Bone marrow is a semi – solid gelatinous tissue which resides within the bony cavities of the axial skeleton. It contains hematopoeitic cells (red marrow), stromal cells, and fat (yellow marrow).
The bone that surrounds the marrow is composed of a thick layer of compact bony material referred to as cortical bone. This bone is covered by fibrocartilaginous tissue (periosteum). The space inside the cortical bone is called the medullary cavity. The medullary cavity itself contains a lattice – like network of thin bone referred to as trabecular or cancellous bone. Hematopoietic cells reside in the intervening spaces of trabecular bone.
Components of b one marrow
1. Bone marrow stroma
The bone marrow stroma is the supporting matrix for hematopoietic cells. The bone marrow stroma is composed primarily of; Macrophages, Specialized bone marrow stromal cells and Fat cells (adipose tissue)
Specialized bone marrow stromal cells
• Bone marrow stromal cells have the important ability to differentiate into other mesenchymal tissues such as bone, cartilage and fat.
• Stromal cells also provide important micro – anatomical niches supporting hematopoiesis and hematopoietic stem cells.
Fat cells (adipose tissue)
• The function of marrow adipose tissue in regulating hematopoiesis or osteogenesis is unclear. Marrow fat secretes cytokines that can influence granulopoiesis and T cell and monocyte function.
• There is an inverse relationship between amount of marrow fat and erythropoiesis and bone density not seen with subcutaneous adipose tissue.
• Marrow adipose tissue also differs from peripheral adipose tissue in that it is not lost in starvation.
The marrow vasculature consists of a nutrient artery that penetrates the bone and branches into smaller and smaller divisions (arterioles) ultimately forming open vascular channels, a sinusoidal network, in the medullary cavity. Sinusoids are dilated thin walled channels at the capillary – venous junction.
Morphologically, on tissue sections, sinusoids are generally not apparent since they are often collapsed. Sinuoids drain into venous sinuses which connect to ultimately form the comitant vein. The comitant vein exits the marrow through the same channel as the nutrient artery.
• Sinusoids are lined on the outside by adventitial cells.
• Adventitial cells serve as an additional niche supporting hematopoiesis.
• Adventitial cells likely make reticulin, a form of collagen, which can increase in disease states.
3. Hematopoietic cells
Hematopoietic cells lie in groups or cords in the inter – trabecular spaces and enter the circulation by migrating through the sinusoidal endothelium. Hematopoietic cells are not randomly distributed through the marrow but are present in small niches.
On histologic sections these zones are not always evident:
(i) Erythroid progenitors localize in small collections around marrow macrophages.
(ii) Granulocytes generally reside adjacent to adipose cells with immature granulocytes near bony trabeculae and adventitial cells.
(iii) Megakaryocytes lie abutting marrow sinusoids.
(iv) The ratio of marrow space occupied by hematopoietic cells to the total available marrow space is referred to as the marrow cellularity.
(v) Marrow hematopoietic cellularity varies with age. As a rule of thumb marrow cellularity is equivalent to 100 minus the patient’s age in years. In reality, marrow cellularity decreases to roughly 50% by age 30 and declines slowly thereafter until late adulthood.
(vi) Marrow cellularity may change rapidly in response to a peripheral cytopenia, infection, or growth factors.