Tunicate Embryos and Cell Specification

Abstract

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.

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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]