Tunicate Embryos and Cell Specification


Tunicates are marine invertebrate chordates and closest relatives of vertebrates. The most common tunicates are ascidians. The fertilised egg of ascidians develops quickly into a tadpole‐type larva. The larva is composed of approximately 2600 cells and has distinct organs including the nervous system, endoderm and mesenchyme in the trunk, and muscle and notochord in the tail. The larval surface is covered by an epidermis. This configuration represents a basic body plan of chordates. Every blastomere of early embryos up to the gastrula stage is distinguishable, and invariant lineage of embryonic cells is well documented. Cell fate restriction occurs relatively early in ascidian embryos. Maternal factors are responsible for differentiation of muscle, endoderm and epidermis, whereas the notochord and nervous system are specified by cellular interactions. Genes that are involved in the specification of embryonic cells have been identified. The tunicate is an emerging model for studies of embryonic cell specification.

Key Concepts:

  • In ascidians, invariant lineage of embryonic cells is completely described up to the gastrula stage.

  • Developmental fates of most embryonic cells are determined during the blastula formation.

  • Maternal factors are responsible for differentiation of muscle, endoderm and epidermis.

  • The notochord and nervous system are specified by cellular interactions.

  • Genes that are responsible for the specification of embryonic cells have been identified.

  • The tunicate is an emerging model for studies of embryonic cell specification.

Keywords: tunicates; ascidians; tadpole larvae; invariant lineage; developmental fate restriction; embryonic cell specification; maternal factors; cell–cell interaction; transcription factors; signalling molecules

Figure 1.

The tunicate ascidian Ciona intestinalis. (a) Adults with incurrent and excurrent siphons. The white duct is the sperm duct and the orange duct paralleling it is the egg duct. (b–l) Embryogenesis. Embryos were dechorionated to show their outer morphology clearly. (b) Fertilised egg, (c) 2‐cell embryo, (d) 4‐cell embryo, (e) 16‐cell embryo, (f) 32‐cell embryo, (g) gastrula (approximately 150 cells), (h) neurula, (i–k) tailbud embryos, and (l) tadpole larva. (m) A juvenile a few days after metamorphosis, showing the internal structures: ds, digestive system; en, endostyle: ht, heart; os, neuronal complex; and pg, pharyngeal gill.

Figure 2.

A phylogenetic tree to show the position of tunicates (urochordates) among dueterostomes and chordates. Species names of several ascidians are also shown with phylogenetic position among tunicates.

Figure 3.

Distribution of cytoplasmic determinants for muscle (left), endoderm (middle), and epidermis (left) in unfertilised eggs (top), during the first and second phases of ooplasmic segregation (middle two) and 8‐cell embryos (bottom). Coloured areas represent location of cytoplasmic determinants. Animal pole is up and vegetal pole is down, which is distinguishable by the position of the polar body. Anterior is to the left and posterior is to the right. Reproduced from Nishida, .

Figure 4.

Cleavage, cell lineage, and gradual restriction of developmental fates of ascidian embryonic cells. Blastomeres are named according to Conklin's nomenclature and coloured when the developmental fate is restricted to give rise to a single type of tissue. (a) Fate restriction and determination. From top to down. An 8‐cell embryo, lateral view. Animal pole is up and vegetal pole is down. Anterior is to the left and posterior is to the right. A 16‐cell embryo, viewed from animal (left) and vegetal pole (right). Anterior is up and posterior is down. A 32‐cell embryo, animal (left) and vegetal (right) views, respectively. A 64‐cell embryo, animal (left) and vegetal (right) views, respectively. A 110‐cell embryo, animal (right) and vegetal (left) views, respectively. (b) Fate map of the 110‐cell embryo, animal hemisphere (upper) and vegetal hemisphere (lower). (c) Schematic drawing showing tissues and organs of the tailbud embryo. Midsagittal section (left) and sagittal section (middle) of the embryo, and transverse section of the tail (right). TLC, trunk lateral cells. TVC, trunk ventral cells. Reproduced from Nishida, .

Figure 5.

Cell lineage and segregation of developmental fates in ascidian embryos. Because the lineage is bilaterally symmetrical, only the left half of the embryo is shown. When developmental fate of a certain blastomere is restricted to give rise to one type of tissue, further division of the blastomere is abbreviated.



Bertrand V, Hudson C, Caillol D, Popovici C and Lemaire P (2003) Neural tissue in ascidian embryos is induced by FGF9/16/20, acting via a combination of maternal GATA and Ets transcription factors. Cell 115: 615–627.

Chabry L (1887) Contribution a l'embryologie normale et teratogique des Ascidies simples. Journal of Anatomical Physiology (Paris) 23: 167–319.

