Glycosylation of the male genital ducts and spermatozeugmata formation in the clearnose skate Raja eglanteria

Progesterone receptors in extratesticular ducts of the Amazonian stingray Potamotrygon wallacei: A potential role in sperm maturation and aggregate formation

In cururu stingray (Potamotrygon wallacei Carvalho, Rosa and Araújo 2016) males, plasma progesterone (P4) levels appear to be associated with spermiation events. However, the specific contribution of P4 in sperm maturation via extratesticular ducts in this stingray species is unknown. With the aim of filling this knowledge gap, this study examined the morphology and the presence of progesterone receptors (PR) in the ducts, and analyzed the relationship of progesterone (P4) with sperm maturation and formation of aggregates. Morphological analysis showed that a columnar pseudostratified epithelium with stereocilia lined all the attached ducts. In active males, the secretory cells of the epididymis and the Leydig glands presented PR; however, these receptors were not found in the distal region of the epididymis (essential for nurturing and capacitation events) of regressing males. In the seminal vesicles of active males, the spermatozoa are parallelly aligned and embedded in a matrix to form the spermatozeugmata. The matrixes are formed by proteins secreted by the ducts and Sertoli cell cytoplasts. These structures presented PR, which suggests that P4 engages in sperm metabolism during storage. Our findings allude to the potential role of P4 in regulating the development and function of the attached ducts in different reproductive phases. Furthermore, P4 seems to be an essential component for regulating sperm progress, protein secretion, aggregate formation, and maintenance of sperm during storage in this freshwater stingray.

Sialylation: fate decision of mammalian sperm development, fertilization, and male fertility†

Sperm development, maturation, and successful fertilization within the female reproductive tract are intricate and orderly processes that involve protein translation and post-translational modifications. Among these modifications, sialylation plays a crucial role. Any disruptions occurring throughout the sperm's life cycle can result in male infertility, yet our current understanding of this process remains limited. Conventional semen analysis often fails to diagnose some infertility cases associated with sperm sialylation, emphasizing the need to comprehend and investigate the characteristics of sperm sialylation. This review reanalyzes the significance of sialylation in sperm development and fertilization and evaluates the impact of sialylation damage on male fertility under pathological conditions. Sialylation serves a vital role in the life journey of sperm, providing a negatively charged glycocalyx and enriching the molecular structure of the sperm surface, which is beneficial to sperm reversible recognition and immune interaction. These characteristics are particularly crucial during sperm maturation and fertilization within the female reproductive tract. Moreover, enhancing the understanding of the mechanism underlying sperm sialylation can promote the development of relevant clinical indicators for infertility detection and treatment.

The role of alkalinization-induced Ca2+ influx in sperm motility activation of a viviparous fish Redtail Splitfin (Xenotoca eiseni)

Mechanisms regulating sperm motility activation are generally known in oviparous fishes, but are poorly understood in viviparous species. The mechanism of osmotic-shock induced signaling for oviparous fishes is not suitable for viviparous fishes which activate sperm motility within an isotonic environment. In addition, the presence of sperm bundles in viviparous fishes further complicates study of sperm activation mechanisms. The goal of this study was to establish methodologies to detect intracellular Ca2+ signals from sperm cells within bundles, and to investigate the signaling mechanism of sperm activation of viviparous fish using Redtail Splitfin (Xenotoca eiseni) as a model. Motility was assessed by classification of bundle dissociation and computer-assisted sperm analysis, and intracellular Ca2+ was assessed using the fluorescent probe Fura-2 AM. Bundle dissociation and sperm motility increased with extracellular Ca2+ and pH levels. Intracellular Ca2+ signals were detected from sperm within bundles, and increased significantly with extracellular Ca2+ and pH levels. Major channel blockers known to inhibit Ca2+ influx (NiCl2, ruthenium red, GdCl3, SKF-96365, nimodipine, verapamil, methoxyverapamil, mibefradil, NNC 55-0396, ω-Conotoxin MVIIC, bepridil, and 2-APB) failed to inhibit Ca2+ influx, except for CdCl2, which partially inhibited the influx. We propose a novel mechanism for motility regulation of fish sperm: an alkaline environment in the female reproductive tract opens Ca2+ channels in the sperm plasma membrane without osmotic shock, and the Ca2+ influx functions as a second messenger to activate motor proteins controlling flagella movement.

Development of germplasm repositories to assist conservation of endangered fishes: Examples from small-bodied livebearing fishes

Germplasm repositories are a necessary tool for comprehensive conservation programs to fully preserve valuable genetic resources of imperiled animals. Cryopreserved germplasm can be used in the future to produce live young for integration into other conservation projects, such as habitat restoration, captive breeding, and translocations; thus compensating for genetic losses or negative changes that would otherwise be permanent. Although hundreds of cryopreservation protocols for various aquatic species have been published, there are great difficulties in moving such research forward into applied conservation projects. Successful freezing of sperm in laboratories for research does not guarantee successful management and incorporation of genetic resources into conservation programs in reality. The goal of the present review is to provide insights and practical strategies to apply germplasm repositories as a real-world tool to assist conservation of imperiled aquatic species. Live-bearing (viviparous) fishes are used as models herein to help explain concepts because they are good examples for aquatic species in general, especially small-bodied fishes. Small live-bearing fishes are among the most at-risk fish groups in the world, and need urgent conservation attention. However, development of germplasm repositories for small live-bearing fishes is challenged by their unusual reproductive characteristics, such as formation of sperm bundles, initiation of spermatozoa motility in an isotonic environment, internal fertilization and gestation, and the bearing of live young. The development of germplasm repositories for goodeids and Xiphophorus species can provide examples for addressing these challenges. Germplasm repositories must contain multiple basic components, including frozen samples, genetic assessment and information systems. Standardization and process generalization are important strategies to help develop reliable and efficient repositories. An ideal conservation or recovery program for imperiled species should include a comprehensive approach, that combines major concerns such as habitat (by restoration projects), population propagation and maintenance (by captive breeding or translocation projects), and preservation of genetic diversity (by repository projects). In this context, strong collaboration among different sectors and people with different expertise is a key to the success of such comprehensive programs.

