Endostyle

Abstract: Thyroid-related functions in organisms devoid of follicular thyroid tissue have been reviewed. In the lamprey, a primitive vertebrate, the larva concentrates iodide and synthesizes thyroid hormones (TH) by iodoperoxidase (IP)-mediated iodination of a thyroglobulin (TG)-like molecule in a subpharyngeal afollicular endostyle. The endostyle is the thyroid homolog, and it reorganizes into a follicular thyroid at metamorphosis to the adult. Ascidians and amphloxus, invertebrate protochordate relatives of vertebrates, also concentrate iodide and synthesize TH in a subpharyngeal afollicular endostyle, but the endostyle never transforms to follicles. Ascidian plasma contains L-thyroxine and its more biologically active derivative 3,5,3'-triiodo-L-thyronine, and TH receptors exist, but TH effects are poorly understood. No other invertebrates possess an endostyle. Several invertebrates concentrate iodide at other sites and form protein-incorporated iodohistidines and iodotyrosines; however, de novo iodothyronine biosynthesis through IP-mediated TG iodination has not been established. Nevertheless, TH occur in invertebrates, and exogenous iodothyrosines or iodothyronines have effects on jellyfish, insects, and sea urchins. Furthermore, gut bacteria metabolize TH, and plants may synthesize TH by nonenzymatic oxidative iodination. Thus, TH occur in many organisms and, after ingestion and enteric absorption, can enter the food chain. Indeed, sea urchin larvae obtain TH required to induce metamorphosis from plant diatoms. Thyroid hormones can therefore have vitamin-like effects and, in conjunction with vitamin D, and possibly with other steroids, may be more aptly termed vitamones. Availability of exogenous TH has implications for models of invertebrate and vertebrate TH metabolism and iodine salvaging, and it may explain the prominent and probable ancestral role of peripheral mechanisms in regulating thyroidal status.

Abstract: Following the reading of its draft genome sequence and the collection of a large quantity of cDNA information, Ciona intestinalis is now becoming a model organism for whole-genome analyses of the expression and function of developmentally relevant genes. Although most studies have focused on larval structures, the development of the adult form is also very interesting in relation to tissues and organs of vertebrate body. Here we conducted detailed observations of the development of tissues and organs in Ciona intestinalis larva and juveniles until so-called the 2nd ascidian stage. These observations included examination of the oral siphon, tentacle, oral pigments and atrial pigments, atrial siphon, ganglion and neural gland, longitudinal muscle, stigmata, transverse bar and languet, longitudinal bar and papilla, heart, digestive organ, gonad, endostyle, and stalk and villi. The findings from these observations make a new staging system for juvenile development possible. Based on the development of the internal organs, we propose here nine stages (stage 0-stage 8) starting with swimming larvae and proceeding through juveniles until the 2nd ascidian stage. These descriptions and staging system provide a basis for studying cellular and molecular mechanisms underlying the development of adult organs and tissues of this basal chordate.

Abstract: Cell lineages during ascidian embryogenesis are invariant. Developmental fates of larval mesodermal cells after metamorphosis are also invariant with regard to cell type of descendants. The present study traced developmental fates of larval endodermal cells after metamorphosis in Halocynthia roretzi by labeling each endodermal precursor blastomere of larval endoderm. Larval endodermal cells gave rise to various endodermal organs of juveniles: endostyle, branchial sac, peribranchial epithelium, digestive organs, peripharyngeal band, and dorsal tubercle. The boundaries between clones descended from early blastomeres did not correspond to the boundaries between adult endodermal organs. Although there is a regular projection from cleavage stage and larval stage to juvenile stage, this varies to some extent between individuals. This indicates that ascidian development is not entirely deterministic. We composed a fate map of adult endodermal organs in larval endoderm based on a statistical analysis of many individual cases. Interestingly, the topographic position of each prospective region in the fate map was similar to that of the adult organ, indicating that marked rearrangement of the positions of endodermal cells does not occur during metamorphosis. These findings suggest that fate specification in endoderm cells during metamorphosis is likely to be a position-dependent rather than a deterministic and lineage-based process.

Abstract: The endostyle is a pharyngeal organ for the internal filter feeding of urochordates, cephalochordates, and larval lamprey. This organ is also considered to be homologous to the follicular thyroid gland of higher vertebrates. Thyroglobulin (Tg) and thyroid peroxidase (TPO) are specifically expressed in the thyroid gland of higher vertebrates, and they play an important role in iodine metabolism for the synthesis of thyroid hormones. Previous histochemical observations showed that iodine-concentrating and peroxidase activities were detected in zones 7, 8, and 9 of the ascidian endostyle, suggesting that these zones contains cells that are equivalent to those in the vertebrate follicular thyroid. In order to investigate the molecular developmental mechanisms involved in the formation and function of the endostyle, with special reference to the evolution of the thyroid gland, in the present study, we isolated and characterized cDNA clones for TPO genes, CiTPO from Ciona intestinalis and HrTPO from Halocynthia roretzi. Northern blot and in situ hybridization analyses revealed that the expression of the ascidian TPO genes was restricted to zone 7, one of the elements equivalent to the thyroid. These results provide the first evidence at the gene expression level for shared function between a part of the ascidian endostyle and the vertebrate follicular thyroid gland. J. Exp. Zool. ( Mol. Dev. Evol. ) 285:158–169, 1999. © 1999 Wiley-Liss, Inc.