Protective role of compensatory growth in the development of mammalian inner ear.

The inner ear in rats is ectodermally derived. It appears as a placode on day 8 of a 22-day gestational period. In the developing inner ear most of the cells undergo terminal mitosis between gestational days 13 and 15. During this critical period of development exposure to toxic substances may result in malformations of the adult inner ear. To test this hypothesis, rat fetuses were exposed to 5-f luoro2’deoxyuridine (FUdR), an antimitotic substance, on gestational days 12-16. The animals also received one injection ofH-thymidine 1 h prior to autopsy. The inner ear specimens were prepared for morphological observations and bioche mical assays. Four hours after the administration of FUdR there was a substantial decrease in the number of otic cells. This trend was seen until 24 h after treatment. When examined on day21 of gestation, the inner ears from the fetuses exposed to FUdR during gestational days 12-15 almost completely recovered from the toxic effects of the drug. The phenomenon of compensa ory growth observed during embryonic development may represent an important protective feature. During early stages of organogenesis, the otocyst perhaps has a remarkable capacity for recovery resulting in relatively fewer congenital anomalies of the inner ear. (JPMA 36: 308, 1986).