Bone Marrow Stem Cells, Aging, and Age-Related Diseases

Abstract

Bone marrow is a soft, gelatinous, and dynamic tissue present in the central cavity of long bones such as the femora and humeri. Bone marrow is a large reservoir of pluripotent stem cells such as hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs), and multipotent adult progenitor cells (MAPCs). Apart from stem cells, it also has bone marrow stromal cells comprising endothelial cells (ECs), osteoclasts, osteoblasts, fibroblasts, tissue macrophages, and adipocytes along with soluble components such as cytokines, chemokines, growth factors, and hormones. Bone marrow has a unique capability to proliferate and differentiate into unspecified lineage of all types of cells of the body and provide immunity to the body. Hence it serves as an organ of the immune system. However, the life-span of tissues depends on the replacement of damaged cells and supply of new cells. With advancing age, bone marrow and stem cells are inefficient to maintain the homeostasis for the delivery of new cells, because of alterations in bone marrow and bone marrow stem cells, which lead to aging process. Aging is a universal process. All cells, tissues, organs, and organisms undergo changes with age. Age-related bone marrow alterations, which include deterioration of bone marrow cellularity, fat cell deposition, and contracted hematopoietic tissue are strictly associated with many age-related diseases such as cancer, altered B lymphopoiesis, osteoporosis, and age-related macular diseases. Various theories explain that the aging process is associated with the bone marrow. Theories such as stem cell theory of aging, gene expression theory, epigenetic mechanism, reactive oxygen species (ROS) theory, metabolic theory, and telomere theory of aging are helpful to understand the process of aging. However, the exact mechanism of aging is still unclear. The extreme consequences of aging are tissue failure, failure of regeneration processes, diseases, and lastly death. In recent years, advanced medical science such as bone marrow transplantation has increased health span and life-span and showed great potential towards the recovery from age-related diseases such as type-2 diabetes, osteoporosis, and Alzheimer’s diseases. However, we have many questions such as the following: What is the principal rule of aging? Can aging be prevented? and How can we increase life-span? These questions are remaining to be answered.