Resorption is a biological process involving the breakdown and assimilation of tissue by specialized cells. This controlled mechanism removes old, damaged, or unnecessary mineralized tissue, ensuring the body can adapt to changing structural and chemical demands. While often associated with skeletal and dental health, resorption describes the body’s natural cycle of tissue removal and renewal.
Resorption in Skeletal Health
Resorption in bone is an integral part of the lifelong process called bone remodeling, which continually replaces discrete sections of old bone tissue with new material. This process is carried out by specialized, multinucleated cells known as osteoclasts, which secrete acid and enzymes to dissolve the mineralized bone matrix. The resulting release of minerals, primarily calcium and phosphate, is then transferred from the bone tissue back into the bloodstream.
The balance between bone resorption by osteoclasts and bone formation by osteoblasts is tightly regulated to maintain both skeletal integrity and calcium homeostasis. When blood calcium levels drop, the parathyroid glands release parathyroid hormone (PTH), which stimulates osteoclast activity to release stored calcium from the bone. Conversely, when calcium levels are too high, the thyroid gland releases calcitonin, which transiently inhibits osteoclast activity and reduces the rate of bone breakdown.
This dynamic remodeling cycle ensures that the skeleton remains strong, repairs microdamage, and adapts to mechanical stress. An imbalance where resorption outpaces formation can lead to reduced bone density and conditions like osteoporosis, where bones become fragile.
Dental Resorption Explained
Resorption also affects dental structures, specifically the hard tissues of the tooth root: cementum and dentin. Unlike the continuous, necessary resorption in bone, root resorption in permanent teeth is generally considered a pathological process, though it occurs physiologically in primary teeth to allow for shedding. Specialized cells are responsible for breaking down the tooth structure.
Dental resorption is classified based on its origin, with external resorption starting on the root’s outer surface and internal resorption originating within the pulp chamber. External resorption often results from chronic inflammation triggered by dental trauma, infection, or pressure from orthodontic movement. It begins by damaging the protective layers of cementum and the periodontal ligament, exposing the underlying dentin to the resorptive cells.
Internal resorption is a rarer condition that involves the loss of dentin from the inside of the root canal system. This process is linked to inflammation of the pulp tissue following a physical injury or bacterial invasion. If the resorptive process advances unchecked, it can lead to a hollowed tooth structure, weakening the tooth and potentially causing tooth loss.
Distinguishing Resorption from Reabsorption
The terms resorption and reabsorption describe distinctly different biological actions. Resorption refers to the process where the body breaks down a solid tissue or substance and assimilates the components, such as the cellular degradation of bone or tooth material.
Reabsorption, in contrast, describes the process of absorbing a substance that has already been absorbed once before, then secreted or filtered. The most recognized example occurs in the kidneys, known as tubular reabsorption. Here, substances like water, glucose, and ions are filtered out of the blood into the kidney tubules, but the body selectively reclaims about ninety-nine percent of these beneficial components back into the bloodstream before they are excreted as urine.