Neurogenic and non‐neurogenic placodes in ascidians

Abstract

The late differentiation of the ectodermal layer is analysed in the ascidians Ciona intestinalis and Botryllus schlosseri, by means of light and electron microscopy, in order to verify the possible presence of placodal structures. Cranial placodes, ectodermal regions giving rise to nonepidermal cell types, are classically found exclusively in vertebrates; however, data are accumulating to demonstrate that the nonvertebrate chordates possess both the genetic machinery involved in placode differentiation, and ectodermal structures that are possible homologues of vertebrate placodes. Here, the term "placode" is used in a broad sense and defines thickenings of the ectodermal layer that can exhibit an interruption of the basal lamina where cells delaminate, and so are able to acquire a nonepidermal fate. A number of neurogenic placodes, ones capable of producing neurons, have been recognised; their derivatives have been analysed and their possible homologies with vertebrate placodes are discussed. In particular, the stomodeal placode may be considered a multiple placode, being composed of different sorts of placodes: part of it, which differentiates hair cells, is discussed as homologous to the octavo-lateralis placodes, while the remaining portion, giving rise to the ciliated duct of the neural gland, is considered homologous to the adenohypophyseal placode. The neurohypophyseal placode may include the homologues of the hypothalamus and vertebrate olfactory placode; the rostral placode, producing the sensorial papillae, may possibly be homologous to the placodes of the adhesive gland of vertebrates.

Retinoic acid signaling in development: Tissue‐specific functions and evolutionary origins

“…It is thus interesting to assess, whether RA signaling affects the development of these invertebrate tissues. Such tissues include, for example, the adhesive papillae (or palps) in the anteriormost general ectoderm of ascidians, which is thought to be homologous either to the vertebrate olfactory placode (Mazet et al, 2005) or to vertebrate cement and hatching glands (Manni et al, 2004). The vertebrate cement and hatching glands might not be placodes in a strict sense, because they do not share a common developmental origin with placodes (reviewed in Schlosser, 2005).…”

Ciona intestinalis: Chordate development made simple

“…Ascidians also appear to develop structures homologous to vertebrate placodes and their sensory organ derivatives (Manni et al, 2004). In the larva of H. roretzi, HrPax-258 is expressed in the atrial primordia, ectodermal thickenings that are precursors of the atrial siphon (Wada et al, 1998).…”

Formation of Nervous Systems and Neural Stem Cells in Ascidians

“…Recently, the notion about the formation of adult central nervous system in ascidians is greatly changed; earlier view is that the neural complex comprising the cerebral ganglion and the neural gland is formed from the neurohypophyseal duct positioned at left side of the cerebral ganglion (Wiley, 1893;Mackie and Burighel, 2005;Manni et al, 2004;Manni et al, 2005). However, Takamura (2002) using neuron-specific antibody and Horie et al (2011) using light-labeled fluorescent protein clearly revealed that the larval central nervous system contributes to the formation of the adult central nervous system.…”

“…Generation of peripheral neurons is a unique phenomenon that does not conform to the central nervous system origin of most neurons. The possible presence of a neurogenic placodal structure in invertebrate chordates has long been debated (Manni et al, 2004(Manni et al, , 2005(Manni et al, , 2006Mackie and Burighel, 2005;Mazet et al, , 2006Schlosser, 2005). Recently, it was directly shown that, using one of the biggest solitary ascidian Halocynthia roretzi (Fig.1), GnRH neurons are generated in the dorsal strand and migrate into the cerebral ganglion (Terakado, 2009).…”

“…Recent results have demonstrated certain morphological, developmental (Burighel et al, 1998;Manni et al, 2004;Terakado, 2009), and molecular (such as Six, Pitx, Eya, Pax, Coe, Dach, POUIV gene families) commonalities between urochordates and vertebrates (Bassham and Postlethwait, 2005;Boorman and Shimeld, 2002;Christiaen et al, 2002;Schlosser, 2005), but information on the localization of these transcription factors and on the sites of emergence of neuroendocrine/endocrine cells remains controversial.…”