What conditions are needed to turn embryonic stem cells into brain, liver and heart muscle cells
About a year and a half ago, the journal "Science" awarded the title of discovery of the year to research in the field of human embryonic stem cells. These cells are extracted from young embryos that are in a very early developmental stage, even before implantation in the uterus. They have not yet differentiated into the various body tissues, and it is currently assumed that these cells have the potential to differentiate and acquire the properties of a variety of tissues.
Prof. Nissim Benvanisti, from the Department of Genetics at the Hebrew University, studies human embryonic stem cells, and the environmental conditions that allow them to differentiate and become a cell with the properties of a specific tissue.
"An embryonic stem cell is capable of becoming a muscle cell, a liver cell, a nerve cell, and other types of cells," says Prof. Benvanisti, "and apparently, the composition of the molecules in the environment of the developing embryo is what dictates and directs the differentiation of these cells into the various body tissues."
Prof. Benvanisti followed the spontaneous differentiation of the embryonic stem cell under laboratory conditions. The cells were initially grown in culture plates, and then in suspension, until the formation of "embryonic bodies" containing a large number of cells. "One morning we approached the cultures," says Benvanisti, "and discovered with great excitement that some of the cells of the 'embryonic body' began to beat together. These are, of course, heart muscle cells, and what we saw in the plate were human heart cells in an early developmental stage."
Based on known markers, a tissue classification was made for the cells of the "fetal body", and in addition to the heart muscle cells, cells from more than ten tissues were identified, including nerve cells, kidney, pancreas, cartilage, liver cells and blood cells.
The first human tissues develop in the very early stages of fetal life, and the growth of embryonic stem cells in culture allows, among other things, to study these stages of development and to also try to understand the causes of "spontaneous abortions" that occur in approximately 20% of pregnancies.
A few months ago, eighty Nobel laureates approached President George W. Bush with a request to fund research on human embryonic stem cells. In the long run, they claim, stem cell transplants will be a treatment for many diseases, and may also serve as a substitute, in some cases, for organ transplants.
"Instead of injecting insulin into severe diabetics," says Professor Benvanisti, "we will implant pancreatic cells derived from embryonic stem cells that have been grown in culture. And in Parkinson's patients, in addition to administering drugs that compensate for the lack of nerve cells in the brain, we will also transplant healthy cells that are also derived from embryonic stem cells."
In order to produce healthy nerve or pancreatic cells, the researchers will have to take stem cells and make them differentiate specifically into the desired tissue cells. The main proteins that direct the differentiation of cells into the various tissues are called growth factors, and many of them have been known for more than two decades. The different concentrations of the growth factors in the environment of the embryo ensure the differentiation of the stem cells into the body tissues and the formation of the whole person.
Prof. Benvanisti and his research students, in collaboration with Prof. Yosef Itzkovitz from Rambam Hospital, tested the effect of eight key growth factors on the differentiation of stem cells in culture. In an environment with a high concentration of nerve growth factor, for example, the stem cells differentiated into an "embryonic body" whose 55% cells were identified as nerve cells. In order to perform a nerve cell transplant, a homogeneous cell population is required, 100% of whose cells are nerve cells, and therefore filtering of the desired cells is performed from all the cells in the culture. The nerve cells are colored using a specific fluorescent marker, and with the help of advanced technology "fishing" is performed for the colored cells. At the end of the experiment, a culture containing only nerve cells will be obtained.
In the same way Benvanisti is trying to define the dosages and types of growth factors needed to grow pancreas, liver, muscle, and other cell types in culture. Transplanting these cells in the body of patients is a bold move that requires dealing with the main problem that doctors face after organ transplants: the body's immune response to the transplant. In many cases the immune system recognizes the implant as a foreign invader, attacks it, and the patient can die as a result of the rejection of the implant and its side effects. "In order to solve the immune problem due to cell transplantation", says Benvanisti, "in the future, they will produce stem cells originating from the patient himself" A cell will be taken from the patient who is a candidate for transplantation, and after its decomposition in the laboratory, its genetic material will be inserted into an egg. The fertilized egg will divide spontaneously, and some of the resulting stem cells will contain the same genetic load as that present in the patient's body cells. The appropriate stem cells will be grown in culture, and with the help of growth factors their differentiation will be regulated. "If it is a liver cell transplant, we will add the relevant growth factors to the culture, in the appropriate dose, and make the stem cells divide and become liver cells," says Benvanisti. "If it is nerve cells, we will add other growth factors to the culture and cause differentiation in the direction of nerve cells." The cells will then be transplanted into the patient's body. The hope is that the immune system will treat them as it treats the original body cells.
