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The lifespan of formed elements in the blood is relatively short. While certain leukocytes, known as memory cells, can persist for years, most erythrocytes, leukocytes, and platelets typically have a brief existence ranging from a few hours to a few months. Consequently, the body continually and rapidly generates new blood cells and platelets, a process known as hemopoiesis or hematopoiesis (derived from the Greek roots "haima-" meaning "blood" and "-poiesis" meaning "production").
Hematopoiesis occurs at different sites throughout various stages of life. It begins in the yolk sac of the developing embryo and continues in the fetal liver, spleen, lymphatic tissues, and eventually the red bone marrow before birth. After birth, most hematopoiesis takes place in the red marrow of spongy (cancellous) bone tissue. In children, hematopoiesis may also occur in the medullary cavities of long bones. In adults, however, it is primarily confined to specific bones such as the skull, pelvis, vertebrae, sternum, and the proximal epiphyses of the femur and humerus. Although the liver and spleen retain the ability to produce blood cells throughout adulthood, they typically play a minimal role in hematopoiesis under normal conditions.
Formed elements in the blood originate from stem cells located within the red bone marrow. When a stem cell divides, one daughter cell remains a stem cell to sustain the stem cell pool, while the other differentiates into a specialized cell type. Stem cells follow a hierarchical structure, where their ability to give rise to different cell types decreases with each developmental stage. Totipotent stem cells, the most versatile, can produce all cell types in the human body, including those that form the fetal membranes. Next in the hierarchy are pluripotent stem cells, capable of producing many, but not all, body cell types. Below them are mesenchymal stem cells, which develop into various connective tissues, though not into epithelial, muscle, or nervous tissue. Hematopoietic stem cells (HSCs), also known as hemocytoblasts, represent the next level and are responsible for generating all blood cell types (Figure 2).
Hematopoiesis begins when HSCs are exposed to specific chemical signals, collectively called hematopoietic growth factors, which stimulate their division and differentiation. One daughter cell remains an HSC to ensure the ongoing production of blood cells, while the other differentiates into a more specialized stem cell. These specialized stem cells include lymphoid stem cells, which give rise to lymphocytes—such as T cells, B cells, and natural killer (NK) cells—essential for immune defense. Myeloid stem cells, on the other hand, produce erythrocytes (red blood cells), megakaryocytes (platelet-producing cells), and myeloblasts, which further differentiate into monocytes and three types of granular leukocytes: neutrophils, eosinophils, and basophils.
It is important to note that lymphoid and myeloid stem cells do not directly develop into mature blood cells. Instead, they progress through several intermediate precursor stages, often identified by names ending in "-blast." For example, megakaryoblasts mature into megakaryocytes (Figure 2 and Figure 10), and proerythroblasts become reticulocytes, which then lose their nucleus and most organelles to mature into erythrocytes (Figure 3).
Hemopoietic Growth Factors
The progression of stem cells into fully developed cells is driven by various hemopoietic growth factors, including erythropoietin (EPO), a glycoprotein hormone composed of 193 amino acids. EPO's primary role is to stimulate the production of erythrocytes in the bone marrow. Around 85 percent of circulating EPO is produced by interstitial fibroblast cells in the kidneys, with the liver contributing the remaining 15 percent. EPO production increases in response to low oxygen levels, such as those experienced at high altitudes. Similarly, regular aerobic exercise, which elevates oxygen demand, prompts the kidneys to release more EPO.
When RBCs are depleted due to severe or prolonged bleeding or are underproduced due to illness or malnutrition, the kidneys compensate by producing additional EPO to boost RBC production. In cases of renal failure, the loss of EPO production leads to anemia, which hampers the body's ability to meet oxygen demands, even under normal conditions.
EPO is used therapeutically to treat certain types of anemia where increasing RBC counts and oxygen levels is beneficial. Within 24 hours of EPO administration, RBC production increases, potentially leading to erythrocytosis (an elevated number of RBCs), which eventually reduces further EPO production as oxygen levels normalize.
Thrombopoietin is a glycoprotein hormone synthesized by the liver and kidneys. Its primary role is to promote the development of megakaryocytes into platelets, which play a critical role in blood clotting and wound healing.
Cytokines constitute a category of glycoproteins secreted by various cell types, including red bone marrow cells, leukocytes, macrophages, fibroblasts, and endothelial cells. They function as local signaling molecules, operating in autocrine or paracrine modes. Cytokines stimulate the proliferation of progenitor cells and contribute to the enhancement of both nonspecific and specific immune responses. Two major cytokine subtypes are colony-stimulating factors (CSFs) and interleukins. Some CSFs drive the differentiation of myeloblasts into granular leukocytes, specifically neutrophils, eosinophils, and basophils. Another type of CSF stimulates the production of monocytes. Synthetic forms of these hormones are commonly administered to cancer patients undergoing chemotherapy to restore WBC counts.
Initially believed to be secreted exclusively by leukocytes to communicate with other leukocytes, interleukins are now known to be produced by various cells, including those in the bone marrow and endothelium. Beyond their role in communication between immune cells, interleukins have broader functions in cell differentiation and maturation, immune system modulation, and inflammation regulation. Over a dozen interleukins have been identified to date, with more anticipated discoveries in the future.6
This page titled 8.7: Hematopoiesis was last modified on Wed, 19 Mar 2025 23:12:51 GMT and is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Chaya Gopalan (Consortium of Academic and Research Libraries in Illinois (CARLI)) .
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