Cloney L (1978) Ascidian metamophosis; review and analysis. In: Chia F‐S and Rice ME (eds) Settlement and Metamorphosis of Marine Invertebrate Larvae, pp. 255–282. Amsterdam: Elsevier.

Conklin EG (1905a) The organization and cell lineage of the ascidian egg. Journal of Academy of National Science (Philadelphia) 13: 1–119.

Conklin EG (1905b) Organ forming substances in the eggs of ascidians. Biological Bulletin 8: 205–230.

Dehal P, Satou Y, Campbell RK et al. (2002) The draft genome of Ciona intestinalis: insights into chordate and vertebrate origins. Science 298: 2157–2167.

Driesch H (1891) Entwicklungsmechanische Studien. I. Der Werth der beiden ersten Furchungszellen in der Echinodermenentwicklung. Experimentelle Erzeugung von Thielund Doppelbildungen. Zeitschrift fur Weiss Zoologie 53: 160–178.

Gyoja F, Satou Y, Shin‐i T et al. (2007) Analysis of large scale expression sequenced tags (ESTs) from the anural ascidian, Molgula tectiformis. Developmental Biology 307: 460–482.

Hotta K, Takahashi H, Satoh N and Gojobori T (2008) Brachyury‐downstream gene sets in a chordate, Ciona intestinalis: integrating notochord specification, morphogenesis and chordate evolution. Evolution and Development 10: 37–51.

Imai K, Takada N, Satoh N and Satou Y (2000) β‐Catenin mediates the specification of endoderm cells in ascidian embryos. Development 127: 3009–3020.

Imai KS, Levine M, Satoh N and Satou Y (2006) Regulatory blueprint for a chordate embryo. Science 312: 1183–1187.

Imai KS, Satoh N and Satou Y (2002a) Early embryonic expression of FGF4/6/9 gene and its role in the induction of mesenchyme and notochord in Ciona savignyi embryos. Development 129: 1729–1738.

Imai KS, Satoh N and Satou Y (2002b) An essential role of a FoxD gene in notochord induction in Ciona embryos. Development 129: 3441–3453.

Jeffery WR and Swalla BJ (1992) Evolution of alternate modes of development in ascidans. Bioessays 14: 219–226.

Jiang D and Smith WC (2007) Ascidian notochord morphogenesis. Developmental Dynamics 236: 1748–1757.

Kowalevsky A (1866) Entwicklungsgeshichte der einfachen Ascidien. Memory l'Acadmy St. Petersbourg, Serires 7: 101–119.

Kumano G and Nishida H (2007) Ascidian embryonic development: an emerging model system for the study of cell fate specification in chordates. Developmental Dynamics 236: 1732–1746.

Lemaire P (2009) Unfolding a chordate developmental program, one cell at a time: invariant cell lineages, short‐range inductions and evolutionary plasticity in ascidians. Developmental Biology 332: 48–60.

Lemaire P, Smith WC and Nishida H (2008) Ascidians and plasticity of the chordate developmental program. Current Biology 18: R620–R631.

Marikawa Y, Yoshida S and Satoh N (1995) Muscle determinants in the ascidian egg are inactivated by UV irradiation and the inactivation is partially rescued by injection of maternal mRNAs. Roux's Archiv Developmental Biology 204: 180–186.

Meinertzhagen IA and Okamura Y (2001) The larval ascidian nervous system: the chordate brain from its small beginnings. Trends in Neurosciences 24: 401–410.

Munro EM, Robin F and Lemaire P (2006) Cellular morphogenesis in ascidians: how to shape a simple tadpole. Current Opinion in Genetics & Development 16: 399–405.

Nakatani Y and Nishida H (1994) Induction of notochord during ascidian embryogenesis. Developmental Biology 166: 289–299.

Nakatani Y, Yasuo H, Satoh N and Nishida H (1996) Basic fibroblast growth factor induces notochord formation and the expression of As‐T, a Brachyury homolog, during ascidian embryogenesis. Development 122: 2023–2031.

Nishida H (1987) Cell lineage analysis in ascidian embryos by intracellular injection of a tracer enzyme. III. Up to the tissue restricted stage. Developmental Biology 121: 526–541.

Nishida H (2002) Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules. International Review of Cytology 217: 227–276.

Nishida H (2005) Specification of embryonic axis and mosaic development in ascidians. Developmental Dynamics 233: 1177–1193.

Nishida H and Sawada K (2001) macho‐1 encodes a localized mRNA in ascidian eggs that specifies muscle fate during embryogenesis. Nature 409: 724–729.

Passamaneck YJ and Di Gregorio A (2005) Ciona intestinalis: chordate development made simple. Developmental Dynamics 233: 1–19.

Pordon F, Yamada L, Shirae‐Kurabayashi M, Nakamura Y and Sasakura Y (2007) Postoplasmic/PEM RNAs: a class of localized maternal mRNAs with multiple roles in cell polarity and development of ascidian embryo. Developmental Dynamics 236: 1698–1715.

