisen two centuries earlier when the English scientist Robert Hooke looked through an early microscope at a thin slice of cork and saw rectangular chambers that reminded him of monks' cells in a monastery. He called the microscopic voids "cells" (Figure 1.1). Hooke's cells, it later turned out, were the empty spaces inside the dead and dried cell walls of a cork tree. Schleiden and Schwann revived the term for microscopic chambers of living matter, applying it not to Hooke's empty spaces but to the packages of living stuff within the spaces.

Schleiden even went so far as to perceive one of the fascinating philosophical questions of cell theory: If a cell is a unit of life, then is a multicellular organism really a colony of one-celled individuals?

Some plants and animals, after all, live part of their lives as free-roaming individual cells and only later congregate by the thousands to form colonies that look like multicelled organisms. One well-studied example is the cellular slime mold called Dictyostelium, which has one of the more remarkable life cycles in nature. Individual cells of Dictyostelium roam the forest floor as free-living amoebas, but if food supplies turn scarce, they congregate in huge masses and assemble themselves by the thousands into a much larger multicelled organism that looks and acts like a slug. The slug creeps about for a while, then settles down and metamorphoses into something that looks more like a fungus. The blob of cells sprouts a thin stalk that projects upward perhaps a quarter of an inch. Then other cells of the slug climb up the stalk and organize themselves into a ball balanced on top. The outer cells harden into a shell; the inner cells shrivel into dustlike spores. Eventually the shell cracks open and the spores are scattered to the wind. Spores that land in wet places change into amoebas and slither off to begin the cycle anew.

Another challenge to Schleiden's definition of an organism is the living sea sponge. Push one through a fine-mesh sieve and its cells will separate from one another, turning clear aquarium water into a thick, cloudy liquid, like pea soup. Wait a few hours, however, and the cells will gradually find one another, stick together, and reassemble themselves into a whole sponge. Although sponges come in many species, each with a distinctive appearance, the individual sponge cells will invariably rebuild the correct architecture for its species. In fact, the disaggregated cells of two different sponge species can be mixed, and the cells will sort themselves out and reassemble only with their own kind, re-creating sponges of the original two species. In other words, contained within each individual cell is a part of the "knowledge" of how its species is built.

Schwann compared an animal made of cells to a hive of bees, suggesting that each hive was a kind of superorganism in which the individual cells, or individual bees, had a separate identity and could have an autonomous existence. In the early days of cell theory, incidentally, philosophical speculation about the possible colonial nature of the human body quickly found its way into debates in political philosophy about the role of the individual in relation to society as a whole.

One error in Schwann's thinking was his idea that cells arose anew out of some generalized body fluid. In the 1850s a major advance in cell theory laid this idea to rest. Rudolf Virchow, a German pathologist and a strong advocate of the cell theory, suggested instead that all cells are formed by the division of preexisting cells. It was a crucial modification of cell theory, for it opened the door to an understanding of how organisms develop, growing from one cell to many. Virchow also advanced the concept of the cellular basis of disease, the view that all disease arises because of afflictions within or among cells. Virchow was an early proponent of the idea that life is an essentially mechanical process—that it can he explained entirely by the workings of the laws of physics and chemistry and without any need of the vitalists' supernatural forces.

In their battle with the vitalists, the mechanists did not mind stating their views in provocative terms. "A man is what he eats," was one slogan, noting that every atom of the human body has been extracted from food. (To be totally accurate, the mechanists should have included water and air as sources of atoms in the human body.) Not even the stuff of the mind, the mechanists argued, had a mystical source. "The brain," some liked to say, "secretes thought as a kidney secretes urine."

Over the next few decades Virchow's ideas encouraged more detailed study of cell division. Improved microscopes and better methods of staining cells to make their normally colorless insides visible led to a slowly improving knowledge of both the internal architecture of the cell and the events of cell division—the process that gave rise to all existing cells. Still, much would remain unknown into the late twentieth century. By 1882 Walther Flemming, yet another German, produced the first detailed description of cell division, including the central phenomenon of mitosis, the creation of two identical sets of genetic material (that is, two sets of chromosomes), each an exact copy of the one set in the parent cell.

