Bone marrow is a kind of tissue that found in our bone. As I am going to do the bone marrow cancer research soon, I do a lot of reading about the bone marrow. Before we talk about the bone marrow, let us explore the organization of the bone marrow first. There are 3 parts of the bone marrow that we need to discuss: bone, other connective tissues, hematopoietic and other cells.
Bone Marrow Organization – Bone
Bones are composed of cortex and medulla. The function and the histological appearance are different for these two parts of bone. The classification of bone can be divided to two ways:
i. Macroscopic appearance
- Compact bone with small interstices that is invisible macroscopically.
- Cancellous bone with large and visible interstices.
ii. Histologically appearance
- Well-organized osteons in which a central Haversian canal is surrounded by concentric lamellae. These are composed of parallel bundles of fibrils or we called lamellar bone.
- The fibrils of bone which are in disorderly bundles or we called woven bone.
The cortex is a solid layer of bone that mainly supports the bone. It is composed largely of lamellar bone with small amount of woven bone too. The medulla is honeycomb of cancellous bone. It is less organized than the cortex although it consists of lamellae too. The interstices of medulla form the medullary cavity that contains bone marrow.
Periosteum has an outer fibrous layer and an inner osteogenic layer. It covers the outer surface of most of the cortical bone. On the other hand, the bone is covered with cartilage but not periosteum at articular surfaces. The endosteal cells which include the osteoblasts and osteoclasts cells line the bony trabeculae and inner surface of the cortex. Osteoblasts are mesenchymal origin where as osteoclasts are originated from hematopoietic stem cell.
Bone Marrow Organization – Other Connective Tissue Elements
There are many other connective tissues inside the bone too. The connective tissue stroma resides in the intertrabecular spaces of the medulla. Most of the hematopoietic cells of the bone marrow are attached in the stroma. Besides the stroma tissue, there are other connective tissues that have not discussed in this article due to the complexity. We might touch on those tissues in the future post in Cytogenetics and Cancer Research blog.
Bone Marrow Organization – Hematopoietic and Other Cells
Hematopoietic cells occupy in cords between the sinusoids. The hematopoietic cell is confined to the interstitium in normal hematopoiesis in human except at extramedullary sites for some thrombopoiesis. The hematopoiesis can occur within sinusoids in pathological conditions. The matured hematopoietic cells will go into the circulation system via sinusoidal endothelial cells. Furthermore, there are lymphoid cells, plasma cells and mast cells in bone marrow too.
In conclusion, knowing the organization of bone marrow is important when we wish to carry out the research on it. It seems like very complex, but I wish that I can really explore it deeply so that I can deal with it during my cancer research on leukemia. We will discuss about the formation of the bone in the next article. Stay tuned! 
(Reference: Bone Marrow Pathology written by Barbara J. B., David M. C., Irvin A. L. and Bridget S. W.)
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