You are on page 1of 10

The Architecture of Life

A universal set of building rules seems to guide

the design of organic structures—from simple
carbon compounds to complex cells and tissues
by Donald E. Ingber

L ife is the ultimate example of complexity at work. An

organism, whether it is a bacterium or a baboon, de-
velops through an incredibly complex series of in-
teractions involving a vast number of different components.
These components, or subsystems, are themselves made up
of smaller molecular components, which independently ex-
hibit their own dynamic behavior, such as the ability to cat-
alyze chemical reactions. Yet when they are combined into
some larger functioning unit—such as a cell or tissue—utterly
new and unpredictable properties emerge, including the abil-
ity to move, to change shape and to grow.
Although researchers have recognized this intriguing fact
for some time, most discount it in their quest to explain life’s
fundamentals. For the past several decades, biologists have
attempted to advance our understanding of how the human
body works by defining the properties of life’s critical materi-
als and molecules, such as DNA, the stuff of genes. Indeed,
biologists are now striving to identify every gene in the com-
plete set, known as the genome, that every human being car-
ries. Because genes are the “blueprints” for the key molecules
molecules from which a human is created. Understanding
does little to explain how the whole system works, regardless
of whether the complex system is a combustion engine or a organelles, which self-assem-
cell. In other words, identifying and describing the molecular ble into cells, which self-assemble
puzzle pieces will do little if we do not understand the rules into tissues, which self-assemble into
for their assembly. organs. The result is a body organized hierar-
That nature applies common assembly rules is implied by chically as tiers of systems within systems. Thus, if
the recurrence—at scales from the molecular to the macro- we are to understand fully the way living creatures form
scopic—of certain patterns, such as spirals, pentagons and and function, we need to uncover these basic principles
triangulated forms. These patterns appear in structures rang- that guide biological organization.
ing from highly regular crystals to relatively irregular proteins Despite centuries of study, researchers still know relatively
and in organisms as diverse as viruses, plankton and hu- little about the forces that guide atoms to self-assemble into
mans. After all, both organic and inorganic matter are made molecules. They know even less about how groups of mole-
of the same building blocks: atoms of carbon, hydrogen, oxy- cules join together to create living cells and tissues. Over the
gen, nitrogen and phosphorus. The only difference is how past two decades, however, I have discovered and explored
the atoms are arranged in three-dimensional space. an intriguing and seemingly fundamental aspect of self-as-
This phenomenon, in which components join together to sembly. An astoundingly wide variety of natural systems, in-
form larger, stable structures having new properties that could cluding carbon atoms, water molecules, proteins, viruses,
not have been predicted from the characteristics of their indi- cells, tissues and even humans and other living creatures, are
vidual parts, is known as self-assembly. It is observed at constructed using a common form of architecture known as
many scales in nature. In the human body, for example, large tensegrity. The term refers to a system that stabilizes itself
molecules self-assemble into cellular components known as mechanically because of the way in which tensional and

48 Scientific American January 1998 Copyright 1997 Scientific American, Inc.

compressive forces are distributed and balanced within the tures are mechanically stable not because of the strength of
structure. individual members but because of the way the entire struc-
This fundamental finding could one day have practical ap- ture distributes and balances mechanical stresses. The struc-
plications in many areas. For example, new understanding of tures fall into two categories. Structures in one category, which
tensegrity at the cellular level has allowed us to comprehend includes the geodesic domes of Buckminster Fuller, are basical-
better how cellular shape and mechanical forces—such as ly frameworks made up of rigid struts, each of which can bear
pressure in blood vessels or compression in bone—influence tension or compression. The struts that make up the frame-
the activities of genes. At the same time, deeper understand- work are connected into triangles, pentagons or hexagons, and
ing of natural rules of self-assembly will allow us to make each strut is oriented so as to constrain each joint to a fixed
better use—in applications ranging from drug design to tissue position, thereby assuring the stability of the whole structure.
engineering—of the rapidly accumulating data we have about The other category of tensegrity structures encompasses
molecules, cells and other biological components. An expla- those that stabilize themselves through a phenomenon known
nation of why tensegrity is so ubiquitous in nature may also as prestress. This type of structure was first constructed by
provide new insight into the very forces that drive biological the sculptor Kenneth Snelson. In Snelson’s elegant sculptures,
organization—and perhaps into evolution itself. structural members that can bear only tension are distinct from
those that bear compression. Even before one of these struc-
What Is Tensegrity? tures is subjected to an external force, all the structural mem-
bers are already in tension or compression—that is, they are

