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Introduction In studies of reproduction of marine organisms, the electron microscopy has allowed describing details of morphology and function of cells involved in different stages of gametogenesis, given its higher resolution power than conventional histology. In marine bivalves in particular, electron microscopy has helped characterizing ultrastructure of spermatogenesis in various species of economic value (Thielley et al., 1993; Ortíz-Ordoñez et al., 2003; Ortíz-Ordoñez et al., 2006), providing a wider and clearer perspective of the cellular dynamics of key processes involved in spermatogenesis. Such as synthesis and storage of energy reserves, differentiation of germ cells, development, ripening, spawning and lysis of gametes, and resorption of nutrients. In species such as lion’s paw scallop Nodipecten subnodosus, there is a paucity of information relative to ultrastructure of gametogenesis, despite this species is likely the largest member of the Pectinidae family, as well as an important and overexploited resource in northwestern Mexico. This study describes the ultrastructure of spermatogenesis of N. subnodosus, identifying type, number, and distribution of organs and structures involved in the formation of spermatozoa. Material and methodsThe scallops were obtained from a commercial culturing venture of lion’s paw scallop in Bahía Tortugas, Baja California Sur, Mexico. Samples of testis were fixated in 2.5% glutaraldehyde (pH 8) during 6 h, washed with sodium phosphate (2.5%), and afterward post-fixated with osmium tetroxide (1%) during 1 hr. Thereafter, samples were dehydrated in ascending ethanol series and passed through propylene oxide solutions twice. Samples were infiltrated in a 1:1 propylene oxide:epoxy resin solution, embedded in EPON 812, and then mounted in copper slides. Slides were contrasted with uranyl acetate and lead citrate to be observed under a Jeol-100SX TEM. Results Spermatogonia are the largest cells from the male germ linage (area = 38.9 µm² ±4.43), which have a large, spherical nucleus in central position and few heterochromatin granules of different size scattered throughout the nucleoplasm. Spermatogonia have few organelles within the cytoplasm, but abundant mitochondria. Primary spermatocytes are smaller (area = 15. 5 µm² ±1.01) than spermatogonia and show a larger nucleus in central position and a higher proportion of condensed heterochromatin (occupation = 55% ±4). This increase is related to the increase in synthesis of ribonucleic proteins coming from the cytoplasm which now yield a lower number of mitochondria with respect to spermatogonia. Secondary spermatocytes (area = 7.48 µm² ± 0.10) have a higher proportion of condensed heterochromatin (occupation = 68% ± 0.45) and a typical donut-like arrangement of the nucleus. This structure is smaller when compared with that of primary spermatocyte. Spermatids are characterized by having a spherical, central nucleus, which in this stage occupies almost entirely the cytoplasm (2.33 µm² ± 0.31), leaving 4 mitochondria which only that now start to migrate towards the basal pole of the nucleus and reduce their size (area = 0.09 µm² ± 0.01). In spermatids with a greater grade of differentiation, the cytoplasm remains as a small halo around the nucleus only. Additionally, mitochondria area located in the basal part and heterochromatin occupies almost entirely the nucleus (90% approximately). Spermatids go through the spermiogenesis process and develop into spermatozoa that migrate towards the lumen of seminal tubules. The head, neck, and tail (flagellum) are clearly visible. The head contains the nucleus having a spherical shape and a large proportion of granular, condensed heterochromatin occupying the totality of the matrix (area = 2.04 µm² ± 0.16). The acrosome is pyramidal, convex towards the apical part, and concave towards the nucleus, corresponding to previous reports of organisms having external fertilization (Franzen, 1955). The space between the acrosome and nucleus is occupied by a granular, dense sub-acrosome substance, which is consistent with the typical pattern of most marine bivalves (Suwanajat, 1999; Ortíz et al., 2003). The centriole is found among mitochondria and is constituted by a constant number of microtubules arranged in a central pair and nine peripheral pairs. The centriole forms the tail or flagellum, which also has the same arrangement of microtubules.

Palabras clave: Gametogenesis; Pectinidae; reproducción; Espermatozoides

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