The interstitial cells of the trout testis (Oncorhynchus mykiss): ultrastructural characterization and changes throughout the reproductive cycle

Multi-Parametric Portfolio to Assess the Fitness and Gonadal Maturation in Four Key Reproductive Phases of Brown Trout

Simple Summary

Brown trout is a freshwater fish with economic importance and with a great potential to be used as an environmental biosensor species. Despite being selected as a model species in distinct scientific contexts, in cultured specimens, there is a surprising lack of works investigating the morpho-physiological changes associated with the reproductive cycle; particularly concerning the gonads. In this study, a multi-parameter portfolio of biometric, biochemical, hormonal, and morphological analysis was established, which allowed a seasonal and sex characterization of the gonad status of adult brown trout males and females. Sampling included four reproductive phases: spawning capable (December), regressing (March), regenerating (July), and developing (November). Sex- and season-specific changes were described. The discriminative parameters characterized here stand now as normal baseline values against which abnormal patterns can be compared with. These parameters have the potential to be used as tools for the environmental monitoring of the reproductive status of wild populations and for the control of breeding stocks in aquaculture.

Abstract

Brown trout is an environmental freshwater sentinel species and is economically important for recreational fishing and aquaculture. Despite that, there is limited knowledge regarding morpho-physiological variations in adults throughout the reproductive cycle. Thus, this study aimed to analyze the fitness and gonadal maturation of cultured adult brown trout in four reproductive phases (spawning capable—December, regressing—March, regenerating—July, and developing—November). The systematic evaluation of males and females was based on biometric, biochemical, and hormonal parameters, along with a histomorphological grading of gonads and the immunophenotype location of key steroidogenic enzymes. The total weight and lengths reached the lowest levels in December. Gonad weights were higher in December and November, while the opposite pattern was found for liver weights. The lowest levels of cholesterol and total protein were also noted during those stages. The 11-ketotestosterone (11-KT) and testosterone (T) for males, and estradiol (E2) and T for females, mostly explained the hormonal variations. The immunohistochemistry of cytochrome P450c17 (CYP17-I), aromatase (CYP19), and 17β-hydroxysteroid dehydrogenase (17β-HSD) showed sex and site-specific patterns in the distinct reproductive phases. The sex- and season-specific changes generated discriminative multi-parameter profiles, serving as a tool for environmental and aquaculture surveys.

Ultrastructural feature of spermatogenic cells and spermatozoon in cultured burbot Lota lota

Testis development and ultrastructure of spermatogenic cells and spermatozoa of burbot Lota lota, a commercially important cold freshwater fish, were studied by light and transmission electron microscopy. Spermatogonia, spermatocytes, spermatids, and spermatozoa are distributed along the seminiferous tubules. Electron-dense bodies appear in germ cells from primary spermatogonia to secondary spermatocytes. We identified three distinct stages of spermatid cell differentiation based on chromatin condensation, development of the flagellum, formation of a nuclear fossa, and elimination of excess cytoplasm. Spermatozoa were anacrosomal and characterized by location of the centrioles outside the nuclear fossa and incomplete perpendicular arrangement of the centrioles. The sperm flagellum displayed an axoneme with nine doublets of peripheral microtubules and two central microtubules. These results provide valuable information for burbot taxonomy and may clarify the process of spermatogenesis for this species.

Ultrastructural investigations on spermatogenesis and spermatozoan morphology in the endangered Adriatic sturgeon, Acipenser naccarii (Chondrostei, Acipenseriformes)

Spermatogenesis was investigated in the Adriatic sturgeon, Acipenser naccarii, by light and electron microscopy. The testis of the unrestricted type had a germinal compartment composed of lobules containing germ cells and Sertoli cells, and separated by a basal lamina from the interstitial compartment, in which Leydig and myoid cells were detected for the first time in Acipenseridae. Spermatogenesis occurred in spermatocysts produced when Sertoli cells became associated with type A spermatogonia of subsequent generations, which produced a clone of synchronized aligned spermatogonia. In primary spermatocytes at zygo-pachytene stage, the large spherical nucleus contained synaptonemal complexes. The smaller secondary spermatocytes were ovoid with a central round nucleus and scarce cytoplasm. Spermatids were interconnected by cytoplasmic bridges until early spermiogenesis. Chromatin initially condensed as long, twisted, and nonhomogeneous fibers and finally as a compact structure made of thick filaments. Early spermatids showed the flagellum, the primordia of centriole complex and of "implantation fossa," followed by the acrosomal vesicle formed by Golgi complexes and a fibrous body associated to centriole complex. The spermatozoan head had 10 postero-lateral projections and a trapezoidal nucleus, a cylindrical midpiece with six to eight mitochondria, the centriole complex, and a "9 + 2" flagellum with a pair of lateral fins. Three helical endonuclear canals crossed the nucleus from the acrosome base to the implantation fossa; their spiralization and that of chromatin fibers suggest a spiral twisting of the nucleus during spermiogenesis. The Sertoli cells performed phagocytosis of degenerating spermatids and spermatozoa. Significant interindividual differences were detected in most morphological parameters of spermatozoa. Data on spermatogenesis in A. naccarii and morphometric measurements on mature spermatozoa provide information about the reproductive biology of the species useful not only for phylogenetic studies but also for evaluation of sperm quality for artificial reproduction projects and restocking of this and other critically endangered sturgeon species.

