By Lilly Su, MD Candidate, Class of 2023, Penn State College of Medicine
Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells, through a process called hematopoiesis. There are many organs and tissues within the body that are involved with hematopoiesis, such as the bone marrow, liver, and the spleen. However, as hematopoiesis starts as early as the first weeks of embryonic development, during this stage of blood cell production, also known as “primitive hematopoiesis,” only red blood cells are produced, which transports oxygen to the growing fetus, and occurs solely in the yolk sac. It isn’t until the embryo develops further, does hematopoiesis move to the other organs and tissues mentioned previously. Additionally, as more organs and tissues are involved in the production of blood cells, there are more types of blood cells produced. In infants and children, hematopoiesis of red blood cells and platelets occurs mainly in the spleen and liver, but once they become adults, hematopoiesis primarily occurs in the bone marrow. On the other hand, white blood cells are consistently produced in the lymph system, which involves organs such as the spleen, lymph nodes, and thymus.
HSCs are possibly the most complex type of stem cells due to the variety of blood cells it is able to produce. There are more than 10 different cell types that exist in the blood that come from the original hematopoietic stem cell, and they can be broken up into a few categories: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). Yet, the process the hematopoietic stem cell must go through to become one of these cell types is a bit less straight-forward than that.
Starting out as the hematopoietic stem cell, there are two “paths” that the stem cell can choose to go down, either become a common myeloid progenitor, or a common lymphoid progenitor. From the common myeloid progenitor, the cell has even more options, it can either become a megakaryocyte that will differentiate into a thrombocyte, an erythrocyte, a mast cell, or a myeloblast that can then further differentiate into either a basophil, neutrophil, eosinophil, or monocyte that ultimately differentiates into a macrophage. If it chooses to go down the common lymphoid progenitor path, it can differentiate into a natural killer cell or small lymphocyte and from small lymphocyte, be further differentiated into either T or B lymphocyte. From the B lymphocyte, it can then differentiate into a plasma cell.
Although each cell type has its own specific role and function within the body, we will only be going through the cell types that are important when looking at lab values. Erythrocytes help transport oxygen and hemoglobin throughout the body. T and B cells are the major players in the immune system and help fight pathogens, specifically viruses. Neutrophils are also a component of the immune system, but target mainly bacteria and fungi. Thrombocytes are what help the blood to clot.
Transplantation of HSCs are actually used as curative therapy for diseases such as leukemia, however, there is still much room for improvement regarding stabilizing HSC culture and expansion.