M y interest in tensegrity dates back to my undergraduate

years in the mid-1970s at Yale University. There my
studies of cell biology and also of sculpture led me to realize
prestressed. Within the structure, the compression-bearing rigid
struts stretch, or tense, the flexible, tension-bearing members,
while those tension-bearing members compress the rigid struts.
that the question of how living things form has less to do These counteracting forces, which equilibrate throughout the
with chemical composition than with architecture. The mol- structure, are what enable it to stabilize itself.
ecules and cells that form our tissues are continually removed Tensegrity structures of both categories share one critical
and replaced; it is the maintenance of feature, which is that tension is continuously transmitted across
pattern and architecture, I rea- all structural members. In other words, an increase in tension
soned, that we call life. in one of the members results in increased tension in mem-
Tensegrity struc- bers throughout the structure—even ones on the opposite

TENSEGRITY—an architectural system in which


structures stabilize themselves by balancing the

counteracting forces of compression and tension—
gives shape and strength to both natural and artifi-
KENNETH SNELSON (foreground);

cial forms. The cytoskeleton of a living cell (back-

ground) is a framework composed of interconnect-
ed microtubules and filaments. The dynamic re-
lation of these structural elements is reminiscent of
a sculpture (at center) by Kenneth Snelson, in
which long struts are joined with cables.

Copyright 1997 Scientific American, Inc. Scientific American January 1998 49

a given load with a minimum

©ALLEGRA FULLER SNYDER Buckminster Fuller Institute, Santa Barbara

amount of building materials.

side. This global increase in tension is balanced by an increase cells behave when placed on different surfaces. It had been long
in compression within certain members spaced throughout the known that cells spread out and flatten when they attach to a
structure. In this way, the structure stabilizes itself through a rigid glass or plastic culture dish. In 1980, however, Albert K.
mechanism that Fuller described as continuous tension and lo- Harris of the University of North Carolina at Chapel Hill
cal compression. In contrast, most buildings derive their stabili- showed that when affixed to a flexible rubber substrate, cells
ty from continuous compression because of the force of gravity. contract and become more spherical. This contraction bunch-
The tension-bearing members in these structures—whether es up, or puckers, the underlying rubber.
Fuller’s domes or Snelson’s sculptures—map out the shortest It occurred to me that a view of the cell as a tensegrity
paths between adjacent members (and are therefore, by defini- structure could easily explain such behavior. I modeled a cell
tion, arranged geodesically). Tensional forces naturally trans- as such a structure; it consisted of six wood dowels and some
mit themselves over the shortest distance between two points, elastic string. I arranged the dowels—which bore the com-
so the members of a tensegrity structure are precisely posi- pressive stress—in three pairs. Each pair was perpendicular to
tioned to best withstand stress. For this reason, tensegrity the other two, and none of the wood struts actually touched
structures offer a maximum amount of strength for a given one another. A tension-bearing elastic string connected to the
amount of building material. ends of all the dowels, pulling them into a stable, three-di-
mensional form. I also placed a smaller, spherical tensegrity
From Skeleton to Cytoskeleton model, representing the nucleus, within the larger one that
represented the rest of the cell. Then, to mimic cytoskeletal

W hat does tensegrity have to do with the human body?

The principles of tensegrity apply at essentially every
connections between the nucleus and the rest of the cell, I
stretched elastic strings from the surface of the large tensegrity
structure to the smaller one inside [see illustration at top right
206 bones that constitute our skeleton are pulled up against on opposite page].
the force of gravity and stabilized in a vertical form by the pull To understand how my experiment worked, it is necessary
of tensile muscles, tendons and ligaments (similar to the ca- to know that pushing down on a tensegrity model of the kind
bles in Snelson’s sculptures). In other words, in the complex I built forces it into what appears to be a flattened pile of sticks
tensegrity structure inside every one of us, bones are the com- and string. As soon as the pressure is removed, the energy
pression struts, and muscles, tendons and ligaments are the stored in the tensed filaments causes the model to spring back
tension-bearing members. At the other end of the scale, pro- to its original, roughly spherical shape. To simulate how cells
teins and other key molecules in the body also stabilize them- behave when placed on a surface, I mimicked a solid culture
selves through the principles of tensegrity. My own interest substrate of glass or plastic by stretching a piece of cloth taut
lies in between these two extremes, at the cellular level. and pinning it firmly to a piece of wood below. I affixed the
As a graduate student working with James D. Jamieson at
Yale, I focused on how the components of biological sys-

