Chicken Lateral Septal Organ and Other Circumventricular Organs Form in a Striatal Subdomain Abutting the Molecular Striatopallidal Border S.M. BARDET, 1 I. COBOS, 2 E. PUELLES, 3 M. MARTI ´ NEZ-DE-LA-TORRE, 1 AND L. PUELLES 1 * 1 Department of Human Anatomy and Psychobiology, Medical School, University of Murcia, Murcia E30071, Spain 2 Nina Ireland Laboratory of Developmental Neurobiology, Center for Neurobiology and Psychiatry, University of California San Francisco, San Francisco, California 94143-2611 3 Institute of Neuroscience of Alicante, UMH-CSIC, Unidad de Neurobiologı ´a del Desarrollo; Universidad Miguel Herna ´ ndez, Campus de San Juan, San Juan E03550, Alicante, Spain ABSTRACT The avian lateral septal organ (LSO) is a telencephalic circumventricular specialization with liquor-contacting neurons (Kuenzel and van Tienhoven [1982] J. Comp. Neurol. 206:293–313). We studied the topological position of the chicken LSO relative to molecular borders defined previously within the telencephalic subpallium (Puelles et al. [2000] J. Comp. Neurol. 424:409 – 438). Differential expression of Dlx5 and Nkx2.1 homeobox genes, or the Shh gene encoding a secreted morphogen, allows distinction of striatal, pallidal, and preoptic subpallial sectors. The chicken LSO complex was characterized chemoarchitectonically from embryonic to posthatching stages, by using immunohistochemistry for calbindin, tyrosine hydroxylase, NKX2.1, and BEN proteins and in situ hybridization for Nkx2.1, Nkx2.2, Nkx6.1, Shh, and Dlx5 mRNA. Medial and lateral parts of LSO appear, respectively, at the striatal part of the septum and adjacent bottom of the lateral ventricle (accumbens), in lateral continuity with another circumventricular organ that forms along a thin subregion of the entire striatum, abutting the molecular striatopallidal boundary; we called this the “striatopallidal organ” (SPO). The SPO displays associated distal periventricular cells, which are lacking in the LSO. Moreover, the SPO is continuous caudome- dially with a thin, linear ependymal specialization found around the extended amygdala and preoptic areas. This differs from SPO and LSO in some molecular aspects. We tentatively identified this structure as being composed of an “extended amygdala organ” (EAO) and a “preoptohypothalamic organ” (PHO). The position of LSO, SPO, EAO, and PHO within a linear Dlx5-expressing ventricular domain that surrounds the Nkx2.1-expressing pallidopreoptic do- main provides an unexpected insight into possible common and differential causal mechanisms underlying their formation. J. Comp. Neurol. 499:745–767, 2006. © 2006 Wiley-Liss, Inc. Indexing terms: preoptic area; septum; subpallium; calbindin; BEN protein; Nkx2.2; Nkx6.1; Shh; Dlx5; Nkx2.1 This report aims to correlate the topography of some forebrain circumventricular organs, including the conven- tional lateral septal organ (LSO; Kuenzel and van Tien- hoven, 1982), relative to molecularly characteristic re- gional boundaries within the avian subpallium (Puelles et al., 2000; Cobos et al., 2001a). Some developmental genes are known to be expressed conservatively in the brain of vertebrates within characteristic ventricular or mantle zone domains (Puelles and Rubenstein, 1993, 2003; Puelles et al., 2000, 2004). Published functional analyses of the same genes in mice suggest that they contribute in various ways to brain regionalization, that is, to progressive, Grant sponsor: Spanish Ministry of Education and Science (MEC; to S.B.); Grant number: PB2002-3668 (to L.P.); Grant number: BFU2005- 09378-C02-01/BFI (to L.P.); Grant sponsor: MEC and Alicante Institute of Neuroscience (UMH-CSIC; to E.P.). *Correspondence to: L. Puelles, Department of Human Anatomy and Psychobiology, Medical School, University of Murcia, Murcia E30071, Spain. E-mail: puelles@um.es Received 7 March 2006; Revised 27 April 2006; Accepted 14 June 2006 DOI 10.1002/cne.21121 Published online in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 499:745–767 (2006) © 2006 WILEY-LISS, INC.