Posted by When I was a baby, I loved looking up at the mobile hung above my bed as I went to sleep. The planets of our solar system drifted around, where I could dream of the incredible depth of outer space. But why would my parents want me to dream of the rings of Saturn, while I could be dreaming of the various white blood cells and their functions, the function of red blood cells and hemoglobin, and the other various components of blood and their key structural functions? I’m sure other medical science students and professionals have had similar thoughts. So, I decided to build a custom mobile that displays our various blood cell types throughout their process of development.
Aside from a small amount of primitive blood cells produced during early stages of fetal development, all our blood cells originate in the red marrow of our bones. Hematopoietic stem cells (HSCs), are formed from the mesoderm in embryonic development and move into the bones around 10 weeks after fertilization. As the progenitor cell type, these HSCs can develop into every variety of blood cell in the human body (Jagannathan-Bogdan & Zon, 2013).
White blood cells are typically split into two categories, granulocytes (basophils, neutrophils, and eosinophils) and agranulocytes (monocytes and lymphocytes) based on if granules of concentrated enzymes are visible on the surface of the cells when a certain stain is applied for microscopic analysis. This categorization is misleading for two reasons. First, the agranulocytes do have granules on their surface, they are just harder to see under a microscope. Second, monocytes come from the same progenitor myeloblast cells as the granulocytes, while lymphocytes come from a completely separate developmental path (Glenn & Armstrong, 2019).
Lymphocytes start their development in the red bone marrow when HSCs differentiate into lymphoblasts. Some lymphoblasts continue to develop in the bone marrow and become B-lymphocytes. These B-cells provide a core function in our acquired immune response – recognizing a foreign antigen and dividing many times to create plasma-B cells that produce antibodies that bind to foreign cells. They also create additional memory-b cells so there is a faster antigen recognition and response when we meet that pathogen again. If the lymphoblasts migrate to the thymus, they develop into T-lymphocytes. These also play a similar role to B-cells, recognizing foreign antigens and mounting a defense against them. B-cells are unique because they need antigens to be presented to them by another white blood cell (usually a WBC that has phagocytized a foreign cell). Helper-T cells will release interleukin to promote a larger immune response, cytotoxic-T cells will kill host cells that have been infected with the pathogen, and memory-T cells will preserve our immunity for future encounters (Minors, 2004).
HSCs can also become myeloid stem cells (MSCs), which branch off into many, many cell lineages. Many become erythroblasts (also called pronormoblasts) as they develop in the bone. These cells lose their nucleus, become filled with hemoglobin, shrink somewhat as they develop into erythrocytes over about a week (Glenn & Armstrong, 2019). MSCs can also develop into megakaryocytes, large cells that split off into many small Thrombocytes (platelets). There are about 10 times as many red blood cells than platelets in the blood, and about 50-100 times more platelets than white blood cells (Minors, 2004).
These MSCs are also a progenitor to all non-lymphocyte white blood cells. Neutrophils, basophils, and eosinophils all share a common development pathway taking place in the bone marrow, then moving to the bloodstream. Monocyte development is also similar, but they spend relatively little time in the bloodstream, migrating into the tissues and becoming macrophages (Glenn & Armstrong, 2019).
Much like evolutionary taxonomy, the development pathways of our cells can tell us a lot about how they relate to each other. Blood cells provide a complex mix of basic nutrient transportation and multi-layered defense from harmful pathogens, and it is impressive how so many distinct cells can rise from a single type of stem cell.
Sources
Al-Dulaimi, K., Banks, J., Chandran, V., Tomeo-Reyes, I., & Nguyen, K. (2018).
Classification of white blood cell types from microscope images:Techniques and challenges. Microscopy science: Last approaches on educational programs and applied research, Microscopy Book Series (8) 17-25. Formatex Research Center, Spain. https://eprints.qut.edu.au/121783.
Glenn, A. & Armstrong C.E. (2019). Physiology of red and white blood cells.
Anesthesia & Intensive Care Medicine, 20(3), 170-174, https://doi.org/10.1016/j.mpaic.2019.01.001.
Jagannathan-Bogdan, M., & Zon, L. I. (2013). Hematopoiesis. Development, 140 (12), 2463–2467. https://doi.org/10.1242/dev.083147.
Minors, D.S. (2004). Physiology of red and white blood cells. Anaesthesia & Intensive Care Medicine, 5(5), 174-178, https://doi.org/10.1383/anes.5.5.174.34003.Shrestha, R.P., Horowitz, J., Hollot, C.V. et al. (2016). Models for the red blood cell lifespan. Journal of Pharmacokinetics and Pharmacodynamics, 43, 259–274. https://doi.org/10.1007/s10928-016-9470-4
Soren’s STEAM project is inspired by a traditional baby mobile but turns it into an educational model portraying blood cell development. Motivated by his interest in how stem cells differentiate into various blood cells, he built a mobile that visually represents the development pathways of white blood cells, red blood cells, and platelets. Using colorful, hand-drawn images, Soren illustrates the journey of hematopoietic stem cells (HSCs), which originate in embryonic development and settle in the bone marrow by 10 weeks gestation. From these HSCs, cells can become either lymphoid or myeloid stem cells. Lymphoid cells form lymphocytes—B-cells (which produce antibodies and memory cells) and T-cells (which kill infected cells and regulate immune responses). Myeloid cells give rise to red blood cells (which transport oxygen), platelets (which assist in clotting), and other white blood cells including neutrophils, eosinophils, basophils, and monocytes (which later become macrophages in tissues). Soren’s creative talent and scientific understanding are clearly demonstrated in this project that any baby would be lucky to have hanging over their crib!