By 1900 cell theory had led to a coherent view of how a complex organism arises. It was clear that the organism begins as a single cell, formed at conception by union of the father's sperm and the mother's egg, or ovum. It was known that sperm and egg each carried a set of hereditary factors, or genes, from one parent. And it was understood that the combined genetic endowments were duplicated during each round of cell division and one complete set of genes passed on to each new cell.

In 1912 the mechanist movement received a major boost from a German-born biologist named Jacques Loeb, who had immigrated to the United States and began working summers at the Marine Biological Laboratory in the Cape Cod fishing village of Woods Hole, Massachusetts. In 1912 Loeb published a landmark book entitled The Mechanistic Conception of Life. In it he described his own experiments on sea urchin eggs. Loeb had found a way to remove the eggs from a female urchin and make them start their embryonic development, just as if they had been fertilized by sperm—but without sperm. Loeb found that a simple dose of certain lifeless chemicals would launch one of the most dramatic phenomena in all of biology, the development of an organism from a single cell. Loeb offered his research findings as confirmation of the mechanistic view.

Nowadays the procedure is quite routine and is performed even in high school biology classes to demonstrate the early stages of embryonic development. But in Loeb's day word of his experiments captivated the public. Newspaper headlines, reflecting the naivete of early science writing, virtually claimed that Loeb could create life in a test tube. Some compared it to reproducing "virgin birth" in the laboratory, starting living cells on a course that had long been regarded as miraculous. Loeb's experiments, in a seaside laboratory and on marine organisms, became so mixed up in the popular mind with the allegedly mysterious powers of the sea that unmarried women were advised not to bathe in the ocean. Childless couples, on the other hand, rushed hopefully to beach resorts. Loeb even received letters from desperate couples asking him to give them a child.

As the inheritor of a German tradition in which great thinkers in science felt free to pontificate on all manner of social and political issues, Loeb wrote and spoke widely. Like Virchow he espoused the society of cells as a model of social cooperation. Loeb hobnobbed with the literary figures of his day—Thorstein Veblen, H. L. Mencken, even the young Gertrude Stein, who studied marine biology at the Woods Hole lab for a summer. Sinclair Lewis made Loeb the model for Max Gottlieb, the wise scientific mentor of Martin Arrowsmith, the idealistic hero of his famous book Arrowsmith.

Loeb's thinking was also influenced by the rediscovery in the early twentieth century of Gregor Mendel's lost writings on the breeding of garden peas in his monastery garden. In the mid-nineteenth century, Mendel had discovered powerful evidence that hereditary traits are passed from one generation to the next in discrete units, parcels of heredity that today we call genes. But the monk's publications were little noticed nor long remembered. They were found again, however, in the opening years of the twentieth century and seized upon by the mechanists as evidence that molecules—which are, of course, discrete units—govern heredity. In 1911 a prescient Jacques Loeb grasped the significance of Mendel's findings and knew from more recent experiments that at every cell division the chromosomes were duplicated and parceled out in equal numbers to the daughter cells. The biochemist's chief job, Loeb said, was to find "the chemical substances in the chromosomes which are responsible for the hereditary transmission of a quality." Loeb was calling for the discovery of the structure of DNA and the genetic code.

It took longer than Loeb probably anticipated, but in the next halt: century biochemists would do exactly that. By applying a purely reductionist approach to the study of life, they would vindicate the mechanist view spectacularly with the discovery of the double helix of DNA and the cracking of the genetic code. While the biochemists were pursuing a largely molecular approach to life, a parallel branch of research, cell biology, was studying the forms and functions of whole cells. Eventually, in the 1970s and 1980s, the molecular approach and the cellular approach would merge into a relatively unified, and utterly mechanist, approach to the study of life itself:

But Loeb went even further. He was confident that the mechanics of life would prove simple enough that it should be possible to create life in the laboratory. "We must either succeed in producing living matter artificially, or we must find the reasons why this is impossible," Loeb wrote. Today, as we shall see, many of the phenomena of life—many of the events within cells that, together, make for life—can be made to happen spontaneously under artificial conditions in the test tube. Whether that would have satisfied Loeb is not clear. Most biologists today think it will be a very long time before anything like an artificial cell could be synthesized.