Paracrine role of macrophage produced-nitric oxide (NO) in Leydig cell steroidogenesis in a teleost, Clarias batrachus: Impact of gonadotropin, growth hormone and insulin on NO production by testicular macrophages

The present in vitro study for the first time demonstrates the role of extragonadal hormones in regulation of NO production by testicular macrophages in vertebrates and paracrine role of NO in Leydig cell steroidogenesis in fishes. N-nitro L-arginine methyl ester (L-NAME - a NOS inhibitor) treatment substantially reduced NO production by testicular macrophages suggesting that testicular macrophages are one of the sources of testicular NO in the catfish, Clarias batrachus. Significant decline in NO production was also recorded following treatment of macrophages with the gonadotropin (GtH), growth hormone (GH) and insulin indicating that macrophage-produced NO is under endocrine inhibitory control. Treatment of Leydig cells with sodium nitroprusside (SNP) decreased testosterone (T) production. SNP treatment also remarkably suppressed the GtH, GH and insulin-stimulated T production by Leydig cells indicating that Leydig cell steroidogenesis is sensitive to exogenous NO. Further, effect of conditioned medium of testicular macrophages incubated with medium alone (non-treated TMCM) or GtH (GtH-treated TMCM) or GH (GH-treated TMCM) or insulin (insulin-treated TMCM) were also observed on Leydig cell T production. Non-treated TMCM as well as hormone-treated TMCM stimulated T production by Leydig cells; hormone-treated TMCM were more effective in stimulating T production than non-treated TMCM and/or hormones alone. These experiments altogether suggest that testicular macrophage secrete some factors, which influence Leydig cell steroidogenic activity through paracrine mechanism, and these paracrine secretions are under the endocrine control. Decline in NO in hormone-treated TMCM might also be one of the reasons for more stimulation in T production than that of hormones alone.

Androgen-induced masculinization in rainbow trout results in a marked dysregulation of early gonadal gene expression profiles

Background

Fish gonadal sex differentiation is affected by sex steroids treatments providing an efficient strategy to control the sexual phenotype of fish for aquaculture purposes. However, the biological effects of such treatments are poorly understood. The aim of this study was to identify the main effects of an androgen masculinizing treatment (11β-hydroxyandrostenedione, 11βOHΔ4, 10 mg/kg of food for 3 months) on gonadal gene expression profiles of an all-female genetic population of trout. To characterize the most important molecular features of this process, we used a large scale gene expression profiling approach using rainbow trout DNA microarrays combined with a detailed gene ontology (GO) analysis.

Results

2,474 genes were characterized as up-regulated or down-regulated in trout female gonads masculinized by androgen in comparison with control male or female gonads from untreated all-male and all-female genetic populations. These genes were classified in 13 k-means clusters of temporally correlated expression profiles. Gene ontology (GO) data mining revealed that androgen treatment triggers a marked down-regulation of genes potentially involved in early oogenesis processes (GO 'mitotic cell cycle', 'nucleolus'), an up-regulation of the translation machinery (GO 'ribosome') along with a down-regulation of proteolysis (GO 'proteolysis', 'peptidase' and 'metallopeptidase activity'). Genes considered as muscle fibres markers (GO 'muscle contraction') and genes annotated as structural constituents of the extracellular matrix (GO 'extracellular matrix') or related to meiosis (GO 'chromosome' and 'meiosis') were found significantly enriched in the two clusters of genes specifically up-regulated in androgen-treated female gonads. GO annotations 'Sex differentiation' and 'steroid biosynthesis' were enriched in a cluster of genes with high expression levels only in control males. Interestingly none of these genes were stimulated by the masculinizing androgen treatment.

Conclusion

This study provides evidence that androgen masculinization results in a marked dysregulation of early gene expression profiles when compared to natural testicular or ovarian differentiation. Based on these results we suggest that, in our experimental conditions, androgen masculinization proceeds mainly through an early inhibition of female development.