tems—especially of cells—interacted mechanically. At this

time, in the late 1970s, biologists generally viewed the cell as
a viscous fluid or gel surrounded by a membrane, much like
a balloon filled with molasses. Cells were known to contain
an internal framework, or cytoskeleton, composed of three
different types of molecular protein polymers, known as mi-
crofilaments, intermediate filaments and microtubules. But
their role in controlling cell shape was poorly understood.
Another mystery in those days concerned the way isolated

LIVING CELLS crinkle a thin rubber substrate be-

cause they exert tractional forces where they adhere.

50 Scientific American January 1998 Copyright 1997 Scientific American, Inc.

tensegrity model to the substrate by
flattening it and sewing the ends of
some of the dowels to the cloth.


These attachments were analogous
to the cell-surface molecules, now
known as integrins or adhesion re-
ceptors, that physically connect a cell
to its anchoring substrate.
With the dowel ends sewed to the
tightly pinned cloth, the model re-
mained flat, just as a real cell does on
a hard substrate. When I lifted the
pins to free the cloth from the wood, however, thereby mak-
ing the cell’s anchoring surface flexible, the tensegrity model
popped up into its more spherical form, puckering the cloth TENSEGRITY MODEL of a cell
underneath. Furthermore, I noticed that when I stretched the was built with dowels and elastic cords.
model flat by connecting it to the cloth substrate, the cell and Like a living cell, it flattens itself and its nu-
nucleus inside it extended in a coordinated manner. The nu- cleus when it attaches to a rigid surface (left) and
cleus model also moved toward the bottom of the simulated retracts into a more spherical shape on a flexible sub-
cell. Soon thereafter, I showed experimentally that living cells strate, puckering that surface (right).
and nuclei spread and polarize in a similar manner when
they adhere to a substrate. Thus, with my highly simplified
construction, I showed that tensegrity structures mimic the
known behavior of living cells. filaments—a key element of the cytoskeleton—extends through-
out the cell, exerting tension. In other words, it pulls the cell’s
Hard-Wiring in Cells membrane and all its internal constituents toward the nucleus
at the core. Opposing this inward pull are two main types of

I n the years since my modeling experiment, biologists have

learned a great deal about the mechanical aspects of cells,
and their findings seem to confirm that cells do indeed get
compressive elements, one of which is outside the cell and the
other inside. The component outside the cell is the extracellu-
lar matrix; the compressive “girders” inside the cell can be ei-
their shape from tensegrity. Further, just as the models predict, ther microtubules or large bundles of cross-linked micro-
most cells derive their structure not only from the cytoskele- filaments within the cytoskeleton. The third component of
ton’s three major types of filaments but also from the extra- the cytoskeleton, the intermediate filaments, are the great in-
cellular matrix—the anchoring scaffolding to which cells are tegrators, connecting microtubules and contractile micro-
naturally secured in the body. filaments to one another as well as to the surface membrane
Inside the cell, a gossamer network of contractile micro- and the cell’s nucleus. In addition, they act as guy wires, stiff-

CYTOSKELETON of a cell consists of microfilaments

CELL SURFACE (bottom left), microtubules (bottom center) and inter-
mediate filaments (bottom right), all of which are

NUCLEUS nanometers wide. The rounded shape near the center

in each of these photographs is the cell nucleus. The

three components interconnect to create the cytoskele-

tal lattice, which stretches from the cell surface to the
nucleus (top left). The molecular structure of each
CYTOSKELETON component is shown above the corresponding photo-
graph and is color coded to the top left illustration.




ROBERT D. GOLDMAN Northwestern University Medical School

Copyright 1997 Scientific American, Inc.



