Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
SAF Archives  search ask the scientists in the classroom cool science
scientists from previous shows
cool careers in science
ask the scientists

Gordana Vunjak Novakovic Gordana Vunjak Novakovic as seen on Never Say Die: How to Make a Nose

Click on Gordana's photo to read a brief bio.

q Wow! I was fascinated by your work with human tissue. I plan on looking further into the subject. I was wondering if you could answer a question for me: How do you stimulate the stem cells to develop into specific tissues? Thank you for your time, keep up the good work. Thanks, Courtney

A Nice question. Selective stimulation of primitive (stem) cells is one of the most important, most difficult and least known problems of tissue engineering. During natural embryonic development, stem cells differentiate into specific cell types (muscle, bone...), and this process is modulated by a number of various regulatory factors, biochemical and physical; examples include growth factors and physical forces such as tension or compression. In adults, most body cells (but not all: nerves, teeth...) are being gradually replaced by new cells in processes involving the actions of undifferentiated cells from bone marrow.

At this time, we have very limited knowledge about the factors themselves and their actions. For example, we know how to stimulate mesenchymal stem cells that can be found in bone marrow to develop cartilage or bone. Depending on the specific "cocktail" of growth factors, hormones and metabolites we use in culture, the same cell population will make one or the other tissue after being cultured on a polymer scaffold in a bioreactor. For most other tissues, including for example the cardiac muscle and nerves, many research groups are working on the identification of specific factors that can induce cell commitment and differentiation in a desired direction. Our ability to make functional equivalents of damaged/lost human tissues will largely depend of the success of these efforts.

q It was exciting to see the cartilage and heart muscle cells your lab is engineering. I'm curious why you started with these two particular tissues. Were they the least complicated to engineer? (Question from Ben)

A Cartilage and cardiac muscle were chosen as two paradigms of engineered tissues, two tissues that are very different from each other and representative of a broader range of skeletal and cardiovascular tissues.

Cartilage was the first tissue to engineer, for the reasons you stated yourself, and one more important reason: cartilage does not repair by itself, and in case of injury or disease needs to be replaced. There is about a million patients per year in the United States who need cartilage replacement.

Adult cartilage is a tissue containing only one cell type (chondrocyte), the cells are present at relatively low concentration (only 5% of volume) and there are no blood vessels or nerves. Therefore, cartilage can be maintained in culture at relatively low rates of nutrient and oxygen supply (metabolic needs of the cells are low, due to their low density), and was an ideal and obvious candidate for our early tissue engineering efforts. However, the regeneration of functional cartilage in culture depends on a variety of factors (biochemical and physical regulatory signals) in the cell microenvironment, and we and others had to solve a number of problems in order to grow functional cartilage substitutes using cells, polymer scaffolds and bioreactors.

Cardiac muscle is a composed of muscle fibers with longitudinally positioned contractile units and contains a dense network of blood vessels. The fibers themselves consist of cells that are joined end-to-end by specilaized junctions. Cardiac muscle is both highly organized and highly active. During exercise, skeletal and cardiac muscle receive about 75% of the total cardiac output, as compared to only 10% received by cartilage and bone. Engineered cardiac muscle is thus expected to require substantially higher rates of nutrient and oxygen supply than cartilage or bone. Over the last 4-5 years, we have extended the methods and approaches established for cartilage tissue engineering to the engineering of cardiac muscle. At this time, we have engineered a cardiac muscle which expressed several structural and functional features of a natural cardiac muscle (for example, the engineered muscle sustains continuous propagation of electrical impulses, which is a sign of electro-mechanical coupling of the cells). The problem of vascularization of such engineered muscle still needs to be solved.

q It said on the show that you're working with animal heart muscle cells right now. What kind of animal? When will you begin working with human heart tissue, and do you think it will be a more complicated process? (Question from Terry)

A We have used hearts harvested from embryonic chick (i.e. fertilized eggs) and neonatal rats to isolate cells which we seed onto polymer scaffolds and culture in bioreactors. The use of human tissue is certainly much more complicated, not only for scientific reasons (adult cardiac muscle cells do not multiply in culture) but also due to the ethical issues that are associated with its use for scientific purposes. For research, we can use cell lines which are a convenient experimental system; for actual tissue engineering applications, we'll need to look into other cell sources. Ongoing efforts to define the conditions which can induce stem cells (e.g. obtained from bone marrow) to differentiate into cardiac and other tissue cells will largely determine the future success of tissue engineering for use in human medicine.

q If we can make body parts like noses and parts of hearts from stem cells, eventually in the future it might be possible to create a whole human being. Do you think there is ever a point when science goes too far in creating human tissue? (Question from Rachel D.)