Morphofunctional changes in Leydig cells throughout the continuous spermatogenesis of the freshwater teleost fish, Serrasalmus spilopleura (Characiformes, Characidae): an ultrastructural and enzyme study

The freshwater fish Serrasalmus spilopleura (piranha) has a continuous type of reproduction; gametes are constantly produced and released during the reproductive cycle. The testes do not undergo seasonal morphological changes but exhibit two constant regions throughout the year: the medullar region (involved with spermatogenesis) and the cortical region (involved with spermiation and sperm storage). We have evaluated the ultrastructure of the Leydig cells and the activity of 3beta-HSD (an essential enzyme related to steroid hormone biosynthesis) and acid phosphatase (AcPase; lysosomal marker enzyme) in these two regions. The activity of 3beta-HSD is stronger in the medullar region, and the Leydig cells in this region have a variety of cytological features that reflect differences in hormone synthesis and/or that could be linked to steroidogenic cells under various degrees of hormonal activity. In the cortical region, 3beta-HSD activity is weak and the Leydig cells exhibit signs of degeneration, as confirmed by their ultrastructure and intense AcPase activity. These degenerative signs are indicative of cytoplasmic remodelling to degrade steroidogenic enzymes, such as 3beta-HSD, that could lead to senescence or even to autophagic cell degeneration. S. spilopleura thus constitutes an interesting model for increasing our understanding of steroidogenesis control in freshwater teleost fish.

Immunohistochemical distribution of hyperacetylated histone H4 in testis of paddlefish Polyodon spathula: ultrastructural correlation with chromatin condensation

The acetylation of core histones has been correlated with the deposition of free histones onto newly replicated DNA, transcriptional activity and the displacement of histones by protamines during spermiogenesis. The aim of the present study was to investigate the immunohistochemical distribution of hyperacetylated H4 during spermatogenesis in Polyodon spathula and to correlate these findings with the pattern of chromatin condensation in spermatids. In immature testis, the Sertoli cells showed more intense immunoreactivity for highly acetylated H4 than that of most primary spermatogonia. The testis of paddlefish at the beginning of spermatogenesis possessed early secondary spermatogonia and late secondary spermatogonia/preleptotene primary spermatocyte with intense nuclear immunoreactivity for highly acetylated H4. In seminiferous tubules in which secondary spermatogonia nuclei were intensely immunostained, Sertoli cell nuclei were unstained. Testes in late spermatogenesis contained tubules in which the immunohistochemical reaction for highly acetylated H4 was positive in the nuclei of preleptotene primary spermatocytes and secondary spermatocytes and in spermatids at the beginning of the elongation process. No immunostaining was found in round spermatids and spermatozoa. During the resting stage, immunostaining was confined to the nuclei of spermatogonia and the cells from the interstitial tissue. Transmission electron microscopy revealed that early spermatids had a round nucleus with filaments of fine chromatin that were dispersed or condensed in clumps. In later stages of maturation, the spermatids had slightly oval nuclei and homogeneous granular chromatin. The chromatin of advanced spermatids was organized into thick fibres. At the end of spermiogenesis, spermatozoan nuclei consisted of homogeneous highly compacted chromatin.

Morphological changes of Sertoli cells during the male reproductive cycle of the teleost Piaractus mesopotamicus (Holmberg, 1887)

An investigation of the histological and ultrastructural changes of Sertoli cells during the male reproductive cycle in Piaractus mesopotamicus was made. The results showed that the Sertoli cell development is closely related with germ cell maturation. Therefore, these cells may have some role in germ cell maturation during the reproductive cycle of this species, whether in forming a tissue framework for the developing spermatogenic cysts, aiding in testes reorganization for a new reproductive cycle, in addition to other possible functions discussed in the text.

Testicular interstitial cells, and steroidogenic detection in the protogynous fish, Synbranchus marmoratus (Teleostei, Synbranchidae)

The swamp eel, Synbranchus marmoratus, is a freshwater protogynic diandric species. Primary males develop directly as males while secondary males arise from the sex reversal of females. Fishes from Argentine and Brazil inland waters were collected, examined and compared for this study. In order to characterize the interstitial testicular compartment, light and electron microscopy techniques and an enzyme histochemical examination for steroidogenic cells detection were used. The interstitial compartment of S. marmoratus is composed of Leydig and myoid cells, collagen fibers, blood cells, macrophages,and amyelinic nerves. At the ultrastructural level, no differences were observed in the interstitial tissue, either between specimens from the different sampling sites or between primary and secondary males. Leydig cells are present in all testes examined throughout the year. A cytoplasmatic reaction of 3beta-HSD was detected only in Leydig cells during sex reversal and in both type of males, mainly during the regressed and early maturation classes (autumn and winter). Leydig cells possess the typical fine structural characteristics associated with steroidogenesis. Furthermore, in both type of males, during sex reversal and after the spawning period, the number of granulocytes and macrophages present in the testes increased, suggesting that they could be involved in phagocytosis and resorption of damaged cells.

