Ultrastructural Studies on Oocyte Differentiation and Vitellogenesis in the Oocytes of Female Kareius bicoloratus in Western Korea

Abstract

Ultrastructural studies on oocyte differentiation and vitellogenesis in the oocytes of female Kareius bicoloratus were investigated by transmission electron microscopy. The Golgi complex in the cytoplasm is involved in the formation of yolk vesicles that contain yolk carbohydrates in the yolk vesicle of oocytes in the early vitellogenic phase. In this phase, many pinocytotic vesicles (PVs), which are formed by pinocytosis, contain yolk precursors (exogenous substances). These substances are associated with exogenous heterosynthetic vitellogenesis. In yolked oocytes in the late vitellogenic phase, two morphologically different bodies, which formed by modified mitochondria, appear in oocytes. One is a multivesicular body (synthesized by autosynthetic vitellogenesis), and the other is a yolk precursor (an exogenous substance formed by heterosynthetic vitellogenesis). The multivesicular bodies (MVB) are taken into the yolk precursors (YP) and are transformed into primary yolk globules. However, after the YP mix with exogenous PVs near the zona pellucida, they are transformed into primary yolk globules. Vitellogenesis of this species occurs via endogenous autosynthesis and exogenous heterogenesis. Vitellogenesis occurs through endogenous autosynthesis, which involves the combined activity of the Golgi complex, mitochondria and MVB formed by modified mitochondria. However, heterosynthesis involves pinocytotic incorporation of extraovarian precursors (such as vitellogenin in the liver) into the zona pellucida (via granulosa cells and thecal cells) of the yolked oocyte.

Ultrastructural changes in mitochondria during oogenesis in two phylogenetically close fish species

“…Differences in the structure of mitochondria between primary and secondary growth oocytes have been observed in many species, such as the guppy, Lebistes reticulatus, where early oocyte mitochondria were large and contain tubular and shelf-like cristae, whereas after the onset of secondary growth oocytes, mitochondria became thinner in diameter and increased in number, while tubular cristae disappeared (Droller & Roth, 1966). In some species, modified mitochondria appeared during late vitellogenesis and participate in the formation of multivesicular bodies involved in the formation of yolk granule precursors (Chung et al, 2009;Jun et al, 2018;Kunz, 2004;Pfannestiel & Grünig, 1982). In sturgeon and paddlefish, mitochondria with various degrees of cristae distortion were assumed to play a role in the formation of lipid droplets in previtellogenic oocytes ( _ Zelazowska & Fopp-Bayat, 2019;_ Zelazowska & Kilarski, 2009).…”

“…Both the Golgi apparatus and endoplasmic reticulum are involved in the development of yolk generating vacuoles, which contributes to the expansion of yolk globule size (Anderson, 1968). Furthermore, some studies have shown that mitochondria and multivesicular structures play a role in the production of yolk granule precursors (Chung et al, 2009;Jun et al, 2018). According to the observations in sardine, the close relation of endoplasmic reticulum, and the electrontranslucent yolk bodies (YB1), as well as the absence of YB1, when more advanced electron-dense yolk bodies (YB2) and yolk granules develop in the cytoplasm, and also the presence of mitochondria with distorted cristae during the appearance of crystalline content in the yolk granules, lead to the assumption that mitochondria are involved in yolk granule formation.…”

“…Specifically, oocytes absorb yolk components through pore canals of the thickened zona radiata, which includes long microvilli and numerous actin filament bundles that provide a more advanced level of controlled mobility (Kjesbu et al, 1996;Kobayashi, 1985;Mooseker, 1985). Although the number of microvilli gradually increases during vitellogenesis, in mature oocytes the microvilli disappear and the pits of zona radiata close, preventing the entrance of nutrients (Chung et al, 2009;Jun et al, 2018;Kjesbu et al, 1996).…”

“…Nevertheless, the role of organelles in yolk formation is not universal across species. For example, although it was indicated that Golgi is involved in yolk synthesis, in some species mitochondria with altered cristae are assumed to take part as well (Anderson, 1968;Chung et al, 2009;Jun et al, 2018).…”