Morphological comparison of apoptotic with non-apoptotic dying cells in the developing inner ear of mouse embryos.

Dying cells studied by the TdT-mediated dUTP nick end-labeling (TUNEL) method have been classified as "apoptotic" and "non-apoptotic" cells. In this study, in which 12-day-old mouse embryos were used because of a high frequency of "natural cell death" due to changing inner ear morphology [Kaufman, M.H., 1992. The Atlas of Mouse Development, first ed., Academic Press, London, p. 147], the percentages of "apoptotic" and "non-apoptotic" dying cells (ADC and NADC) among total dying cells in the inner ear were calculated. Observation of consecutive paraffin sections showed about 90% of the dying inner ear cells to be ADC and about 10% to be NADC. ADC and NADC TUNEL positive dying cells in resin sections observed by light microscopy were examined again by transmission electron microscopy using a re-embedding procedure. ADC and NADC were then analyzed based on the classification of dying cells (types 1, 2, 3A, and 3B) as described by Clarke [Anat. Embryol. 181 (1990) 195]. It was clear that ADC were the equivalent of type 1 (apoptotic) dying cells and NADC were the equivalent of type 2 (autophagic) dying cells. We consider these findings to be important baselines for determining the process underlying abnormal development of the inner ear and its functional disorders such as hearing loss.