NERVE CELL has long extensions, called neurites, that connect and restricting their growth. The same microfilaments, however,
electrically with neighboring nerve cells (above left and top are connected to other ones (orange) that extend forward to the
right). Neurites extend from the cell (views at right), for exam- points where the cell anchors to its underlying substrate (center).
ple, during the repair of an injury, by elongating internal molec- When the microfilaments pull themselves forward against these
ular fibers known as microtubules (purple). Contractile micro- adhesions, they enable the microtubules to elongate, and the
filaments (red) surround the microtubules, compressing them neurite extends (bottom).

ening the central nucleus and securing it in place. Although The tensegrity model suggests that the structure of the cell’s
the cytoskeleton is surrounded by membranes and penetrated cytoskeleton can be changed by altering the balance of phys-
by viscous fluid, it is this hard-wired network of molecular ical forces transmitted across the cell surface. This finding is
struts and cables that stabilizes cell shape. important because many of the enzymes and other substances
If the cell and nucleus are physically connected by tensile that control protein synthesis, energy conversion and growth
filaments and not solely by a fluid cytoplasm, then pulling on in the cell are physically immobilized on the cytoskeleton.
receptors at the cell surface should produce immediate struc- For this reason, changing cytoskeletal geometry and mechanics
tural changes deep inside the cell. Recently Andrew Maniotis, could affect biochemical reactions and even alter the genes
who was in my group at Children’s Hospital of Harvard Med- that are activated and thus the proteins that are made.
ical School, demonstrated this directly. By binding micro- To investigate this possibility further, Rahul Singhvi and
pipettes to adhesion receptors on the surface of living cells and Christopher S. Chen in my group, working with George M.
pulling outward, Maniotis caused cytoskeletal filaments and Whitesides, also at Harvard, developed a method to engineer
structures in the nucleus to realign immediately in the direction cell shape and function. They forced living cells to take on
of pull. Thus, as my early experiments suggested, cells and nu- different shapes—spherical or flattened, round or square—by
clei do not behave like viscous water balloons. placing them on tiny, adhesive “islands” composed of extra-
cellular matrix. Each adhesive island was surrounded by a
How Mechanics Controls Biochemistry Teflon-like surface to which cells could not adhere.
By simply modifying the shape of the cell, they could switch

T ensegrity can be invoked to explain more than the stabi-

lization of cellular and nuclear form. For example, Steven
R. Heidemann, working with Harish Joshi and Robert E.
cells between different genetic programs. Cells that spread flat
became more likely to divide,
whereas round cells that were
Buxbaum of Michigan State University in the mid-1980s, prevented from spreading
found that tensegrity can explain how nerve cells extend activated a death program
long, thin projections called neurites, which are filled with mi- known as apoptosis. When STEFANIE KAECH, BEAT LUDIN AND ANDREW MATUS

crotubules and transmit electrical signals in the nervous sys- cells were neither too extend-
tem. This growth is required for repair of nerve damage. ed nor too retracted, they
Heidemann’s group found that microtubules are com- neither divided nor died. In-
pressed at their ends by the pull of surrounding contractile stead they differentiated
microfilaments inside the neurites. More important, the re- themselves in a tissue-specific
searchers discovered that microtubule assembly (elongation)— manner: capillary cells formed
and, hence, neurite extension—is promoted by shifting com- hollow capillary tubes; liver
pressive loads off the microtubule and onto the cell’s attach- cells secreted proteins that the
ments to its extracellular matrix. In other words, the existence liver normally supplies to the
of a tensegrity force balance provides a means to integrate blood; and so on.
mechanics and biochemistry at the molecular level. Thus, mechanical restruc- les under compression in these time-
Very recently, Andrew Matus of the Friedrich Miescher In- turing of the cell and cyto- lapse video images. The buckling oc-
stitute in Basel added a vivid footnote to this story. By mak- skeleton apparently tells the curs when the microtubule elongates
ing cells that produce fluorescent microtubules, Matus actually cell what to do. Very flat cells, and pushes against other components
viewed those microtubules buckling under compression. with their cytoskeletons of the cell’s skeleton.

52 Scientific American January 1998 The Architecture of Life

Copyright 1997 Scientific American, Inc.
stretched, sense that more cells are needed to cover the sur- LINEAR STIFFENING
rounding substrate—as in wound repair—and that cell divi- occurs in a tensegrity
sion is needed. Rounding indicates that too many cells are structure because struc-
competing for space on the matrix and that cells are prolifer- tural members reorient
ating too much; some must die to prevent tumor formation. themselves to lie more
in the direction of ap-
In between these two extremes, normal tissue function is es-
plied stress (downward
tablished and maintained. Understanding how this switching in the right-hand view).
occurs could lead to new approaches to cancer therapy and

tissue repair and perhaps even to the creation of artificial-tis-
sue replacements. Working with Dimitrije
Stamenovic of Boston
Making Cells Do the Twist University, we developed
a mathematical model

