Stomodeal and Neurohypophysial Placodes in Ciona Intestinalis: Insights into the Origin of the Pituitary Gland LUCIA MANNI*, ALBERTO AGNOLETTO, GIOVANNA ZANIOLO, and PAOLO BURIGHEL Dipartimento di Biologia, Universita`di Padova, via U. Bassi 58/B, I–35121 Padova, Italy ABSTRACT The ascidian larva has a central nervous system which shares basic characteristics with craniates, such as tripartite organisation and many developmental genes. One difference, at metamorphosis, is that this chordate-like nervous system regresses and the adult’s neural complex, composed of the cerebral ganglion and associated neural gland, forms. It is known that neural complex differentiation involves two ectodermal structures, the neurohypophysial duct, derived from the embryonic neural tube, and the stomodeum, i.e. the rudiment of the oral siphon; nevertheless, their precise role remains to be clarified. We have shown that in Ciona intestinalis, the neural complex primordium is the neurohypophysial duct, which in the early larva is a short tube, blind anteriorly, with its lumen in continuity with that of the central nervous system, i.e. the sensory vesicle. The tube grows forwards and fuses with the posterior wall of the stomodeum, a dorsal ectodermal invagination of the larva. The duct then loses posterior communication with the sensory vesicle and begins to grow on the roof of the vesicle itself. The neurohypophysial duct differentiates into the neural gland rudiment; its dorsal wall begins to proliferate neuroblasts, which migrate and converge to build up the cerebral ganglion. The most anterior part of the neural gland organizes into the ciliated duct and funnel, whereas the most posterior part elongates and gives rise to the dorsal strand. The hypothesis that the neurohypophysial duct/stomodeum complex possesses cell populations homologous to the craniate olfactory and adenohypophysial placodes and hypothalamus is discussed. J. Exp. Zool. 304B:324–339, 2005. r 2005 Wiley-Liss, Inc. INTRODUCTION Neurogenic placodes and neural crest are ectodermal cell types, which were classically considered to have evolved together and to be a characteristic of craniates (Northcutt and Gans, ’83). However, recent studies on protochordates (amphioxus and tunicates) have suggested that these structures arose by stages and that they arose before the divergence of tunicates from the stem chordate group. For amphioxus, considered the closest living relative to craniates, some findings suggest possi- ble homology to craniate placodes: it has been hypothesized that some epidermal sensory cells are homologues of the olfactory organs (which derive from the olfactory placode) and taste buds; moreover, the anterior ectoderm of the head of amphioxus expresses genes that are considered specific markers of the craniate olfactory epithe- lium (Holland and Holland, ’99; Lacalli et al., ’99; Satoh et al., 2002; Lacalli, 2004). In addition, it has been proposed that Hatschek’s pit, an ectodermal pocket of the dorsal stomodeum, is homologous to the adenohypophysis in craniates (deriving from the adenohypophysial placode), on the basis of developmental and molecular data (Nozaki, ’92; Candiani and Pestarino, ’98; Holland and Holland, 2001; Boorman and Shimeld, 2002; Holland and Yu, 2002; Kawamura et al., 2002; Satoh et al., 2002). Ascidians, the main group of tunicates, possess tripartite organization of the larval brain and, in individual territories along the neuraxis, express orthologues of developmental genes in patterns which closely resemble those that regionalize the vertebrate brain (Wada et al., ’98; Satou et al., 2003). However, after metamorphosis, this Grant support: Ministero della Universita` e Ricerca Scientifica e Tecnologica and the University of Padova to L.M. and P.B. *Correspondence to: Lucia Manni, Dipartimento di Biologia, Universita ` di Padova, via U. Bassi 58/B, I–35121 Padova, Italy. E-mail: lucia.manni@unipd.it Received 13 January 2005; Accepted 4 February 2005 Published online 10 May 2005 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jez.21039. r 2005 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 304B:324–339 (2005)