Bone marrow is a spongy connective tissue found inside bones. Basically it is of two types
1. Yellow bone marrow: It is mostly filled with adipose tissue and located in the shafts (hallow interior portion) of long bones.
2. Red Bone marrow: It is a heamopoietic tissue found at the end of long bones, short and flat bones such as hip bones, skull bones, vertebrae etc.
Bone marrow is a fundamental heamopetic organ and assists in the production of blood cells. Bone marrow stroma is densely packed with various cell types most of them derived from the stem cells. These are also called pluripotent stem cells. These pluripotent stem cells produce other stem cells like lymphoid stem cell and myeloid stem cell from which other cell types of mature blood cells arise. The lymphoid stem cells generate T and B progenitor lymphocytes. The myeloid stem cells generate progenitor stem cells for erythrocytes, granulocytes, monocytes, mast cells and platelets. These committed progenitor cells are capable of giving rise to only one specific type of mature cell.
In healthy human beings the number of each type of stem cell and the offspring originating from it is within certain limits. Proteins such as interleukins and colony stimulating factors play a crucial role in determining whether stem cell will replicate itself or produce offspring that evolve into mature cell types. These proteins also regulate the maturation of precursor cells. If the regulatory mechanism fails this may lead to serious complication of leukemia.
Besides, the heamopoietic stem cells bone marrow also contains Meshynchymal stem cells giving rise to osteoblasts, chondrocytes, myocytes etc. Additionally they contain endothelial stem cells from which endothelial cells develop capable of forming endocytes.
The bone marrow is fragmented into wedge- shaped hematopoietic compartments filled with proliferating and differentiating blood cells in the matrix of connective tissue bordered by venous sinuses. The heamopoietic tissue inside the bone marrow is well traversed with an extensive capillary network of vascular sinusoids that communicate with the peripheral circulation. The marrow tissue and the capillary sinusoid wall form a barrier called bone marrow- blood barrier from which the mature blood cells will pass through. The vascular system is almost closed one with little leakages. Mature cells are transported to venous sinuses through cytoplasmic apertures in endothelium. The release of the cells is regulated by the requirement of the body (for ex. Low oxygen- leads to production of erythropoietin this stimulates the bone marrow cells to produce RBC).
I would like to continue the discussion in the next blog......
- Targeted & Controlled Drug Delivery (Drug Delivery to Bone marrow); pgs: 563--566, S. P. Vyas & R. K. Khar. C. B. S. Publishers. [access date: 15th June, 2010].
- Image source: Lippincott's text book of medical physiology, pg no: 203.
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