T he next level up in the hierarchy of self-assembly is the

formation of tissues, which are created from the joining
of cells to one another and to their extracellular matrix. One
based on these principles. It predicts, for the first time, the lin-
ear-stiffening response of tissues, living cells and even
molecules. We hope to use this model to help design ad-
emergent property of tissues is how they behave mechanical- vanced materials that have the linear-stiffening property and
ly. Many different types of tissue, including muscle, cartilage, that may be useful in such applications as protective clothing
blood vessels and skin, exhibit a response known as linear and artificial body parts. The same mathematical approach
stiffening. If you pull on your skin, for example, you will feel may also be incorporated within computer programs as a
the resistance increase as you tug harder. An increasing exter- shortcut to accelerate molecular modeling and drug design.
nal force is met with increasing resistance. Recent studies In Wang’s magnetic-twisting studies and in Maniotis’s mi-
show that even isolated molecules, such as DNA, exhibit lin- cropipette-pulling experiments, we found that applying stress
ear stiffening, yet until we examined this phenomenon in the to cell-surface receptors involved with metabolism—rather
context of tensegrity, there was no mechanical or mathemat- than adhesion—did not effectively convey force to the inside
ical explanation for this behavior. of the cell. Thus, these studies confirmed that mechanical forces
In 1993 my co-worker Ning Wang, working with James P. are transmitted over specific molecular paths in living cells, a
Butler of the Harvard School of Public Health, developed a finding that provided new insight into how cells sense me-
device that allowed us to twist individual molecules on the chanical stimuli that regulate tissue development. This in-
surface membrane of living cells while simultaneously measur- sight, in turn, may help us better understand a wide variety
ing the cellular response. We found that when we increased of phenomena, from the growth of muscle in response to ten-
the stress applied to integrins (molecules that go through the sion to the growth of plant roots in response to gravity.
cell’s membrane and link the extracellular matrix to the inter-
nal cytoskeleton), the cells responded by becoming stiffer and Molecular Geodesic Domes
stiffer—just as whole tissues do. Furthermore, living cells
could be made stiff or flexible by varying the prestress in the
cytoskeleton by changing, for example, the tension in con-
tractile microfilaments.
A lthough the tensegrity models predicted many cell behav-
iors, one disparity needed explaining. Many cells can
spread and flatten without microtubules—the most important
Although the exact details of the interaction are not all compression struts in the model. If living cells can change from
known, we showed, using a stick-and-string tensegrity mod- spherical to flat without these struts, how can tensegrity ap-
el, that the gist of the response can be discerned from the way ply? Again using an uncomplicated modeling approach, I
in which tensegrity structures respond to stress. Essentially, all found that, incredibly, the microfilament network itself is a
the interconnected structural elements of a tensegrity model tensegrity structure.
rearrange themselves in response to a local stress. Linear stiff- In the cytoskeleton of a living cell, contractile microfilaments
ening results because as the applied stress increases, more of form a lattice that reorganizes locally into different forms,
the members come to lie in the direction of the applied stress. such as large bundles or networks of triangles. To explore the


with flexible joints shows how
contracting microfilament net-
works can rearrange into linear
bundles (top right) or triangulated
geodesic forms, such as the octa-
hedron (bottom right).

Copyright 1997 Scientific American, Inc.

SPL Photo Researchers, Inc

mechanism behind this reorganization, I modeled the micro- GEODESIC FORMS appear in many dif-
filament lattice as a polyhedral framework of soda straws ferent natural structures, including part of
that contained six triangles and four squares [see bottom illus- the cytoskeleton of a mammalian cell
tration on preceding page]. The straws were held together by (above left), an adenovirus (top middle)
a single elastic string that I threaded through all the straws and (clockwise from bottom right) a pollen
and tied to itself. I assumed that each soda straw in the model grain, a buckyball surrounding a potassi-
um ion, a protein enzyme complex and a
represented a single contractile microfilament that could gen- multicellular organism known as a volvox.
erate mechanical tension by shortening itself. It is known that
contractile microfilaments get stiffer when they shorten.
Thus, the internal elastic thread in the model would then framework of squares and triangles converts
mimic the continuous tension in the whole structure that re- into fully triangulated octahedral or tetrahe-
sults from the shortening of all these stiffened filaments. dral forms—or, in other words, into fully tri-
I assumed that this soda-straw model represented one mod- angulated tensegrity structures.
ular cytoskeletal unit that interconnected in all directions with When I interconnected many similar soda-
other similar modules in a round, unattached (suspended) cell. straw models, I found that the individual mod-
The question I was trying to answer was, What would happen ules progressively contracted, resulting in the forma-
to this framework if the cell it supported were to attach to a tion of a geodesic framework composed of alternat-
rigid surface? ing octahedral and tetrahedral forms, closely packed. In

KEN EWARD BioGrafx/Science Source/Photo Researchers, Inc.

