Histological study on maturation, fertilization and the state of gonadal region following spawning in the model sea anemone, Nematostella vectensis

PLoS One. 2017 Aug 10;12(8):e0182677. doi: 10.1371/journal.pone.0182677. eCollection 2017.

Abstract

The starlet sea-anemone Nematostella vectensis has emerged as a model organism in developmental biology. Still, our understanding of various biological features, including reproductive biology of this model species are in its infancy. Consequently, through histological sections, we study here key stages of the oogenesis (oocyte maturation/fertilization), as the state of the gonad region immediately after natural spawning. Germ cells develop in a secluded mesenterial gastrodermal zone, where the developing oocytes are surrounded by mucoid glandular cells and trophocytes (accessory cells). During vitellogenesis, the germinal vesicle in oocytes migrates towards the animal pole and the large polarized oocytes begin to mature, characterized by karyosphere formation. Then, the karyosphere breaks down, the chromosomes form the metaphase plate I and the eggs are extruded from the animal enclosed in a sticky, jelly-like mucoid mass, along with numerous nematosomes. Fertilization occurs externally at metaphase II via swimming sperm extruded by males during natural spawning. The polar bodies are ejected from the eggs and are situated within a narrow space between the egg's vitelline membrane and the adjacent edge of the jelly coat. The cortical reaction occurs only at the polar bodies' ejection site. Several spermatozoa can penetrate the same egg. Fertilization is accompanied by a strong ooplasmatic segregation. Immediately after spawning, the gonad region holds many previtellogenic and vitellogenic oocytes, though no oocytes with karyosphere. Above are the first histological descriptions for egg maturation, meiotic chromosome's status at fertilization, fertilization and the gonadal region's state following spawning, also documenting for the first time the ejection of the polar body.

MeSH terms

  • Animals
  • Female
  • Fertilization
  • Gonads / cytology
  • Male
  • Oocytes / physiology*
  • Oogenesis
  • Reproduction
  • Sea Anemones / anatomy & histology*
  • Sea Anemones / physiology
  • Sperm-Ovum Interactions

Grants and funding

This work was supported by the Ministry of National Infrastructures, Energy and Water Resources in Israel.