Loeb even saw ethics as the product of the mechanical processes of life:

If our existence is based on the play of blind forces and only a matter of chance; if we ourselves are only chemical mechanisms, how can there be an ethics for us? The answer is that our instincts are the root of our ethics and that the instincts are just as hereditary as is the form of our body. We eat, drink and reproduce not because mankind has reached an agreement that this is desirable, but because, machine-like, we are compelled to do so. The mother loves and cares for her children not because metaphysics had the idea that this was desirable, but because the instinct of taking care of the young is inherited. We struggle for justice and truth since we are instinctively compelled to see our fellow beings happy.

The adult human body, according to current estimates, is made up of sixty trillion cells—eleven thousand times as many units of life as there are human beings on Earth. The body also contains various nonliving materials, such as hair, fingernails, and the hard part of bone and tooth, all produced by cells. The most visible part of the body, the skin, is made up of the thickly matted, fibrous "skeletons" of dead skin cells. Even the fat of "middle-aged spread" resides within special fat-storage cells that can swell to huge proportions.

Human cells vary greatly in size, from the tiny red blood cell at 1/25,000th of an inch (0.00004 inch) across (Figure 1.2) to a typical diameter ten times larger of 1/2,500th of an inch (0.0004 inch) for a kidney cell or a liver cell to the gigantic muscle cells that can be thin filaments a few inches long. The record holders are the nerve cells that begin at the base of the spine and run all the way to the tip of the big toe—a distance of several feet.

In the human body there are about 200 different kinds of cells with different shapes and jobs, but all are similar in basic structure and internal workings. And, although each cell is only a minor constituent of a multi-celled organism, much of modern medical progress can be traced to one startling fact: Many kinds of cells can be removed from the body and allowed to live as independent organisms. This is the fact that makes possible the cell cultures at ATCC and in scores of other research laboratories around the world. The discovery of the potential independence of our cells confirms an early speculation of Schleiden's that cells lead what he called "double lives"—their own and that of the organism of which they are a part.

Snip a tiny chunk of tissue from almost any part of your body—a piece of skin, for example, that would fit inside this 0. Treat it briefly with protein-digesting enzymes that break the proteins holding the cells together. Drop the disaggregated cells (there would be thousands in a piece this size) into a teaspoonful of nutrient fluid. The cells that once were loyal, hard-working members of a vast organization of trillions of cells will revert to a way of life much like that of their evolutionary ancestors, the one-celled protozoans. Human cells retain a primordial capacity for independent life, crawling about their culture dishes like amoebas on a pond bottom, feeding on nutrients in the water, reproducing by cell division. Many cells even take on an amoeboid form, becoming sprawling blobs that constantly change shape as they undulate across the bottom of their dishes (Figure 1.3).

Even within the human body there are cells that live rather like protozoans. Certain white blood cells, for example, are part of the immune system and flow as free individual cells with the blood. But they can detect bacterial infections and exit the circulatory system to enter affected tissues, where they crawl around to catch and eat the invaders. Like amoebas, they simply engulf the bacteria and digest them.

One specialized member of the immune system is called the "natural killer cell." Millions of them roam the body, searching for other cells that have turned cancerous. Once the killer cell finds its prey, it presses close and exudes a substance that kills the cell. This newly emerging understanding of natural killer cells, incidentally, is leading to new ideas on the prevention, cause, and cure of cancer. Some researchers suspect cells frequently turn cancerous but are usually killed before they can proliferate into tumors. One reason tumors arise, then, may be defective killer cells, so some researchers are looking for ways to cure cancer by boosting the number of killer cells in the body.