A You raise a question we all ask ourselves quite often. I believe that there is a long way to the point of creating organs, not to mention the whole body. A lot has been achieved, perhaps more than one would expect twenty years ago, but there is still much to be done. In our research, we try learn about tissue development under normal conditions, and then to provide laboratory conditions (e.g. using 3-dimensional scaffolds and bioreactors) to create functional equivalents of natural tissues. An important part of this effort is NOT to go too far, and to make sure we first resolve ethical, social and other related aspects.

q I am a student in 9th grade and I was wondering how long it takes for one of the noses, or anything else for that matter, to grow into the size it could be used. (Question from Curly)

A It depends on the tissue we grow: in most cases several weeks to several months. In all cases, it takes much longer to make a tissue in laboratory, than in human body during normal embryonic development. The reason: we have to make up for the lack of many unknown factors affecting tissue development by prolonged culture time. Over years, as we learn more about tissue development, in vivo and in vitro, we have reduced the cultivation times.

q I would like I know if you think it will ever be possible to replace a body part as delicate and as intricate as a brain or an eye. (Question from Summer)

A Maybe. Ten years ago, one would be skeptical about reconstructing in laboratory even a small patch of cardiac muscle, or a piece of cartilage or bone. The organs you mention are among the most complicated ones. We should revisit this subject in 5 or 10 years and see how much progress has been made. I am among those who think that there is a long way to the time we can make any of the human organs.

q I was wondering how the artificially made heart tissue beat on its own. I was under the assumption that it needed a brain to tell it to beat. (Question from AprilRose, high school student)

A You are right - the heart tissue does need a signal (transmitted by nerves) to beat at a regular pace. At this time, we grow only a heart muscle, which does not contain the nerves, but only heart muscle cells. These cells connect to each other (the process known as electromechanical coupling) such that they can transmit signals and beat synchronously. We pace the engineered tissue to make it beat, as when a pacemaker is used to sustain beating of a human heart. The pacing rate can vary (e.g. 60 to 300 beats per minute), and each stimulus in followed by a response (contraction of the cardiac muscle).

q There is some confusion in our house about where the stem cells used in tissue engineering come from. We thought they come from human embryos, but in this story Mr. Alda said that people may be able in the future to receive new tissue made from their own stem cells. Do we have stem cells when we are adults? Or would stem cells have to be removed from us as babies and stored until we needed to have new tissue grown? And from where do the stem cells come that are used in your research? (Question from Sonia)

A Stem cells can come from human embryos. The potential use of stem cells from this source has been considered for several years now, but there are ethical issues which need to be carefully considered before this cell source can become a reality for scientific research (and eventually for clinical use).

Another source of progenitor or stem cells is our bone marrow. Most tissue in our body (except for nerves, teeth and very few other tissues) undergo constant renewal. Within approximately 7 years, most cells in our body have been gradually replaced by new cells. The source of these new cells is bone marrow, which contains a population of "primitive" or "universal" cells, which can differentiate into any cell type. The "fate" of such cell, and the cell's decision about which direction to take depend on local environmental signals, biochemical (e.g. growth factors) and physical (e.g. stretch, electrical impulses).

It is interesting and important that the progenitor cells from bone marrow remain active throughout the duration of our life. Even in old individuals (e.g. >80 years) these cells, although present at lower concentrations, are as active as in very young individuals. Moreover, they can be multiplied in culture (up to a billion times!).

Tissue engineering can take advantage of the progenitor cells in bone marrow. A small sample of bone marrow stroma (liquid) can be taken by a needle (this is a routine procedure). Progenitor cells can be isolated, multiplied until a sufficient cell number is obtained, and seeded onto 3-dimansional scaffolds. Then, we need to provide local environmental signals (e.g. by using bioreactors) which will "trick" the cells to differentiate in the desired direction. This concept, which can result in autologous implants (i.e. based on the patients own cells) has been demonstrated for some relatively simple tissues, as for example cartilage. However, finding out the procedures for making a variety of tissues starting from bone marrow cells, and in particular the tissue s which do not regenerate in our bodies (e.g. nerves, pancreas) are a challenge and one of the main directions of tissue engineering research.

q Not too long ago, scientists were able to grow a human ear on the back of a lab mouse. I was wondering if the cartilage producing technology used in this experiment was similar, if not the same as, the technology you used to generate a nose. (Question from Patti, Junior at Gill St. Bernards School, Gladstone, NJ)

A It was similar - same concept, different cells and different polymer matrix. This first experiment (ear on the back of a lab mouse) demonstrated two important things: (1) one can use polymer materials to make delicate 3-dimensional scaffolds which can be used to seed cells, and (2) the size and shape of such scaffold are maintained following implantation.

q What are the chances that once a tissue-engineered organ is put in the body, it might not be accepted? Also, is there a great chance that the organ might not work or stop working when in the body? (Question from Maggie, high school biology student)

A We do not know the answers to these questions. The main paradigm of tissue engineering is to use the patients own cells (e.g. progenitor cells from bone marrow) to make implants and avoid any immunological problems which would cause rejection. In addition, some tissues (as for example cartilage) are "immunoprivileged", i.e. cells are embedded in tissue matrix such that surface cell receptors are covered and there is no immunological response.

The other issue you raise, the "lifetime" of engineered implants can be determined in only one way - by conducting systematic long-term studies. The experience available at this time is very limited, and much more work needs to be done before engineered transplants become a routine clinical procedure.

q When you make spare parts, like the nose, where and how do you store them? (Question from Striegeld)

A We do not store them for prolonged periods of time. The ideal situation is to keep such "spare part" in culture until it is needed. But please do not forget that much more work needs to be done before our engineered tissues become "spare parts".


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.