1. Agricultural Research Service (T.H.E., S.K., C.-J.L., W.M.G.), U.S. Department of Agriculture, Beltsville, Maryland 20705
2. Department of Anatomy, Physiology, and Pharmacology, College of Veterinary Medicine (J.L.S.), Auburn University, Auburn, Alabama 36849
3. Department of Neuroanatomy and Cellular Biology (J.R.), Cajal Institute, E-28002 Madrid, Spain