Still another kind of amoebalike cell in the body inhabits bone. When bones are growing, or healing after a fracture, these cells crawl through the hollow spaces something like a snail that leaves a slime trail. The bone cell's slime, however, is a substance that hardens into the mineral part of bone, gradually building the bone's thickness. Other motile bone cells do the opposite, taking up previously deposited bone by dissolving it in their path. Like sculptors, the two kinds of cells work in concert, removing bone here and adding bone there, to remodel the tiny bones of a baby into the big ones of an adult. When bones break, these cells receive a special signal that spurs them to knit the fragments together. Even if the bone fragments grow together in a crooked form, the bone sculptors will continue reshaping the affected area until it becomes normal again.

Skin cells also become motile to heal wounds. Imagine a cut finger or a scraped knee. New skin doesn't grow only at the edges of the wound, repairing it progressively from the edges toward the middle. Special proto-skin cells crawl from the bottom layer of the epidermis and onto the exposed wound surface, where they multiply rapidly and distribute themselves in a thin layer over the entire damaged area. Once the cells have formed a complete layer, they begin dividing with a new result. The daughter cells change form and rebuild the skin's normally multilayered structure.

Surely the most spectacularly motile human cells are sperm. Like many of their primordial ancestors in pond water, they swim by lashing their taillike flagella.

Because individual cells are virtual organisms in their own right, some biologists maintain the view of Schleiden and Schwann and think of each multicelled organism, including each human being, as a community of organisms, a huge colony of extraordinarily selfless citizens, each forsaking independent existence for the good of the colony. The kidney cell patiently resides deep within its own province, performing its specialized job of removing waste materials from the blood. The skin cell clings tightly to its neighbors to protect the body against infection and desiccation, dutifully staying put—unless, of course, an injury requires it to mobilize its healing potentials. The human body is a republic of cells, a society of discrete living beings who have, for the good of the society as a whole, sacrificed their individual freedoms.

So profound is this sacrifice that cells have gone so far as to sign their own death warrants and hand them over to their neighbor cells. Then, should a cell's continued existence become a threat to its society, the neighbors may execute the warrant and leave the cell no choice but to commit suicide. Upon command of its neighbors, the hapless cell initiates a genetic program that carries out its suicide in ritual precision. Although evidence of this extraordinary form of natural cell death has been known for decades, only since the early 1990s have cell biologists accumulated evidence that this "programmed cell death" is an integral part of multicellular life.

This phenomenon, sometimes called apoptosis (from the Greek words that refer to leaves falling off a tree in autumn), is in fact the way "natural killer cells" destroy cells that have turned cancerous. It also plays a key role in embryonic development, performing the cellular equivalent of removing the scaffolding after construction is complete. For example, during the fifth week of human embryonic life, the hands are flat paddles with no distinct fingers. Then, during the sixth week, four waves of programmed cell death roll across the paddle, removing the cells between the fingers. Even in adult life, the death program continues as millions of cells kill themselves every minute in the human body. We'll return to this extraordinary process in detail in later chapters.

Of course, cells have also gained something in the cooperative bargain. By banding together they create an environment that is far more stable and nurturing for each individual than the outside world can provide. By collaborating in various specialized jobs, the cells that make up a human body create huge systems that maintain ideal temperature, protect against drying out, and provide ample supplies of oxygen and nutrients.

For all the evident differences among specialized cell types, one of the most profound insights to emerge from modern cell biology is that all cells, even those from species as different as a yeast and a human, carry out the same, fundamental housekeeping chores of life in exactly the same way. A human brain cell, for example, is not more complicated than a one-celled creature inhabiting pond scum. Indeed, all the fundamental processes needed to sustain life are identical from the amoeba cell to the human cell, just as the fundamental processes of a sedan and a station wagon are identical. The differences are in the added special functions and shapes.

Though animal cells were confined to communal existence billions of years ago when multicelled animals evolved from one-celled ancestors, liberation did not come until 1885 when Wilhelm Roux, still another of the Germans who dominated early basic biology, discovered that a patch of cells removed from a whole organism (from a chicken embryo in this case) could live for several days in a dish of saline solution. Further success in culturing individual cells came in 1907 when the American biologist Ross G. Harrison of Yale University found a way to maintain frog nerve cells in a dish. He found that isolated nerve cells in his dish even sprouted thin filaments that reached out and made contacts with other nerve cells, as if they were trying to form a primitive nervous system.