Cells attach by binding to surface-bound molecules in the a cell, contraction of surrounding microfilament networks
extracellular matrix. But cells are not evenly “glued” to the that interconnect with the cell base would bend this frame-
matrix; rather they are “spot welded” in localized sites known work down over the spherical nucleus, thereby transforming it
as focal adhesions. Contractile microfilaments respond to an- into a highly triangulated dome—specifically, a geodesic dome.
chorage by shortening and increasing isometric tension within Elias Lazarides, then at Cold Spring Harbor Laboratory in
the lattice. The soda-straw models suggested that the increas- New York, and Mary Osborn and Klaus Weber of the Max
ing tension produced by attachment would cause the individ- Planck Institute in Göttingen, Germany, observed these very
ual contractile microfilaments that formed the squares in the transformations in the region of the cytoplasm above the nu-
model to self-assemble into linear bundles stretching between cleus in spreading cells. Significantly, the existence of a
these focal-adhesion sites where integrin receptors anchor the geodesic dome within the cytoskeleton at the molecular level
cell to the matrix. In fact, when living cells spread on a surface, demonstrates conclusively that cells can and do use the archi-
individual contractile microfilaments align in a nearly identi- tecture of tensegrity to shape their cytoskeleton.
cal manner to form bundles called stress fibers.
In contrast, at the top of the cell, there is no adhesive sub- A Universal Pattern
strate to resist the pull of the shortening microfilaments. In
these regions the contraction of each microfilament can be re-
sisted only by the pull and stiffness of its neighboring filaments.
Fuller showed many years ago that inward pulling and twist-
T he geodesic structure found within the cytoskeleton is a
classic example of a pattern that is found everywhere in
nature, at many different size scales. Spherical groups of carbon
ing causes this type of polyhedral structure to undergo what atoms called buckminsterfullerenes or buckyballs, along with
he called a “jitterbug” transformation: the highly flexible viruses, enzymes, organelles, cells and even small organisms,

54 Scientific American January 1998 The Architecture of Life

Copyright 1997 Scientific American, Inc.
all exhibit geodesic forms. Strangely, few researchers seem to tails that extend from neighboring proteins to form a trian-
have asked why this is so. My view is that this recurrent pat- gulated geodesic framework on the nanometer scale. Each
tern is visual evidence of the existence of common rules for joint in this framework self-stabilizes as a result of a balance
SOURCE: Journal of Biological Chemistry (below)

self-assembly. In particular, all these entities stabilize them- between the pull of intermolecular attractive forces (hydro-
selves in three dimensions in a similar way: by arranging gen bonds) and the ability of the individual protein tails to re-
their parts to minimize energy and mass through continuous sist compression and buckling.
tension and local compression—that is, through tensegrity. The same basic scheme is apparent in buckyballs, except
The assembly of viruses, the smallest form of life on the that the building blocks are atoms instead of proteins. In bucky-
earth, involves binding interactions between many similar balls, 60 carbon atoms form a geodesic sphere covered by 20

proteins that come together to form a geodesic viral coat that hexagons interspersed with 12 pentagons: the pattern on a
encloses the genetic materi- soccer ball. In effect, the 90 carbon-carbon bonds in a bucky-
al. During virus formation, ball are the struts in a tensegrity sphere.
linear extensions of the pro- It is more difficult, however, to see that the same building
teins overlap with similar rules also apply to irregular structures, including many bio-
logical molecules, that do not exhibit
geodesic forms. Proteins, on which
cells depend for structure, catalysis and
many other functions, are long strings
of amino acids. Small regions of the
protein’s amino acid backbone fold
into helical forms that stabilize them-
selves through a balance between the
attractive force of hydrogen bonds
(pulling together different regions of
the molecule) and the ability of the
protein coil to resist shortening, or
compression. In other words, these
helical regions stabilize themselves
through tensegrity—as does any heli-
cal molecule, such as DNA.
M. ABBEY Photo Researchers, Inc.
Protein organization also involves
hierarchical assembly. The small re-
gions of a protein that are helically
stiffened are separated from one an-
other by parts of the same amino acid

Copyright 1997 Scientific American, Inc.

