Additional Author(s): Aoba Hosono
Published on SDB CoRe: Jun 29 2012
Tools & Techniques: Microscopy
Early Embryogenesis: Cleavage; Blastula
Morphogenic Movements: Gastrulation; Neurulation
Morphogenesis: Cell Movements
Ectoderm-derived: Nervous System
Stage of Development: Embryo
Ascidians (sea squirts) are invertebrate chordates (subphylum Urochordata or Tunicata, class Ascidiacea). Ascidian eggs develop into tadpole larvae through a relatively simple manner of development with a small number of constituent cells. Halocynthia roretzi embryos are shown at various stages from a fertilized egg to a hatched larva (35 hours of development). From the upper left, 1st row: fertilized egg, 4-cell, 16-cell; 2nd row: 64-cell, 110-cell, gastrula; 3rd row: neural plate stage, neurula with dorsal neural tube closing, initial tailbud; 4th row: middle tailbud, larvae just before hatch; and 5th row: hatched larva (anterior to the left). The larvae represent the basic body plan of chordates, having a dorsal neural tube, notochord flanked by bilateral muscle, and a brain with two sensory pigment cells (anterior end of larva). The notochord, a characteristic feature of chordata, is visible in the tail of the larva, consisting of exactly 40 cells in a single line. The total number of cells of the hatched larva is approximately 3000. Embryogenesis is invariant and cell lineages are well described. Embryos develop within the vitelline membrane. The egg diameter is 280 µm and the larva is 1.5 mm long.
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Lemaire, P., Smith, W.C., and Nishida, H. Ascidians and the plasticity of the chordate developmental program. Curr. Biol., 2008, 18:R620–R631.