Ultrastructure of the oocyte envelopes of some Eurasian acipenserids

The cellular and acellular envelopes of oocytes and follicles at the ultrastructural level are described for some acipenserids. The objective was to identify whether morphological features exist that can assist in discrimination of caviar products in trade originating from various species. The follicular envelopes include one thecal and one follicular cell layer separated by a basal lamina. The oocyte is surrounded by four acellular layers: (i) the internal zona radiata directly in contact with the oocyte plasma membrane; (ii) the external zona radiata; (iii) a layer of alveolar material; and (iv) a thin adhesive layer probably secreted by the follicular cells. These structures are similar in the eggs of genus Acipenser and Huso. Comparison of descriptions and definitions from different authors reveal a confusing terminology and sometimes different terms are used for similar structures. A terminology for the envelopes of Acipenseriform eggs is proposed. The authors conclude that morphological characters of sturgeon eggs are less valuable for discrimination of caviar.