Quality evaluation of sperm from livebearing fishes: Standardized assessment of sperm bundles (spermatozeugmata) from Xenotoca eiseni (Goodeidae)

Standardized evaluation of sperm quality is essential for research, commercial-scale cryopreservation, and induced spawning. However, standardized methods for evaluation of sperm bundles (spermatozeugmata or spermatophores) have not been established. The purpose of the present study was to use Redtail Splitfin (Xenotoca eiseni) as a model for freshwater livebearing fishes to establish initial standardized methods to collect sperm bundles, and quantitatively and qualitatively evaluate quality-related attributes. No sperm or sperm bundles were able to be collected by stripping. Testes were removed, rinsed, weighed, placed in 50 μL of buffer solution on a glass slide, and crushed gently 3-5 times with angled spade-tip forceps. Sperm bundles were released into the buffer solution and collected with a pipette into 1.5-mL centrifuge tubes. To quantify size and shape, images of bundles were captured with a CCD camera connected to a microscope, and measured with computer software. There was no significant correlation between body wet weight and major bundle axis length (P = 0.6759), minor axis length (P = 0.5658), average axis length (P = 0.5869), aspect ratio (P = 0.7839), and observed area (P = 0.5727). The concentrations of sperm bundles, estimated with the three methods (Makler® counting chamber, a hemocytometer, and direct counting) were significantly different (P < 0.0001). Hemocytometers were suitable for estimation of bundles from X. eiseni. To evaluate activation of sperm, bundles were viewed with a microscope, and classified into one of five phases by evaluating morphology of the bundles and motion of sperm within the bundles as Phase 0 through Phase 4 that represented early through late activation stages. The frequencies and duration of each activation phase were used to evaluate dissociation of sperm bundles and motility capability of sperm within the bundles. Within 180 min of activation, all five phases were observed. Overall, this study for the first time established standardized methods to collect and evaluate quality-related attributes of sperm bundles. These standardized evaluations provide a basis for further modification, standardization, and generalization, which are useful in research on livebearing fishes involving male gametes, such as studies on cryopreservation, artificial insemination, and in development of germplasm repositories for imperiled species including goodeids.

Sperm of Galeorhinus galeus (Elasmobranchii, Triakidae) Traverse an Excurrent Duct System Characterized by Pronounced Regionalization: A Scanning Electron and Light Microscopy Study

The transport and subsequent maturation of spermatozoa in the vertebrate excurrent duct require the creation of a series of biochemically defined luminal milieus along the length of the duct. Such specialization is accomplished, among others, by changes in the epididymal histoarchitecture. Here we show that the intratesticular and extratesticular genital ducts of mating Galeorhinus galeus exhibit pronounced regionalization both in terms of epithelial histology and lumen diameter size. Findings also reveal distinct differences in the manner in which the spermatozoa were found in each segment of the duct. Novel scanning electron microscopy evidence is presented showing that the wide lumen ductuli epididymides, which ultimately convey the spermatozoa to the proximal epididymis, show functional specialization as well. The wall of the former consisted of cuboidal ciliated and nonciliated cells whose spatial arrangement in the duct wall resulted in a luminal surface showing lengthy rows of cilia-free areas, with each row bordered on both sides by a single row of cilia. The proximal epididymis comprised several subregions whose epithelial histology varied widely. The distal epididymis and ampulla of the epididymis possessed many fingerlike projections and transverse septa, respectively. As the main storage site for spermatozoa, the ampulla completed the bundling of spermatozoa into spermatozeugmata. These were circular sperm masses in which the heads of the spermatozoa were aligned side by side and embedded in a seminal matrix, while their tails extended outward. These findings of pronounced regionalization differ greatly from the rather uniform epididymal histology seen in some rays.

Ultrastructure of the male genital ducts of the clearnose skate Raja eglanteria

Tissues from the male genital ducts of six specimens of the clearnose skate Raja eglanteria, comprising the Leydig gland, upper and lower epididymis, ductus deferens and seminal vesicle, were fixed and embedded for ultrastructural examination. In the Leydig gland, two types of columnar cells were identified, one bearing microvilli, a basal nucleus and evidence of active secretion with plentiful endoplasmic reticulum and numerous secretory droplets, and the other pyriform with cilia, and swathes of cytofilaments emanating from prominent desmosomes. Occasional crystalloid intramitochondrial inclusions were seen in the first type, with a periodicity of 24 nm. The upper epididymis was composed of cuboidal cells with microvilli and cilia and irregular electron dense granules, some of which were basally situated and extremely large, often within cells resembling intraepithelial leucocytes; such cells were also seen in the stroma underlying the epithelium. The lower epididymis cells also bore microvilli and cilia and were heavily vacuolated with fatty inclusions as well as the granule-laden leucocytes seen previously. In the ductus deferens, cells had masses of long cilia with occasional microvilli; endoplasmic reticulum was well developed, forming complex arrays with sparse secretory droplets and basal mitochondria. In the seminal vesicle there were two cell types, the most common having long cilia and short microvilli and an occasional, paler cell with supranuclear accumulations of small, round mitochondria. These ultrastructural appearances have been related to cell glycosylation and functions including protein secretion, water absorption and waste removal, and illustrate how structure and function vary down the length of the genital tract.