Effects of acute exposures to elevated temperatures on rat embryo growth and development in vitro.

Day 10 rat embryos (8-12 somites) cultured in vitro were exposed to elevated temperatures (41-43 degrees C) for varying lengths of time. A 15-minute exposure to a temperature of 43 degrees C (109.4 degrees F) was sufficient to produce malformed embryos when observed on day 11. Longer exposures at this temperature produce higher incidences of malformed embryos and also more severely affected embryos. Temperatures of 42 degrees C (107.6 degrees F) or 41 degrees C (105.8 degrees F) also produced malformed embryos, but the required length of exposure was increased compared to 43 degrees C. The minimal length of exposure at 42 degrees C was 60 minutes, while at 41 degrees C it was increased to 4 hours. The central nervous system was particularly sensitive to increased temperatures, and embryos exposed to "teratogenic doses" of hyperthermia exhibited primarily microcephaly and microphthalmia. In addition, histological analyses revealed that at 4.5 hours after a 30-minute exposure to 43 degrees C, necrotic debris was prevalent in the neuroepithelium, less prevalent in the surrounding mesenchyme and surface ectoderm, and absent in the tissues of the heart.