Hematopoietic stem cell

Special cells make your blood.

These cells live inside your bones. They are in the red marrow. This marrow is in the core of most bones.

These cells make many parts for your blood. They can make many different kinds of cells. This helps you stay well. They make enough cells for your body every day.

Doctors can use these cells to help sick people. They can move cells from a donor to a patient. This helps treat some types of cancer.

These cells can even travel through your blood. They move from one bone to another bone. They are very busy cells!

Special cells make your blood. We call these hematopoietic stem cells. They are very important. They make all the different parts of your blood. This way of making blood is called haematopoiesis.

In adults, these cells live in red bone marrow. This is in the core of most bones. In a baby, they start in different places. They can be found in the liver or the spleen. These cells can even travel through your blood. They move from one bone to another bone.

This shows the many blood cells made from stem cells.

These cells are very busy. An average person makes over 500 billion blood cells every day. The stem cells can make many types of cells. Some make myeloid cells. Others make lymphoid cells. These cells help your body fight germs.

Doctors use these cells to help sick people. They can move stem cells from a donor to a patient. This is called a transplant. It can help treat some types of cancer. This process helps the body build a new blood system. It can take two to four weeks to work.

Your body is constantly making new blood. This important job is called haematopoiesis. It all starts with special cells called hematopoietic stem cells, or HSCs. These cells are like the master builders of your blood. They are multipotent, which means they can turn into many different kinds of cells. Without these busy cells, your body could not replace its blood. An average person produces more than 500 billion blood cells every single day!

How do these master builders work? HSCs follow two main paths to create blood. One path is called the myeloid line. This path makes cells like red blood cells and platelets. The other path is the lymphoid line. This path creates cells like T cells and B cells to help fight germs. Most of these cells come from a small number of HSCs. These stem cells can also self-renew. This means they can make copies of themselves to keep the supply going.

Scientists have been studying these cells for a long time. People first discovered hematopoietic stem cells in 1961. Since then, we have learned a lot about where they live. In adults, they mostly live in the red bone marrow. This is the soft part inside the core of most bones. In a developing baby, they start in different places. They can be found in the aorta, the placenta, or even the fetal liver.

Doctors use the power of these cells to help sick people. This is often done through a hematopoietic stem cell transplant. In this process, doctors move stem cells into a patient. These cells might come from the patient themselves or a donor. This can help treat serious illnesses like leukemia or other blood cancers. It can take about two to four weeks for the new system to grow. Scientists use special tools like flow cytometry to find these rare cells.

Even though these cells are strong, they can change as we get older. As people age, the DNA inside the stem cells can get damaged. This damage happens because of things like normal metabolism. Scientists are studying how to keep these cells healthy for a long time. They want to understand how the cells stay in a state called quiescence. This is a type of resting state that helps them survive. Understanding this could help us find new ways to treat many diseases.

Hematopoietic stem cells, or HSCs, are the essential foundation of the entire blood system. These unique cells are responsible for a process called haematopoiesis, which is the production of all mature blood cells. HSCs are characterized as multipotent, meaning they have the capacity to differentiate into many different specialized cell types. They also possess the ability for extensive self-renewal, allowing them to create more stem cells to maintain their own population. This balance is vital because the average human produces more than 500 billion blood cells every single day. Without the constant work of HSCs, the body could not regulate the necessary levels of blood cells in circulation.

The mechanism of blood production follows specific biological pathways known as lineages. HSCs divide into two primary lines: the myeloid lineage and the lymphoid lineage. The myeloid line produces various cells, including erythrocytes (red blood cells), megakaryocytes (which become platelets), and several white blood cells like neutrophils, basophils, eosinophils, macrophages, and monocytes. The lymphoid line is responsible for producing T cells, B cells, natural killer cells, and innate lymphoid cells. Both lineages contribute to the formation of dendritic cells. This complex system ensures that every specific type of cell required for oxygen transport, clotting, and immunity is constantly replenished.

HSCs exist in several distinct subtypes and states during their life cycle. Some are classified as colony-forming units (CFUs), which are categorized by the specific cells they produce. Examples include CFU-GEMM, which produces several myeloid types, and CFU-L, which leads to the lymphoid line. There are also more specific units like CFU-E for erythrocytes or CFU-Meg for megakaryocytes. Additionally, HSCs often exist in a state of quiescence, which is a form of reversible growth arrest. This dormant state is a survival mechanism. It allows the cells to endure the hypoxic, or low-oxygen, environment of the bone marrow. When the body experiences cell death or damage, these cells exit quiescence to begin active division.

The history of our understanding of these cells dates back to their discovery in 1961. Since then, scientists have mapped their complex locations throughout development. In vertebrate embryos, the first definitive HSCs arise from the ventral endothelial wall of the embryonic aorta. This occurs in the aorta-gonad-mesonephros (AGM) region through endothelial-to-hematopoietic transition. As development continues, HSCs can be found in the yolk sac, placenta, umbilical arteries, and the fetal liver. In adults, the process shifts primarily to the red bone marrow located in the core of most bones. Recent global discoveries have even identified HSC niches within invertebrate skeletons, challenging the idea that skeletal haematopoiesis is unique to vertebrates.

The clinical significance of HSCs is most evident in hematopoietic stem cell transplantation (HSCT). This medical procedure is used to treat life-threatening conditions like leukemia, multiple myeloma, and various immune system disorders. Transplants can be autologous, using the patient's own cells, or allogeneic, using cells from a donor. They can also be syngeneic, using cells from an identical twin. Because HSCs are rare—making up only about 1:10,000 of cells in myeloid tissue—doctors must use specialized methods to find them. They use flow cytometry to identify specific surface markers, such as CD34, to isolate these cells from the surrounding blood.

As organisms age, the health of the hematopoietic system undergoes significant changes. One major factor is the accumulation of DNA damage within long-term HSCs. The body relies on a repair pathway called non-homologous end joining (NHEJ) to fix double-strand breaks in DNA. This pathway requires specific proteins, including DNA ligase 4 and NHEJ factor 1. If these proteins are deficient, it can lead to a progressive loss of stem cells and premature aging. Research suggests that clonal diversity, or the variety of different stem cell lines, can decrease drastically around age 70. This loss of diversity is a key part of how the blood system changes over a lifetime.

Understanding HSCs connects many different fields of science, from embryology to oncology. The study of the "stem cell niche," the specific environment in the bone marrow, helps researchers understand how molecular signals trigger self-renewal. Scientists are currently investigating how to expand these cell populations in vitro, which means in a controlled laboratory setting. If successful, this could lead to new ways to regenerate tissues or provide more abundant supplies for transplants. By studying how HSCs move through the body and respond to physiological stress, researchers hope to unlock new therapies for both blood diseases and general aging.