What is Bone Marrow?
Bone marrow is a highly vascularized, gelatinous tissue that contains hematopoietic (HSCs) and mesenchymal stem cells (MSCs). These stems cells generate progenitor cells that differentiate into a variety of cell types, including osteocytes (bone cells), adipocytes (fat cells), myocytes (muscle cells), erythrocytes (red blood cells), and white blood cells. Disease-associated variations in bone marrow progenitor cell populations provide insight into the causes of blood and autoimmune diseases. To interpret these changes, normal bone marrow must be understood. This article summarizes the current basic understanding of bone marrow.
What is the Difference between Red and Yellow Bone Marrow?
Two types of bone marrow exist: red and yellow. Both marrows contain the same constituents, but the levels of these components vary, resulting in the unique characteristics and functions of each marrow. Red marrow, also called myeloid tissue, is highly vascularized and primarily produces HSCs, which differentiate into progenitor cells that generate red blood cells (coloring this marrow red), white blood cells, and platelets. These differentiated cells enter the blood circulation through sinusoids.
At birth, all bone cavities contain only red marrow: no yellow marrow is present. Red marrow is converted into yellow marrow throughout an individual's life but is still the primary marrow present in children, residing in the medullary cavity of long bones, such as the femur. By adulthood, roughly equal amounts of yellow and red marrow exist, with flat bones, such as the pelvic girdle and sternum, containing the highest amounts of red marrow.
Yellow marrow, which is predominately located in the axial skeleton, specifically in the diaphysis of long bones, becomes more prevalent with age. In contrast to red marrow, adipocytes are the primary cell type in yellow marrow. The lipid droplets within these adipocytes contain carotenoids, which cause the yellow tinge of this marrow.
How Does Bone Marrow Contribute to Health and Disease?
The lipid stores present in yellow marrow, called marrow fat, are physiologically different from white and brown adipose tissue and increase with age, in a gender-specific manner, and under various disease conditions, such as starvation and spinal cord injury. This increase in marrow fat levels reduces the ratio of HSCs to adipocytes, which is known as bone marrow cellularity. Under other conditions, such as sickle cell disease or high altitude, the number of HSCs is upregulated, increasing bone marrow cellularity.
Like HSCs, MSCs, the other bone marrow stem cell population, play important roles in disease and healing. MSCs, which were originally identified in bone marrow but also exist in other tissues, differentiate into progenitor cells that generate osteocytes, adipocytes, myocytes, and chondrocytes. These stem cells are a powerful resource for regenerative medicine, with the potential to heal skeletal, hematologic, inflammatory, ischemic, and autoimmune diseases, and have recently produced promising effects in neurologic disease. Currently, over 600 clinical trials are being conducted to test mesenchymal stem cell-based therapies, propelled by advances in MSC isolation techniques (see here and here).
Red and yellow marrow have unique age- and disease-specific functions within the body, and the cell populations present within bone marrow provide insight into disease pathology and potential therapeutic solutions. Studies investigating the function of these cell populations in disease have been propelled by recent advances in cell isolation techniques. However, these techniques can still be tedious and time consuming. Scientists interested in bone marrow research may benefit from purchasing bone marrow samples, such as those offered by Folio Conversant.
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Revised on 6/22/18 by Rachel Lane PhD, RD