Oogenesis of microlecithal oocytes in the viviparous teleost Heterandria formosa

Ovarian changes and development of the branchial placenta occurring in Jenynsia lineata (Cyprinodontiformes, Anablepidae)

In viviparous teleosts, intraovarian gestation occurs intrafollicularly, as in poeciliids, or intraluminally, as in goodeids and anablepids. Furthermore, there are two different forms of embryonic nutrition: lecithotrophy and matrotrophy; depending on the species, these can be exclusive or coexist during gestation. In matrotrophic species, nutrients are transmitted from the mother to the embryo and are especially important in species with intraluminal gestation. Jenynsia lineata is a South American viviparous teleost with intraluminal gestation, characterized by eggs with scarce yolk, which is resorbed when embryos are 6 mm long, thus developing a branchial placenta. Using histological, histochemical, and immunohistochemical techniques, the present study describes the characteristics and changes of the ovarian mucosa in J. lineata during gestational and nongestational phases, and analyzes the embryonic pharyngeal epithelium in the branchial placenta. The ovaries of 30 adult female specimens were processed using histological techniques and stained with hematoxylin-eosin, Masson's trichrome, and Alcian Blue pH 2.5/periodic acid Schiff reagent. To detect cell proliferation, we used antiproliferating cell nuclear antigen antibody. In nonpregnant females, eosinophilic granular cells (EGCs) and lymphocytes were identified in the lamina propria of the tunica mucosa, and melanomacrophage centers (MMCs) and fibroblasts were identified adjacent to tissue debris in the ovarian folds'. In the cellular debris, an embryo in resorption was observed. In pregnant females, the ovarian mucosa has thin vascularization branches entering the opercular chamber of the embryos, in close contact with the forming gill processes, thereby establishing a branchial placenta. Active cell replacement was observed in these ovarian branches. The identification of fibroblasts, lymphocytes, EGCs, and MMCs adjacent to tissue debris could indicate that these cell types are involved in the embryonic resorption process. Considering the new data obtained in this study on the branchial placenta of J. lineata, we conclude that cell proliferation could be involved in the development of maternal-embryonic interaction.

Oogonial proliferation and early oocyte dynamics during the reproductive cycle of two Clupeiform fish species

Although oogonial proliferation continues in mature females in most teleosts, its dynamics and the transformation of oogonia to early meiotic oocytes during the reproductive cycle have received little attention. In the present study, early oogenesis was examined throughout the reproductive cycle in two Clupeiform fishes, the Mediterranean sardine, Sardina pilchardus, and the European anchovy, Engraulis encrasicolus. Observations using confocal laser scanning microscopy (CLSM) provided extensive information on markers of oogonial proliferation (mitotic divisions, oogonia nests) and meiotic prophase I divisions of oocyte nests (leptotene, zygotene, pachytene, diplotene) in ovaries of different reproductive phases. In sardine, oogonial proliferation persisted throughout the entire reproductive cycle, whereas in anchovy, it was more pronounced prior to (developing ovaries) and after (resting ovaries) the spawning period. Anchovy exhibited a higher rate of meiotic activity in developing ovaries, whereas sardine exhibited a higher rate in resting ovaries. The observed differences between the two species can potentially be attributed to different seasonal patterns of energy allocation to reproduction and the synchronization between feeding and the spawning season.

Intraovarian Gestation in Viviparous Teleosts: Unique Type of Gestation among Vertebrates

The intraovarian gestation, occurring in teleosts, makes this type of reproduction a such complex and unique condition among vertebrates. This type of gestation of teleosts is expressed in special morphological and physiological characteristic where occurs the viviparity and it is an essential component in the analysis of the evolutionary process of viviparity in vertebrates. In viviparous teleosts, during embryogenesis, there are not development of Müllerian ducts, which form the oviducts in the rest of vertebrates, as a result, exclusively in teleosts, there are not oviducts and the caudal region of the ovary, the gonoduct, connects the ovary to the exterior. The lack of oviducts defines that the embryos develop into the ovary, as intraovarian gestation. The ovary forms the oocytes which may develop different type of oogenesis, according with the storage of diverse amount of yolk, variation observed corresponding to the species. The viviparous gestation is characterized by the possible intimate contact between maternal and embryonic tissues, process that permits their metabolic interchanges. So, the nutrients obtained by the embryos could be deposited in the oocyte before fertilization, contained in the yolk (lecithotrophy), and may be completed during gestation by additional provisioning from maternal tissues to the embryo (matrotrophy). Then, essential requirements for viviparity in poeciliids and goodeids are characterized by: a) the diversification of oogenesis, with the deposition of different amount of yolk in the oocyte; b) the insemination, by the transfer of sperm to the female gonoduct and their transportation from the gonoduct to the germinal region of the ovary where the follicles develop; c) the intrafollicular fertilization; d) the intraovarian gestation with the development of embryos in intrafollicular gestation (as in poeciliids), or intraluminal gestation (as in goodeids); and, e) the origin of embryonic nutrition may be by lecithotrophy and matrotrophy. The focus of this revision compares the general and specific structural characteristics of the viviparity occurring into the intraovarian gestation in teleosts, defining this reproductive strategy, illustrated in this review with histological material in a poeciliid, of the species Poecilia latipinna (Lesueur, 1821) (Poeciliidae), and in a goodeid, of the species Xenotoca eiseni (Rutter, 1896) (Goodeidae).

