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Hippocampus is important for learning and memory. This article reviews the recent progress of the relationship between the toothless condition and the hippocampal functional morphology. Tooth loss early in life was generated by extracting the upper molars shortly after tooth eruption in mice or rat. Morphological and physiological studies showed that early toothlessness, acting as a chronic stress, induced constantly elevated levels of corticosterone, leading to morphological and molecular alterations in hippocampus, accompanied by deficits in spatial learning and memory. The early toothlessness may be a risk factor of cognitive impairment. Adequate dental treatments such as denture or dental implants for defective part of teeth are considered to be important for maintaining the hippocampal functions. The possible mechanism of the hippocampal alterations induced by early toothless condition is also discussed.

Both osteoporosis and tooth loss are health concerns that affect many older people. Osteoporosis is a common skeletal disease of the elderly, characterized by low bone mass and microstructural deterioration of bone tissue. Chronic mild stress is a risk factor for osteoporosis. Many studies showed that tooth loss induced neurological alterations through activation of a stress hormone, corticosterone, in mice. In this study, we tested the hypothesis that tooth loss early in life may accelerate age-related bone deterioration using a mouse model. Male senescence-accelerated mouse strain P8 (SAMP8) mice were randomly divided into control and toothless groups. Removal of the upper molar teeth was performed at one month of age. Bone response was evaluated at 2, 5 and 9 months of age. Tooth loss early in life caused a significant increase in circulating corticosterone level with age. Osteoblast bone formation was suppressed and osteoclast bone resorption was activated in the toothless mice. Trabecular bone volume fraction of the vertebra and femur was decreased in the toothless mice with age. The bone quality was reduced in the toothless mice at 5 and 9 months of age, compared with the age-matched control mice. These findings indicate that tooth loss early in life impairs the dynamic homeostasis of the bone formation and bone resorption, leading to reduced bone strength with age. Long-term tooth loss may have a cumulative detrimental effect on bone health. It is important to take appropriate measures to treat tooth loss in older people for preventing and/or treating senile osteoporosis.