Bone Marrow Structure and Function

Understand “haemopoiesis”: the processes by which blood cells are formed

Haemopoiesis, which I'm pretty sure is also called haematopoiesis, is the process in which new blood cells are formed in the bone marrow. The most primitive cells in the bone marrow are pluripotent haemaopoietic stem cells (HSCs), which can be induced by various signalling molecules such as cytokines to go down a particular growth pathway. For instance, they can become lymphoid stem cells, which become lymphocytes, or myeloid stem cells, which go on to become basically every other type of blood cell (erythrocytes, platelets, basophils, etc.). Usually around 20% of HSCs become erythrocytes (red blood cells), 60% become granulocytes (white blood cells, excluding lymphocytes), 15% become lymphocytes, and less than 0.1% form megakaryocytes which bud off to form the platelets. The remaining HSCs go on to form the other blood cells that I didn't list, such as plasma cells and macrophages.

Haematopoiesis takes place in a range of different locations during development. At around week 3 of gestation, haematopoiesis takes place in the yolk sac. During months 3-7, the liver is the main site of haematopoiesis. During this time, at around months 4-5, the bone marrow commences haematopoiesis, but the bone marrow doesn't reach its full potential until around months 7-8.

From birth onwards, haematopoiesis takes place solely in the bone marrow. However, as we age, the number of bones at which we can perform haematopoiesis becomes more limited. For instance, the tibia and femur stop producing blood by the time we are in our early to mid 20s. As we grow older, haematopoiesis mainly takes place in more central bones, such as the vertebrae and sternum. Therefore, if you want to take a sample of bone marrow from an adult, you generally have to take it from a more central bone- usually the posterior superior iliac spine, as there are fewer important structures around that area.

Normal bone marrow structure

Bone marrow is located in the hard bony cavity of bones. As well as stem cells, it also contains growth factors such as cytokines, metals such as iron, vitamins, and so on. (Aside from that, this lecture didn't really have that much detail on bone marrow structure, so I assume we'll learn more about that as the semester goes on.) We can assess haematopoiesis by looking at the bone marrow and the cells that are present.

Cytokines: cell hormones that influence haemopoiesis

As mentioned earlier, pluripotent stem cells can be induced to go down a certain lineage via signalling molecules such as cytokines. Different cytokines are important for different lineages, and some cytokines have a wider range of effects than others.. For instance, erythropoietin, or EPO for short, is made in the kidney and is needed to increase red blood cell production. Meanwhile, G-CSF and GM-CSF are largely responsible for controlling leukocyte production. Cytokines work by binding to receptors on the cell, leading to downstream signalling pathways and ultimately altering transcription of genes in the nucleus.