Further progress in cell culture methods came slowly, as researchers sought mainly for a better understanding of the environmental conditions and nutrients that cells need to live and multiply. It was not until the early 1950s that cell culture became fairly routine. It was also at that time that the cancerous cells of Henrietta Lacks were removed and established in culture as HeLa cells.

The main thing cultured cells need is to be bathed in a fluid that contains some food—mainly sugar, mineral salts, some vitamins, and some amino acids, which are the building blocks of proteins. The fluid should both supply oxygen and take up the waste product carbon dioxide. When kept warm in such a fluid, the tiny blobs of life thrive. They take in the nutrients, reprocess them to grow and to carry out their specialized functions, and then excrete the waste products. To avoid programmed cell death, cultured cells also must be given a supply of "growth factors" produced by other cells.

As the cells creep about in their plastic dishes, they thrust out a wide, ruffled edge that seems to search the space ahead for cues as to which way to go. Parts of the leading edge stick down to the surface and the cell's body appears to drag its hind parts along. When two cells bump, they cringe, shrink back and quietly slither off in new directions.

Periodically the cells cease their travels, let go of all attachments to the surface, pull themselves into a round ball, and undergo perhaps the most dramatic of life's many astonishing phenomena—cell division. The cell breaks down some of its old internal structures, recycling the components to assemble new ones. The cell's genetic endowment, made of the chemical DNA, is copied into a duplicate set of genes. Then the old cell pinches in half to become two young cells. Through this routine but utterly astonishing process of mitosis, many cells are reincarnated without ever dying.

After several generations of doubling in laboratory containers, the cells become too crowded, and their human keepers must come to their aid. Some of the cells are removed to found new colonies in new dishes of nutritive fluid. The amount removed is critical because different types of cells have different requirements. Some are content as loners, and only a few are needed to start a new population in a new bottle. Others, as one biologist put it, "need to have their friends around. They die if they get too lonely."

Many cells living in culture remember their old lives as parts of tissues in larger organisms (Figure 1.4). Skin cells, for example, will multiply until they form a sheet covering the bottom of their container, just as they used to make skin in their native habitat. Under certain conditions, the layer of skin cells will even develop into proper, multilayered skin—a human tissue assembling itself right in a dish. Today, researchers are learning to control this process in the hope of creating skin farms in the laboratory as sources of human skin that could be grafted onto burn patients who have lost some of their own. Also in the culture dish, cells called fibroblasts will keep on secreting collagen, the tough fibrous protein they once made to give strength to skin and other tissues. Cells of the female breast will manufacture and secrete milk protein. Muscle cells will sometimes weld themselves into large fibers that spontaneously begin twitching in the dish. When cells of the heart muscle do this, they begin beating rhythmically. Even disembodied nerve cells, as Harrison discovered in 1907, will sprout long, thin filaments of living tissue that reach across the surface of the dish, as if seeking another cell to deliver an electrochemical message. And, as recently as 1990, researchers discovered that human brain cells can be induced to multiply in a dish. The cells link up to form synapses, the connections that relay signals from one nerve cell to another. The little knots of brain cells behave as if they were assembling themselves into a primitive brain.

For decades, the only way to maintain human cell cultures was to keep them in incubators heated and thermostatically controlled to maintain a temperature of 98.6 degrees Fahrenheit—human body temperature. Some cell cultures removed from a fully developed human or other animal would proliferate for a number of rounds of cell division, and then mysteriously, the cells would die. Somehow they lost the ability to keep dividing. Cells taken from cancers, on the other hand, proved apparently immortal and would keep on living and dividing indefinitely. The very property that made them deadly to their former hosts now made them invaluable to cell biologists.

Then in 1949 Alan Parkes, a British biologist, developed an entirely new way to maintain cell lines—freezing them. Researchers had been attempting this for decades—especially as a way of storing the semen of prize bulls so that it would be available for artificial insemination after the animal died. Many experiments simply killed the cells. Some found that a few cells revived on thawing but that they were sickly. Finally Parkes found that if he added some antifreeze to the nutrient bath and then cooled the culture very slowly, the cells would enter limbo and could revive on warming. For this antifreeze, Parkes found success with glycerol, a clear, syrupy liquid better known to many as glycerin, which is also used in automobile antifreeze. Nowadays a better substitute is dimethyl sulfoxide, or DMSO (a clear liquid by-product of wood pulp manufacture that has many uses from paint remover to an anti-inflammatory drug).

