Microstructure and ultrastructure observation of eggs and sperms of Hucho bleekeri

【Objective】This study aimed to observe the normal morphological structure of eggs and sperms of Hucho bleekeri， compare differences in ultrastructure of sperms of varying motility， and reveal the intrinsic association between gamete ultrastructure damage and decreased motility in Hucho bleekeri， providing theoretical basis for optimizing its artificial breeding techniques.【Method】Sexually mature female and male Hucho bleekeri reared under artificial culture conditions were used as research subjects. The morphological structure of normal eggs and sperms in Hucho bleekeri was observed through histological analysis with scanning electron microscope and transmission electron microscope. Meanwhile， the differences in ultrastructure of sperms of varying motility were compared using the sperm motility detection system.【Result】In normal eggs of Hucho bleekeri， the nuclear membrane was dissolved， the nucleoli completely disappeared， and vacuoles distributed close to egg membrane. The egg membrane surface was smooth， the pores were uniform in size， the vestibular region of micropyle was flat， and the micropylar canal was relatively deep， belonging to type II. The structure of normal sperms in Hucho bleekeri was of flagellate type， consisting of three parts （head， midpiece， and tail）. The head was elliptical or nearly rounded， without acrosomal structure， and intranuclear space （nuclear vacuole） was visible within the nucleus， with granular or homogeneous chromatins. The midpiece （neck） was short， consisting mainly of the centriolar complex and sleeve. The tail （flagellum） was slender and smooth， with the flagellar axoneme exhibiting the typical “9+2” microtubule arrangement. Scanning electron microscope observations revealed that immotile Hucho bleekeri sperms displayed irregular head contours that were shrunken and collapsed， with an inconspicuous midpiece connecting structure， and the flagella were commonly broken and missing， unable to maintain normal morphology； highly motile sperms exhibited intact structure morphology， with plump ovoid heads， a tightly connected midpiece， slender flagella without obvious damage， and clear structures in all parts. Transmission electron microscope observations showed that in immotile Hucho bleekeri sperms， the mitochondrial structure was disrupted， the membranes were swollen， enlarged， and ruptured， and the internal electron density of mitochondria was decreased， with a wrinkled and uneven head surface， disrupted or disappeared nuclear membrane， and loose and even disintegrated chromatins； in highly motile sperms， the cytoplasm was dense， uniform， and compact， and the plasma membrane was intact with normal structure.【Conclusion】The oolemma of normal eggs in Hucho bleekeri is smooth， and the micropyle belongs to type II， with nucleoli completely disappeared and vacuoles distributed adjacent to egg membrane； normal sperms consist of three parts: the head， midpiece， and flagellum， exhibiting the typical acrosome-free structural characteristic of teleost fish， with the flagellar axoneme in a “9+2” microtubule arrangement. The structure integrity and maturity of the plasma membranes， nuclear membranes， mitochondria， and flagellar structure are directly associated with sperm motility in Hucho bleekeri， serving as key morphological indicators for evaluating semen quality and optimizing artificial breeding techniques.