Abstract
Summary Knowledge of the morphological properties, species specific differences, development, and function of the sturgeon egg envelope is of importance for a better understanding of sturgeon reproduction as well as for improvement of the caviar quality. The structure of the egg envelope is similar among the various species, comprising an external envelope (thecal cells, basal lamina, and follicular epithelium), a five-layered oocyte envelope (adhesive layer, alveolar layer, epilayer, and zona radiata externa and interna) and a layer of oocyte matrix and cortical granules. Egg diameters (1.5–4.9 mm), structure and number of the micropyles (2–52), and the size of the micropylar field distinguish eggs of the various species. The development of the egg envelope within the ovary comprises five stages, with further changes following fertilization with the formation of a fertilization cone. The mechanism of polyspermy block in sturgeon eggs and the role of the micropyle and cortical granules in polyspermy block are also described in this review. This review provides information on the morphology and the developmental stages of the egg envelope that will further the understanding of egg biology of acipenserid species. Standardization of terminology describing the egg envelope would minimize confusion and be helpful for future work on acipenserid eggs.
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