Either way, however, the antifreeze is toxic to cells, so once they have been treated with it, they cannot be allowed to continue in the active state of life for too long. A batch of cells is placed in a vial, or ampoule, along with a watery solution that is 95 percent nutrient medium and 5 percent antifreeze. At ATCC the ampoule is put in a special freezer that cools it from room temperature to 112 degrees below zero over an hour. Faster freezing will kill cells. In many labs the vials are simply placed inside a block of thick styrofoam insulation and put directly in a freezer that maintains this temperature. The insulation slows the rate of heat loss to just the right speed.

What happens is that the water outside the cells freezes before the water inside the cells. This is because all the molecules normally present inside the cell lower the freezing point. The antifreeze keeps the water crystals outside the cell from becoming large enough to pierce the cell membranes. As the outside water slowly crystallizes, however, the liquid water inside the cell is drawn out, diffusing through pores in the cell's membrane and joining the ice forming outside the cell. The cell shrinks, literally deflated by the loss of water. If conditions are right, virtually all of the water will have left the cell before the inside becomes cold enough that it would freeze water into crystals big enough to rupture internal membranes.

As the temperature in the vial falls, all the metabolic processes of each cell—the hundreds of simultaneous but different biochemical reactions that, in the aggregate, amount to life—become slower and slower. When the temperature gets cold enough, they stop altogether. Like a microscopic bear going into hibernation (except that bears keep metabolizing at a slow rate), the cells gradually relinquish the usual textbook criteria for being alive. All that remains of these life-forms is the form itself—the architectural structures that, when warm, are unquestionably alive.

Are frozen cells dead? They do not eat or move. They do not even carry out the slightest form of metabolism. By the usual definitions of life, a frozen cell is not living. And yet, when the cells are warmed, they resume all the activities of life. When that happens, is life suddenly created anew? Does life arise spontaneously from nonliving matter? Is it resurrection ?

Many biologists answer that this is just a semantic trap, and that frozen cells should be considered neither alive nor dead but in a state of suspended animation. The key point is that what remains in the frozen cell is the integrity of its structures, from the unbroken cell membrane to intact forms of all internal architecture and molecules.

So long as an organism maintains its structural integrity, one can argue that it is alive. In this view, life can persist without anything happening, much as a car is still a motor vehicle when it is not running. The crucial factor in both cases is whether the component structures are present and linked in the right ways so that they could interact if given the right conditions.

The convenience of cold storage was immediately evident to officials at the American Type Culture Collection (ATCC), which had been started in 1925 to supply scientists from its reference collections of all forms of life, including bacteria, fungi, protozoans, and plants. It's cell bank was put on ice. Or, more properly, it was put on liquid nitrogen, the clear, colorless, odorless liquid (made in factories by chilling nitrogen gas until it condenses) that always stays a frosty 321 degrees below zero. As long as liquid nitrogen is kept well insulated, it stays cold with no need for motors or compressors.

It is liquid nitrogen that chills ATCC's stainless steel tanks and keeps their scientifically precious contents well within that mysterious state of biological limbo. Each tank is about halfway full of liquid nitrogen. Inside the tank some ampoules, clipped into metal holders, hang from a rack submerged in liquid nitrogen. Some hang from a rack in the nitrogen vapor just above, not as cold, but at 211 degrees below zero, still quite frozen.

"Say a researcher at a laboratory somewhere is working on a disease that involves a certain kind of cell in the body," Hay explains as he stands in the large first-floor room that ATCC researchers call the Tank Farm. More than forty stainless steel freezer tanks crowd the room, thirty-two of them devoted to cell culture and others to microbes and protozoans. If ATCC has what the researcher wants, he or she places an order. Unless the appropriate cells have already been thawed and are growing in special cell culture bottles in an incubator upstairs, a technician will have to revive the frozen cells waiting in an ampoule.

