NBRPホヤ論文オンラインセミナー2026年10月：著者による論文（ https://doi.org/10.1016/j.ydbio.2026.05.002）解説： 齋藤 貴子 (静岡大学)

This seminar was given by Dr. Takako Saito from Shizuoka University, presenting her paper on pre-fertilization egg coat dynamics in the ascidian Phallusia philippinensis. The speaker first explained the general structure of egg coats, which are extracellular matrices covering eggs, and noted that changes in egg coat structure after fertilization are known in many species, but pre-fertilization changes are less understood. In this species, the egg is stored in the oviduct at low pH (5.7) and is released into seawater at pH 8.2. The team observed that the vitelline coat (VC) rises and expands when the egg is placed in seawater, and this elevation is pH-dependent, with faster elevation at higher pH. Quantitative measurements showed the VC diameter increases over time, reaching a maximum about 60 minutes after exposure. They also tested whether the elevation requires fertilization and found that fertilization does not further elevate the already risen coat. Using fluorescent labels and electron microscopy, they found that the folded VC structure unfolds and expands, while the coat thickness remains unchanged, suggesting a structural reorganization. They then investigated the role of osmotic pressure, showing that adding high-molecular-weight molecules (e.g., BSA) inhibits elevation, and that the coat is semi-permeable: low-molecular-weight dyes can pass, but large molecules cannot. This suggests water influx driven by osmotic gradient is a driving force. Finally, they asked whether VC elevation is necessary for fertilization and development. Using PEGs of different molecular weights to control elevation, they found that fertilization still occurs without elevation, but development is affected: embryos with suppressed elevation showed a high proportion of abnormal larvae with bent tails. They also tested protease inhibitors and found that a broad inhibitor cocktail and leupeptin inhibited elevation, suggesting protease involvement, though the specific proteases remain unidentified. The speaker proposed three possible models (swelling, osmotic, and egg coat softening) that may act together. The seminar concluded with a discussion of future work needed to clarify the mechanisms.