The basis for the functioning of a multicellular organism is the specialization of cells aimed at performing a specific function. This cell differentiation begins in the early stages of embryo development. But in our body there are cells that are capable of acquiring various specializations throughout a person’s life. And this fully applies to hematopoietic stem cells, which maintain a constant quantitative and qualitative composition of blood cells.
General information
Hematopoietic stem cells (Hematopoietic Stem Cell, from the Greek words Haima - blood, Poiesis - creation) are stem cells capable of unlimited division and differentiation into blood cells.
They are formed in the red bone marrow and differentiate in four directions:
- Erythroid (in red blood cells).
- Megakaryocytic (in platelets).
- Myeloid (multinucleated phagocytes, white blood cells).
- Lymphoid (in lymphocytes).
Hematopoietic stem cell transplantation (allogeneic - from the donor, autologous - transplantation of own cells) restores the hematopoiesis system, which can be impaired in some diseases, chemotherapy.
The first autologous stem cell transplantation was carried out in 1969 by E. Thomas (Seattle, USA). Modern methods in 80% of cases can defeat blood cancer. At this stage, medicine received at its disposal the methods of fetal medicine, when the donation of hematopoietic stem cells is provided by cord blood, embryonic tissues, bone marrow, adipose tissue.
Features of this cellular material
Hematopoietic stem cells (hemocytoblasts) have two main properties:
- The ability to asymmetric division, during which two daughter cells are formed, identical to the mother. But at the same time, the cells do not undergo differentiation. They remain multipotent hematopoietic stem cells. This means that they can go along any of the above paths of specialization.
- The presence of differentiating potential in hematopoietic stem cells. This means that stem cells divide and daughter cells begin their specialization, turning into highly specialized red blood cells, platelets, lymphocytes, white blood cells.
Hematopoietic stem cells in the bone marrow, like all cells in our body, have an age - a short “childhood”, a quick flying “youth”, when the cells choose “army” or “study”, and a long period of “maturity”.
I’ll go to red blood cells - let them teach me
Most of the hematopoietic stem cells of the bone marrow are at rest - they do not divide. But when the hemocytoblast wakes up, it makes the most important choice - to give rise to a new multipotent stem cell or to start the process of specialization of daughter cells. In the first case, the cell can indefinitely prolong its "childhood", in the second - the cells enter the next period of their lives.
Hematopoietic cells that have matured begin to divide asymmetrically, which leads to their differentiation and specialization. Precursor cells are formed that choose "study" - the myeloid path of development, or the "army" - the lymphoid path of development.
Myeloid hemocytoblasts develop into platelets, erythrocytes, macrophages leukocytes, granulocytes (a type of leukocytes - eosinophils, neutrophils or basophils).
Lymphoid hemocytoblasts will give rise to the body's immune defense cells - T-lymphocytes (recognize foreign antigens), B-lymphocytes (produce antibodies), T-helpers (attack foreign cells), NK-lymphocytes (provide phagocytosis of foreign agents).
Realizing potential
Hematopoietic stem cells, entering the stage of differentiation, lose their multipotency and realize their potential. Several factors influence the choice of the pathway for the development of hemocytoblast:
- Environment - differentiation in different areas of the bone marrow proceeds in different ways.
- Factors acting at a distance. For example, the hormone erythropoietin, which stimulates the formation of red blood cells, is synthesized in the kidneys. All these biologically active substances are called cytokines and growth factors (parathyroid hormone, interleukin).
- Signals of the sympathetic nervous system, which transmit information about the state of the body and the composition of the blood.
Today, the mechanisms of hematopoiesis are not fully understood and are still waiting for their Nobel laureates who will learn to control the fate of the hemocytoblast.
Bone marrow transplantation
This term most often refers to the transplantation of hematopoietic stem cells. This is a widely used method in the treatment of blood diseases, oncological and genetic pathologies. Modern methods of therapy make it possible to use not only donor bone marrow. Today, the donor of hematopoietic stem cells is peripheral blood, umbilical cord blood, and fetal (embryonic) medicine products.
The essence of hemocytoblast transplantation is as follows. At the initial stage, the patient undergoes the conditioning phase (radiation or chemotherapy), in which the functioning of his own bone marrow is suppressed. Then the patient is given a suspension of hematopoietic cells that populate his blood-forming organs and restore hematopoietic functions.
Friends or strangers
Depending on the source of stem cells for transplantation, there are:
- Autotransplantation. With this therapy, a patient is given a suspension of his own hemocytoblasts, which are taken in advance and stored frozen. This type of transplantation is used in the treatment of lymphomas, neuroblastomas, brain tumors and other solid malignant tumors.
- Allotransplantation. In this case, hematopoietic cells of the donor are used, which can be either close relatives of the patient, or selected from the registers of bone marrow donors.
With autotransplantations, there is no cell rejection and immune complications, but this method is not always effective. Allograft is effective in many congenital (Fanconi anemia, severe combined immunodeficiencies) and acquired (leukemia, aplastic anemia, myelodysplastic syndrome) pathologies of the blood and hematopoietic system, but requires careful selection of the donor for histocompatibility.
To summarize
But in any case, bone marrow transplantation is associated with a significant risk to the patient’s health. That is why it is carried out exclusively in case of vital necessity.
Modern bone marrow transplantation techniques have already saved the lives of thousands of patients with blood pathologies.
Cord blood stem cells were first used in 1987, and today these techniques have already saved more than 10 thousand patients. In this case, cord blood stem cell banks develop, because it can be taken no more than 100 ml and only once. In frozen form, the cells remain viable for 20 years, and it is possible to select donor blood in such banks.
Another direction in the development of stem cell transplantology is fetal therapy, which uses embryonic cells. Their source is abortive material. But this is a topic for a completely different article.