From the inert microscopic forms, unmoved and unmoving for perhaps many years, will emerge the long-suppressed pulsations of life. This simple phenomenon, this "resurrection," repeated in laboratories around the world hundreds of times a day, poses a profound challenge to the ancient view of life as a mystical process. On the other hand, "resurrection" is perfectly consistent with the view of modern cell and molecular biology that life is a mechanical process that is, in principle, no harder to understand than an automobile or a computer.

In place of the animating "vital force" of 150 years ago, modern biology confirms the view that all the phenomena that together constitute life can be understood in the terms of chemistry and physics. The frozen cells possess the right chemicals in the right combinations and in the right structural arrangements to live. The cold temperatures simply deny the cells the thermal energy needed for the chemical reactions to proceed. Like a car battery in a Minnesota January, the chemistry simply won't go when it's too cold. But add a little energy in the form of heat, and the chemistry happens.

Science is still a long way from being able to explain every detail of how life works, but in recent years basic biology laboratories have been founts of astonishing new knowledge. The field is racing many times faster than it did a generation ago when James Watson and Francis Crick figured out the double helix structure of DNA. Almost every day, some new detail of life's machinery is revealed.

Cell and molecular biologists can now explain many of life's most intimate workings as the results of purely nonliving events—interactions among atoms and molecules no more mysterious, though often far more complex and wondrous, than the crystallization of water molecules into snowflakes. As beautiful and as mathematically precise and as predictable as snowflakes, their formation is obviously no miracle. Crystallization of water is understood down to the last atom and can be produced in the laboratory at will or, obviously, in the kitchen refrigerator. It happens because certain molecules have the innate property of organizing themselves into predictable arrangements entirely without guidance from an outside intelligence. Many atoms or molecules will, when put into the right environment, link together in regular patterns to form crystals. Some are cubic crystals, some are hexagonal. Others take other forms. The key point is that no outside force need guide the events. The guidance is implicit in the molecules themselves. Certain molecules assemble themselves into specific, predictable structures because their shapes, their own physics and chemistry, will allow them to assemble only in those patterns.

This concept of self-assembly is fundamental in today's cell and molecular biology. Under the right conditions, atoms or small molecules automatically link—like water molecules assembling themselves into snowflakes—to form larger structures of absolutely predictable shape. Life's chemistry generally involves very large, highly complex molecules such as the proteins, but these too assemble themselves into still larger, predictable structures. Incidentally, although many people think of protein as something good to eat, cell and molecular biologists think of proteins as many different kinds of molecules, some of which act as little machines that carry out specific actions in the cell or as units of structure, such as bricks or girders, that can combine to make the architecture of the cell. A typical cell may contain several thousand different types of protein molecules.

The discovery of self-assembly explains many events in cells that once seemed utterly mysterious. It is akin to learning that the steel skeleton of a building will assemble spontaneously once a load of girders is dumped at a construction site. The genes cause a load of protein molecules to be synthesized in the cell, but the proteins, needing no further control, spontaneously assemble themselves into larger structures. The plan of the final result is implicit in the structure of the components (Figure 1.5).

Laboratory experiments have shown that self-assembly is even more amazing. Take batches of several different kinds of subunits and mix them together in what would seem to be a hopeless stew of hundreds of different substances. The molecules sort themselves out flawlessly and assemble perfectly. It is as it the load of girders were mixed up with a load of bricks and pipes and boards and plaster and nails and tiles and still the girders sorted themselves out and assembled spontaneously. And, at the same time, so did the bricks and boards and all the other components of the building.

Similar phenomena, as Jacques Loeb prophesied at the beginning of the twentieth century, lie at the heart of all life. Molecules of a given shape and composition possess the power not only to link themselves into larger structures but to act on entirely different molecules, causing them to break apart in specific ways or to combine with still other molecules in predictable ways. The molecules that act upon other molecules, for example, are intracellular mechanics called enzymes. As catalysts, enzymes make things happen chemically but remain separate from the result, able to work their effects again. An average cell contains many hundreds of different kinds of enzymes, each built to perform a specific job. Like their cousins, the structural proteins, enzyme proteins do their work automatically because of their shapes and the inherent chemical proclivities of their components.