JIM AND JULIE BRUTON Photo Researchers, Inc.

PRESTRESSED TENSEGRITY CANTILEVERS filaments, membrane ion channels and entire living cells and
include the muscle-and-bone neck of a giraffe and a tissues exhibit characteristic resonant frequencies of vibra-
cable-and-beam sculpture by Kenneth Snelson. tion. Very simply, transmission of tension through a tensegrity
array provides a means to distribute forces to all intercon-
nected elements and, at the same time, to couple, or “tune,”
backbone that act as if they were flexible hinges. These strut- the whole system mechanically as one.
like regions fold back on themselves (because of tensile hy-
drogen-bonding forces) in order to stabilize the entire Implications for Evolution and Beyond
molecule. The stiffened helices may be extremely compressed
locally, even though forces are equilibrated across the whole
prestressed molecule. A
lthough changes in DNA generate biological diversity,
genes are a product of evolution, not its driving force. In
Because a local force can change the shape of an entire fact, geodesic forms similar to those found in viruses, enzymes
tensegrity structure, the binding of a molecule to a protein and cells existed in the inorganic world of crystals and miner-
can cause the different, stiffened helical regions to rearrange als long before DNA ever came into existence. Even water
their relative positions throughout the molecules are structured geodesically.
length of the protein. For example, The relevant question is, How did or-
when a signal-bearing molecule binds ganic molecules and cells evolve from
to a receptor that goes through the inorganic components? After all, in
membrane and into a cell, the attach- terms of how emergent properties arise,
ment can cause conformational self-assembly of molecules into or-
changes at the opposite end of the re- ganelles or cells into tissues is not very
ceptor. These conformational changes, different from the self-assembly of atoms
in turn, alter the shape of adjacent pro- into compounds. For example, sodium,
teins and trigger a cascade of molecular an explosive metal, and chlorine, a poi-
restructuring inside that cell. Indeed, sonous gas, combine to form sodium
this is how cells sense and respond to chloride, whose emergent property is
changes in their environment. that it can be used as table salt. The im-
Thus, from the molecules to the portant principle here is the manner in

bones and muscles and tendons of the which a structure shapes itself and
human body, tensegrity is clearly na- holds its subcomponents together in
ture’s preferred building system. Only three-dimensional space; this character-
tensegrity, for example, can explain istic is what defines the way the struc-
how every time that you move your ture as a whole will behave.
arm, your skin stretches, your extracel- More broadly, all matter is subject to
lular matrix extends, your cells distort, the same spatial constraints, regardless
and the interconnected molecules that of scale or position. Thus, given these
form the internal framework of the cell constraints, tensegrity is the most eco-
feel the pull—all without any breakage nomical and efficient way to build—at
or discontinuity. VERTICAL TENSEGRITY sculpture the molecular scale, at the macroscopic
Remarkably, tensegrity may even ex- and molecular model of a cytoskeletal mi- scale and at all scales in between. It is
plain how all these phenomena are so crofilament (above) derive strength from possible that fully triangulated tensegri-
perfectly coordinated in a living crea- the same principle: they stabilize them- ty structures may have been selected
selves through a balance of compression
ture. At the Johns Hopkins School of and tension. In the surface tissue of a fly’s through evolution because of their struc-
Medicine, Donald S. Coffey and Ken- eye (background at right), cells are ar- tural efficiency—their high mechanical
neth J. Pienta found that tensegrity ranged geodesically for the same pur- strength using a minimum of materials.
structures function as coupled harmon- pose—to provide stability through contin- The flexibility exhibited by prestressed
ic oscillators. DNA, nuclei, cytoskeletal uous tension and local compression. tensegrity structures would be advanta-