Testicular morphological changes during the seasonal reproductive cycle in the bullfrog Rana catesbeiana

Spermatogenesis and steroidogenesis undergo seasonal variations during the reproductive cycle in amphibians. Testicular morphological and morphometric seasonal variations as well as interstitial lipidic inclusions and intralobular glycoconjugates were evaluated during seasonal cycle of Rana catesbeiana. Testes of frogs collected during the annual seasons were weighed for calculation of GSI (Gonadosomatic index). Seminiferous lobule diameters (DSL) and volume densities of seminiferous lobules (VvSL), excretory ducts (VvED), and interstitial tissue (VvIT) were analyzed. Semithin sections were submitted to Periodic Acid-Schiff (PAS) and Alcian Blue (AB) methods for detection of glycoconjugates, while lipidic inclusions were detected by Sudan Black B. GSI showed no significant variations during the year. Since VvED and VvIT increased significantly during summer and were inversely proportional to VvSL, a compensatory effect between the testicular compartments may be related to the maintenance of GSI. During autumn/winter, larger lobular diameters were observed in comparison to spring/summer when spermiogenesis and spermiation were commonly observed. The increased VvIT and the numerous lipidic inclusions in the interstitial cells during summer suggest a relationship between spermiogenesis and steroidogenesis. Besides the structural stability variations occurring in the IT and SL, a possible paracrine interaction between ED and IT should be also involved in the IT development during summer. The presence of PAS and AB-positive globular structures were observed in the seminiferous lobules and excretory ducts. These structures containing acid glycoconjugates appear to be Sertoli cell apical portions, which are accumulated in the lumen of the seminiferous lobules mainly during spermiation.

Continuous gametogenesis in the neotropical freshwater teleost, Bryconops affinis (Pisces:Characidae)

The gametogenesis of Bryconops affinis was studied by light, transmission and scanning electron microscopy. The spermatogenesis is semi-cystic and spermatids are released into the lumen of seminiferous tubules, where spermiogenesis is completed. Spermatozoa have an ovoid head, a rudimentary middle piece with a small number of mitochondria and long flagellum (primitive spermatozoa). The Sertoli and Leydig cells show secretory activity during spermatogenesis. By the end of this phenomenon, the Sertoli cells phagocytize the residual spermatozoa, while the Leydig cells show involuted characteristics. With regard to the oogenesis process, the oocyte development was divided into four stages based on the cytological characteristics of the oocyte and its surrounding layers. Ultrastructural analysis revealed that the zona pellucida is formed during the previtellogenic stage. Specializations associated to the outer layer of the zona pellucida may be related to the egg's adherence to the substrata.

Modulation of testicular androgen production in adolescent African catfish (Clarias gariepinus)

At 6 months of age the first spermatozoa appear in the testes of the African catfish considered to be adolescent, since the development to adulthood (12 months of age) is accompanied by further morphological and functional differentiation of Leydig cells. There are increasing plasma levels of 11-ketotestosterone (11-KT) and an increasing responsiveness to luteinizing hormone (LH) of testicular androgen secretion in vitro. Whether treatment of adolescent males with key hormones of the brain-pituitary-gonad axis [gonadotropin-releasing hormone (GnRH), LH, and 11-KT] affects the testicular steroidogenic response to a challenge with LH in vitro 7 days later has been investigated. Injection of GnRH (2.5 microg chicken GnRH-II per kilogram of body weight), LH (25 microg/kg), or a high dose of 11-KT (50 microg/kg) down-regulated basal and LH-stimulated testicular androgen secretion to a minimum of 35% of control values. Treatment with LH was, moreover, associated with changes in the ultrastructure of Leydig cell mitochondria which were either swollen and had a less electron-dense matrix or showed an elongated shape. Conversely, a moderate dose of 11-KT (20 microg/kg) enhanced LH-stimulated, but not basal, androgen secretion in vitro to a maximum of 190% of control values. In view of the generally low LH plasma levels and of the steadily increasing 11-KT plasma levels during puberty, 11-KT may be involved in the up-regulation of the testicular steroidogenic capacity observed during development to full maturity.