It is almost impossible to overstate the significance of self-assembly and the inherent powers of enzymes, for these processes have profound philosophical implications. As the French molecular biologist Jacques Monod put it in his book Chance and Necessity, "... an internal, autonomous determinism guarantees the formation of the extremely complex structures of living beings." Although Monod published in 1970 at what now seems to be the Paleolithic era of molecular biology and few examples of self-assembly were available, he has turned out to be entirely right. So too have Loeb's very similar pronouncements more than half a century earlier. Monod died in 1976, but molecular and cell biologists continue to cite his book as an outstanding exposition of a philosophy that accepts a mechanistic view of life and celebrates its resultant glory—the entire living world of Earth.

Monod also saw that this view could resolve an old debate between two schools of thought about the forces that molded living organisms, including human beings. One group was the preformationists, adherents of an ancient doctrine, who claimed that the anatomical details of every person were formed at the creation and simply bloomed, in a sense, at the right time. The oldest version of preformationism even held that every sperm carried a miniature human being, called a homunculus, in whose sperm there were even tinier humans and so on. The rivals were the epigeneticists, disciples of a newer school that accepted the role of genes (their existence had become evident, though not understood, early in the twentieth century). The epigeneticists insisted that genes dictated only the structure of certain molecules. They argued that molecules needed some additional guidance—which they called epigenetic and which many regarded as supernatural—before the molecules could be assembled into larger structures such as cells and whole organisms.

The discovery of the phenomenon of self-assembly, Monod wrote, should end the debate by showing that both are, in a sense, right but that neither need invoke phenomena beyond those natural processes that can be understood by science. "No preformed and complete structure preexisted anywhere; but the architectural plan for it was present in its very constituents. It can therefore come into being spontaneously and autonomously, without outside help and without the injection of additional information. The necessary information was present, but unexpressed, in the constituents. The epigenetic building of a structure is not a creation; it is a revelation."

Monod's book was deeply disturbing to many because it asserted that no event in the life of a cell or, indeed, in the life of a whole human body, was the result of any supernatural guiding hand. Today, although everything that happens in the living cell still cannot be explained on the basis of known chemical reactions, the progress has been so spectacular that it would be hard to find a cell biologist today who thinks that the unexplained processes will not someday be fully understood.

"The secret of life is not a secret anymore," says Duke University cell biologist Harold Erickson. "We've known for twenty or thirty years now that life is not more mysterious than the chemical reactions on which it is based. There's an incredibly complex set of chemical reactions, but they're all logical and understandable. We don't yet understand them all but we do understand a lot of them and it's not hard to see that eventually we should know them all."

Tom Pollard, a cell biologist who heads the famed Salk Institute in La Jolla, California, and a former president of the American Society for Cell Biology, questions whether nonscientists are prepared to accept the idea that life is just so much chemistry and physics: "What molecular biologists have believed for two generations is now generally regarded as proved beyond any doubt. Life is entirely the result of physics and chemistry inside cells and among cells. I wonder whether the general public is prepared to sign on?"

The old, mystical view of life denied any opportunity for expression of the most wonderful aspect of human life, the intellect. But apply the intellect to the rational contemplation of modern cell and molecular biology and what emerges is the awareness, both chilling and inspiring, that the human body is a consummately wondrous assemblage of cells that are each machines. So are all forms of life. Although some religious people complain that this view of life removes any role for a supernatural creator, it need not do so. Just as most of the world's established religions accept evolution as the process by which their deity created the world, they also may come to see the new cellular and molecular biology as revealing the mechanisms by which that creation made life possible.

Editorial Reviews

"Life Itself is a wonderfully readable digest of everything currently known about the mechanisms by which living cells perform their myriad tasks."—The New York Times Book Review

"Rensberger's wonderment and enthusiasm...literally leaps off the page....it is hard to imagine a better way to convey to students the thrill of looking down a microscope at a living cell ."—Nature

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