Developmental Expression Pattern of Monoamine Oxidases in Sensory Organs and Neural Crest Derivatives TANIA VITALIS, 1 CHANTAL ALVAREZ, 2 KEVIN CHEN, 3 JEAN C. SHIH, 3 PATRICIA GASPAR, 2 AND OLIVIER CASES 2 * 1 Department of Anatomy and Developmental Biology, University College London, WC1E6BT London, United Kingdom 2 Institut National de la Sante ´ et de la Recherche Me ´dicale U106, Ba ˆ timent de Pe ´diatrie, Ho ˆpital de la Pitie ´-Salpe ˆtrie `re, 75013 Paris, France 3 Department of Pharmacology and Toxicology, Pharmaceutical Science Center, University of Southern California, Los Angeles, California 90033 ABSTRACT Serotonin (5-HT) has been shown to act as a morphogen in craniofacial and heart development and in the migration of neural crest derivatives. Some of these structures are capable of capturing 5-HT during development, but nothing is known about the localization of the main monoamine degradation enzymes, monoamine oxidase (MAO) A and B, in these developing tissues. We generated a highly specific antibody to MAOB; immunoreactivity is entirely abolished in brain extracts or brain sections of mice lacking MAOB. From the use of this antibody and specific riboprobes, we report that MAOB is expressed early in a variety of neural crest derivatives, in facial sensory organs, and in the heart. From E11.5 to P0, MAOB was found to be strongly expressed in the following neural crest derivatives: the aorta, cranial mesenchyme (developing bones, sensory neurons of the cranial ganglia, cartilages, thyroid, and striate muscles), dental mesenchyme, several soft palate derivatives, and boundary cap cells (E11.5–P4). Boundary cap cells contribute to the formation of nerve exit– entry points between the central and the peripheral nervous systems. Several facial sensory organs also contained MAOB mRNA, protein, and activity. High MAOB expression was noted in the olfactory placode, the dorsal part of the olfactory epithelium, the olfactory nerve layer (probably the ensheathing glia), the cochlear ganglionic cells, the taste buds, and the Merkel cells in the vibrissae follicles. Finally, we found that MAOB is massively expressed in the pharyngeal organ, heart, liver, and mast cells. In contrast, MAOA expresssion was restricted to the sympathetic ganglia and to the meningeal and capillary blood vessels. The pattern of MAOB expression generally matched the previously reported patterns of expression of the plasma 5-HT transporter expression or of the histamine biosynthetic enzyme L-histidine decarboxylase, suggesting a role for MAOB in fine regulation of the levels of 5-HT and histamine in the developing embryo. J. Comp. Neurol. 464:392– 403, 2003. © 2003 Wiley-Liss, Inc. Indexing terms: serotonin; histamine; plasma serotonin transporter; vesicular monoamine transporter; olfactory epithelium; cranial neural crest Grant sponsor: Institut National de la Sante ´ et de la Recherche Me ´di- cale; Grant sponsor: Wellcome Trust; Grant sponsor: National Institute of Mental Health; Grant number: R37 MH39085; Grant number: R01 MH37020; Grant sponsor: Boyd and Elsie Welin Professorship award (J.C.S.). *Correspondence to: Olivier Cases, INSERM U106, Ba ˆ timent de Pe ´dia- trie, Ho ˆpital de la Pitie ´-Salpe ˆtrie `re, 47 boulevard de l’ho ˆpital, 75651 Paris Cedex 13, France. E-mail: olicases@infobiogen.fr Received 7 January 2003; Revised 24 March 2003; Accepted 24 April 2003 DOI 10.1002/cne.10804 Published online the week of August 4, 2003 in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 464:392– 403 (2003) © 2003 WILEY-LISS, INC.