With their capacity to multiply ad infinitum and to differentiate into different cell types, stem cells offer real therapeutic prospects. Among the different types of existing stem cells, embryonic stem cells are the focus of many hopes, but because they raise ethical questions, their use is strictly regulated by law.

Stem cells are called undifferentiated cells capable of self-renewal, of infinite multiplication and of differentiating into specialized cells. They are in a way the “mother cells” of all the other cells. At the very beginning of life, they allow the full development of a human being, then throughout life, they allow, like a nature reserve, to replenish our stocks of specialized cells.

These stem cells are naturally present in the embryo and in certain adult organs or tissues.

According to their origin and their differentiation potential, there are 3 types of stem cells.

Totipotent stem cells

These stem cells come from the very first divisions of the fertilized egg (between fertilization and the fourth day). They can differentiate into all types of cells in the body and into appendages (placenta, umbilical cord). These are the only stem cells that can give birth to an embryo capable of implanting in the uterus and developing.

Pluripotent stem cells

Also called embryonic stem cell lines, they originate from an embryo 5 to 7 days after in vitro fertilization (IVF). They can give rise to more than 200 types of cells representative of all the tissues of the body. These embryonic stem cells are taken from supernumerary embryos obtained by IVF for a parental project, and whose parents have signed a consent to transfer the embryo free of charge for research. At this stage of development, the embryo is shaped like a small bubble containing another small bubble. It is in this internal cell mass that there are about thirty pluripotent cells which will give all the cells of the organism. These cells are removed and placed in culture where they will then multiply and, depending on their culture medium, differentiate into a particular type of specialized cells: blood cells, brain cells, liver cells, etc.

Multipotent stem cells

Derived from fetal or adult tissue, they can give rise to one or more cell types. In adults, they participate in tissue renewal. mesenchymal stem cells (MSCs). These are adult stem cells present throughout the body in adipose tissue, bone marrow, supporting tissues of organs, bones, cartilage, muscles and can give rise to cartilage cells, bone , fatty, to muscle fibers, etc.

Unipotent stem cells

Coming from adult tissues, they can only give rise to one type of cell. They also participate in the renewal of all our fabrics.

Induced pluripotent stem cells

Also called iPS cells (“induced pluripotent stem cells”), these are adult stem cells that have been genetically reprogrammed to have the characteristics of embryonic stem cells. The technique was developed in Japan in 2006 by researcher Shinya Yamanaka, who also received the Nobel Prize in medicine in 2012 for this fabulous breakthrough. Like embryonic stem cells, IPS cells have this capacity for self-renewal and pluripotency, with the advantage of being easy to access (a simple biopsy is sufficient) and of not posing ethical problems for embryonic stem cells.

Understanding human development and disease

The study of stem cells allows us to understand human development and that of certain diseases.

IPS cells are now used to model certain diseases such as amyotrophic lateral sclerosis, parkinson’s disease, type 1 diabetes, Huntington’s disease, trisomy 21, severe combined immunodeficiency, Lesch-Nyhan syndrome , Gaucher disease, and many more. To do this, stem cells are taken from a donor suffering from the pathology, then reprogrammed to then study the mechanisms of the pathology, try to correct the mutation, test therapeutic molecules.

Repair: cell therapy or regenerative medicine

In the event of illness, certain stem cells can be harvested, cultured and then reinjected in order to restore the function of the failing tissue or organ. This very promising stem cell transplant forms the basis of what is called cell therapy or regenerative medicine.

It has already been put into practice since the 70s with bone marrow transplantation, necessary in certain serious blood diseases such as leukemia, certain non-Hodgkin lymphomas, aplastic anemia, thalassemia and sickle cell anemia in particular. This transplant involves so-called hematopoietic stem cells (HSC). From the bone marrow, these stem cells give rise, throughout our life, to blood cells (red blood cells, white blood cells, platelets).

There are two types of bone marrow transplant:

• autologous transplant or autograft of hematopoietic stem cells: the transplant is performed with the patient’s stem cells. Doctors remove 2-3% of the bone marrow before treatment and inject it back after intensive chemotherapy to destroy as many cancer cells as possible. There is no risk of rejection.
• allogeneic hematopoietic stem cell transplantation: stem cells come from a donor compatible with the patient, who has donated his bone marrow. These hematopoietic stem cells are also found in cord blood or placental blood, taken from a baby’s umbilical cord immediately after birth, when parents wish to donate cord blood.

In both cases, the stem cells are then transfused into the patient by infusion. After a few weeks, the bone marrow again produces enough blood cells.

Skin stem cell transplantation has also been practiced since the 80s to reconstitute the epidermis in severe burns. Stem cells in the eye are used to repair certain damage to the cornea.

Cellular bioengineering work is currently underway to try to regenerate or repair certain tissues, such as cartilage for example, through the transplant of specific stem cells, mesenchymal stem cells (MSCs). These are adult stem cells present throughout the body in adipose tissue, bone marrow, supporting tissues of organs, bones, cartilage, muscles and can give rise to cartilage cells, bone , fatty, to muscle fibers, etc.

Many other studies based on these mesenchymal stem cells are currently being carried out in rheumatology, cardiology, for certain myopathies, diabetes, etc.

IPS cells, on the other hand, are little used in cell therapy because their genetic reprogramming could pose a problem once transplanted into a patient.