1. Département de Physiologie, Université Catholique de Louvain, B‐1200 Bruxelles, Belgium
2. Institute of Toxicology, Faculty of Biology, University of Constance, D‐78457 Constance, Germany
3. Institute of Histology and General Embryology, University of Fribourg, CH‐1705 Fribourg, Switzerland
4. Roche Vitamin Division, Hoffman‐LaRoche, CH‐4070 Basel, Switzerland