Putnam NH, Butts T, Ferrier DE et al. (2008) The amphioxus genome and the evolution of the chordate karyotype. Nature 453: 1064–1071.

Reverberi G (1971) Ascidians. In: Reverberi G (ed.) Experimental Embryology of Marine and Fresh‐water Invertebrates, pp. 507–550. Amsterdam: North Holland.

Roux W (1888) Beitrage zur Entwickelungsmechanik des Embryo. Archiv Pathological and Anatomical Physiology 114: 113–153.

Sardet C, Paix A, Pordon F, Dru P and Chenevert J (2007) From oocytes to 16‐cell stage: cytoplasmic cortical reorganizations that pattern the ascidian embryo. Developmental Dynamics 236: 1716–1731.

Sasakura Y, Nakashima K, Awazu S et al. (2005) Trnsposon‐mediated insertional mutagenesis revealed the functions of animal cellulose synthase in the ascidian Ciona intestinalis. Proceedings of National Academy Science, USA 102: 15134–15139.

Satoh N (1994) Developmental Biology of Ascidians. Cambridge: Cambridge University Press.

Satoh N (2003) The ascidian tadpole larva: comparative molecular development and genomics. Nature Review of Genetics 4: 285–295.

Satoh N, Satou Y, Davidson B and Levine M (2003) Ciona intestinalis: an emerging model for whole‐genome analyses. Trends in Genetics 19: 376–381.

Satou Y, Imai KS and Satoh N (2001) Early embryonic expression of a LIM‐homeobox gene Cs‐lhx3 is downstream of β‐catenin and responsible for the endoderm differentiation in Ciona savignyi embryos. Development 128: 3559–3570.

Satou Y, Kawashima T, Shoguchi E, Nakayama A and Satoh N (2005) An integrated database of the ascidian, Ciona intestinalis: towards functional genomics. Zoological Science 22: 837–843.

Swalla BJ, Makabe KW, Satoh N and Jeffery WR (1993) Novel genes expressed differentially in ascidians with alternate modes of development. Development 119: 307–318.

Swalla BJ and Smith AB (2008) Deciphering deuterostome phylogeny: molecular, morphological, and paleontogical perspectives. Philosophical Transactions of Royal Society B 363: 155–1568.

Takahashi H, Hotta K, Erives A et al. (1999) Brachyury downstream notochord differentiation in the ascidian embryo. Genes and Development 13: 1519–1523.

Van Beneden E and Julin CH (1884) La segmentation chez les ascidens dans ses rapports avec l'organization de la larve. Archiv Biology 5: 111–126.

Whittaker JR (1973) Segregation during ascidian embryogenesis of egg cytoplasmic information for tissue‐specific enzyme development. Proceedings of National Academy Science, USA 70: 2096–2100.

Yagi K, Takatori N, Satou Y and Satoh N (2005) Ci‐Tbx6b and Ci‐Tbx6c are key mediators of the maternal effect gene Ci‐macho1 in muscle cell differentiation in Ciona intestinalis embryos. Developmental Biology 282: 535–549.

Yasuo H and Satoh N (1993) Function of vertebrate T gene. Nature 364: 582–583.

Yoshida S, Marikawa Y and Satoh N (1996) posterior end mark, a novel maternal gene encoding a localized factor in the ascidian embryo. Development 122: 2005–2012.

Further Reading

Jeffery WR and Swalla BJ (1997) Tunicates. In: Gilbert SF and Raunio AM (eds) Embryology, pp. 331–364. Sunderland, MA: Sinauer.

Meinertzhagen IA, Lemaire P and Okamura Y (2004) The neurobiology of the asicdian tadpole larva: recent development in an ancient chordate. Annual Review of Neurosciences 27: 453–458.

Sasakura Y (2007) Germiline transgenesis and insertional mutagenesis in the ascidian Ciona intestinalis. Developmental Dynamics 236: 1758–1767.

Satoh N (2008) An aboral‐dorsalization hypothesis for chordate origin. Genesis 46: 614–622.

Satou Y, Satoh N and Imai KS (2009) Gene regulatory networks in the early ascidian embryo. Biochimica et Biophysica Acta 1789: 268–273.

Shoguchi E, Hamaguchi M and Satoh N (2008) Genome‐wide network of regulatory genes for construction of a chordate embryo. Developmental Biology 316: 498–509.

Tassy O, Dauga D, Daian F et al. (2010) The ANISEED database: Digital representation, formalization, and elucidation of a chordate developmental program. Genome Research 20: 1459–1468.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Satoh, Noriyuki(May 2011) Tunicate Embryos and Cell Specification. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001514.pub2]