Bones contain a relatively small number of cells entrenched in a matrix of organic and inorganic components. Although bone cells compose only a small amount of the bone volume, they are crucial to its function. Four types of cells are found within the bone tissue— osteoblasts, osteocytes, osteogenic cells, and osteoclasts.
Osteoblasts and Osteocytes
The osteoblast is the bone cell responsible for forming new bone tissue. It is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts are non-dividing cells that synthesize and secrete the matrix components. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast becomes trapped within it. It then differentiates into an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. However, they can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi — channels within the bone matrix.
Osteogenic Cells and Osteoclasts
Osteogenic cells are undifferentiated cells with high mitotic activity. In fact, they are the only bone cells that can divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts, thus replenishing the dead cells.
The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or calcium release. The cells responsible for bone resorption, or breakdown, are the osteoclasts— large, multinucleated cells found on bone surfaces. Unlike the other cells, which are all derived from the mesenchymal stem cell lineage, osteoclasts are formed by the fusion of precursor cells from the hematopoietic stem cell lineage. Osteoclasts are continually breaking down old bone, while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of a bone.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 6.3: Bone Structure
