TUNEL staining of inner ear structures may reflect autolysis, not apoptosis

A recent study (Usami et al., 1997) using the TUNEL method has suggested that age-related cell death in the senescence-accelerated mouse inner ear is due to apoptosis. TUNEL staining detects not only apoptosis but also late necrosis or autolysis because it detects DNA breaks. Autolysis may occur in inner ear structures during fixation. To determine whether or not age-related cell death is due to apoptosis, TUNEL staining of the inner ear of normal mice should be understood. However, studies of TUNEL staining of the normal inner ear have not yet been reported. We investigated whether the fixation method or the interval between the death of normal mice and the initiation of fixation influences the results of TUNEL staining of the inner ear. Marginal cells of the stria vascularis and hair cells of the saccule were TUNEL-positive, irrespective of the fixation method or the interval between death and fixation. Interdental cells, Reissner membrane cells, fibrocytes in the suprastrial region, and inner and outer hair cells were also occasionally stained. Transmission electron microscopy showed no morphological characteristics of apoptosis in the hair cells of the saccule. Moreover, patterns of TUNEL staining in the normal and senescence-accelerated mouse inner ear were similar. These stained tissues may require a high level of oxygen, making them more susceptible to autolysis. We concluded that the results of TUNEL staining in the inner ear require confirmation by morphological studies.