56 Scientific American January 1998 The Architecture of Life

Copyright 1997 Scientific American, Inc.
geous because it allows structures to take on different shapes. The emergence of DNA and genes gave rise to a new
For example, if a molecule or cell were able to transform into mechanism for generating structural diversity that accelerated
a shape that was more stable at a certain temperature or evolution. Yet throughout all this time the rules guiding the
pressure, or more efficient metabolically, then its lifetime process of hierarchical self-assembly remained essentially un-
would have been extended. It would have been more likely to changed. So it is no surprise that the basic arrangement of
interact with other, similar entities and then to self-assemble bones and muscles is remarkably similar in Tyrannosaurus
once again. rex and Homo sapiens; that animals, insects and plants all rely
Researchers now think biological evolution began in layers on prestress for the mechanical stability of their bodies; and
of clay, rather than in the primordial sea. Interestingly, clay is that geodesic forms, such as hexagons, pentagons and spirals,
itself a porous network of atoms arranged geodesically with- predominate in natural systems.
in octahedral and tetrahedral forms. But because these octa- Finally, more philosophical questions arise: Are these
hedra and tetrahedra are not closely packed, they retain the building principles universal? Do they apply to structures
ability to move and slide relative to one another. This flexibil- that are molded by very large scale forces as well as small-
ity apparently allows clay to catalyze many chemical reactions, scale ones? We do not know. Snelson, however, has proposed
including ones that may have produced the first molecular an intriguing model of the atom based on tensegrity that takes
building blocks of organic life. off where the French physicist Louis de Broglie left off in
Over time, different molecular collectives self-assembled to 1923. Fuller himself went so far as to imagine the solar sys-
form the first structures with specialized functions—the fore- tem as a structure composed of multiple nondeformable rings
runners of present-day organelles—which then combined with of planetary motion held together by continuous gravitational
one another to create the first simple cells. These cells then tension. Then, too, the fact that our expanding (tensing) uni-
produced proteins that self-assembled to form extracellular verse contains huge filaments of gravitationally linked galaxies
matrix–anchoring scaffolds that, in turn, promoted self-as- and isolated black holes that experience immense compres-
sembly of multicellular tissues. Organs developed from the sive forces locally can only lead us to wonder. Perhaps there
self-assembly of tissues, and complex organisms arose through is a single underlying theme to nature after all. As suggested
combination and progressive remodeling of different organs. by early 20th-century Scottish zoologist D’Arcy W. Thompson,
Indeed, the development of an embryo from a sperm and an who quoted Galileo, who, in turn, cited Plato: the Book of Na-
egg recapitulates all these stages of self-assembly. ture may indeed be written in the characters of geometry. SA

The Author Further Reading

DONALD E. INGBER, who holds B.A., On Growth and Form. Revised edition. D’Arcy W. Thompson. Cambridge University
M.A., M.Phil., M.D. and Ph.D. degrees from Press, 1942 (reprinted 1992).
Yale University, is an associate professor of Movement and Self-Control in Protein Assemblies. Donald L. D. Caspar in Biophys-
pathology at Harvard Medical School and ical Journal, Vol. 32, No. 1, pages 103–138; October 1980.
a research associate in the departments of Clay Minerals and the Origin of Life. Edited by A. Graham Cairns-Smith and Hyman
surgery and pathology at Children’s Hospi- Hartman. Cambridge University Press, 1986.
tal in Boston. He is also a member of the Cellular Tensegrity: Defining New Rules of Biological Design That Govern the
Center for Bioengineering at the Mas- Cytoskeleton. Donald E. Ingber in Journal of Cell Science, Vol. 104, No. 3, pages
sachusetts Institute of Technology. In addi- 613–627; March 1993.
tion to his work on cell structure, Ingber has Mechanotransduction across the Cell Surface and through the Cytoskeleton.
contributed to the study of tumor angio- Ning Wang, James P. Butler and Donald E. Ingber in Science, Vol. 260, pages 1124–1127;
genesis, including the discovery of an anti- May 21, 1993.
cancer drug now in clinical trials. He is the Geometric Control of Cell Life and Death. Christopher S. Chen, Milan Mrksich, Sui
founder of Molecular Geodesics, Inc., a Huang, George M. Whitesides and Donald E. Ingber in Science, Vol. 276, pages

Cambridge, Mass., company that creates 1425–1428; May 30, 1997.

advanced materials with biologically in- Tensegrity: The Architectural Basis of Cellular Mechanotransduction. Donald
spired properties. E. Ingber in Annual Review of Physiology, Vol. 59, pages 575–599; 1997.

The Architecture of Life Scientific American January 1998 57

Copyright 1997 Scientific American, Inc.