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.
Highlights
It is evident that this sea anemone has no true ovaries [5], whereas a “gonad” in N. vectensis is the determined gametogenic area of the mesentery, which is located between the retractor muscles and the mesenterial filaments (Fig 1B)
In this study key stages of the oogenesis of Nematostella vectensis and the state of the gametogenic region at the mesentery immediately after spawning are studied using light microscopy with different staining approaches. These stages of oogenesis are described for the first time, as are the identification of the chromosomal status at the moment of fertilization and the documentation of the polar bodies. These findings supplement the information currently available regarding the reproductive biology of N. vectensis, a model species used in studies on metazoan development
N. vectensis has no true ovary and its sex cells develop in a gametogenic region of the mesentery [5], like other anthozoans [41, 42]
Summary
Histological characteristics of maturation and fertilization in Nematostella and the southeastern coast of England [1,2,3]). N. vectensis has emerged as a model organism in developmental biology and metazoan evolution [4,5,6,7,8,9], as well as in ecological and toxicological studies [10,11,12]. Our understanding of various biological features, including reproductive biology characteristics of this powerful model species are in its infancy. Under different combinations of photoperiods, temperatures and diets it may spawn continuously throughout the year, forming mature eggs every 7–8 days [29]. Oocytes, which develop and mature in the mesenteries, are extruded through the mouth while immersed in a jelly coat as a part of the egg masses [5, 28, 29]
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