Comparative transcriptomic analysis of gonadal development and renewal in the ovoviviparous black rockfish (Sebastes schlegelii)

BACKGROUND

The black rockfish (Sebastes schlegelii) has an ovoviviparous reproductive pattern and long-term sperm storage, resulting in asynchronous gonadal development between the sexes. However, the comprehensive understanding of gonadal development in black rockfish has not yet been achieved. Here, we studied gonadal development and germ cell renewal using histology and RNA-seq.

RESULTS

In this study, RNA-seq was performed on testes and ovaries to characterize key pathways and genes that are active during development and gamete maturation in black rockfish. Differentially expressed genes (DEGs) were identified and annotated in 4 comparisons (F_III vs. F_IV, F_IV vs. F_V, M_III vs. M_IV and M_IV vs. M_V). Based on analysis of DEGs enriched in the testis, 11 and 14 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were mapped to the M_III vs. M_IV group and the M_IV vs. M_V group, respectively. DEGs in ovarian development were also classified into 10 groups according to their biological functions. The expression patterns of the selected genes determined by qPCR were significantly correlated with the RNA-Seq results, supporting the reliability and accuracy of the RNA-Seq analysis. E2 levels showed down regulation from previtellogenesis to mature stage in female and T level showed down regulation from spermatogenesis to regressed stage in the male.

CONCLUSIONS

The categories "intercellular interaction and cytoskeleton", "molecule amplification" and "repair in the cell cycle" were revealed to be crucial in testis development and spermatogenesis, as was the biosynthesis of a series of metabolites. Our results provide comprehensive insight into black rockfish gonadal development and provide a basis for further study of reproductive physiology and molecular biology in ovoviviparity teleosts.

Serotonin (5-hydroxytryptamine)-immunoreactive neurons in the brain of the viviparous fish Gambusia affinis

The monoaminergic neurotransmitter serotonin (5-HT) acts as a neuromodulator and is associated with a wide range of functions in fish. In this investigation, 5-HT immunoreactivity was studied in the central nervous system (CNS) of the viviparous mosquitofish Gambusia affinis. 5-HT-immunoreactive (5-HT-ir) cells/fibres were observed throughout the subdivisions of ventral and dorsal telencephalon including the olfactory bulb. Several intensely stained 5-HT-ir cells and/or fibres were detected in different areas of the hypothalamus as well as the proximal pars distalis of the pituitary gland. 5-HT-ir cells were restricted to the dorsal and ventral part of the pretectal diencephalic cluster, but only fibres were detected in the anterior, ventromedial and posterior subdivisions of the thalamic nucleus and in the preglomerular complex. In the mesencephalon, 5-HT-ir perikarya, and fibres were seen in the optic tectum, midbrain tegmentum and torus semicircularis. A cluster of prominently labelled 5-HT-ir neurons was observed in the superior raphe nucleus, whereas numerous 5-HT-ir fibres were distributed throughout the rhombencephalic divisions. In addition, a bundle of rostrocaudally running 5-HT-ir fibres was noticed in the spinal cord. This is the first detailed neuroanatomical study in a viviparous teleost, reporting a widespread distribution of 5-HT-ir somata and fibres in the CNS. The results of this study provide new insights into the evolutionarily well conserved nature of the monoaminergic system in the CNS of vertebrates and suggest a role for 5-HT in regulation of several physiological, behavioural and neuroendocrine functions in viviparous teleosts.

Morphology of yolk and pericardial sacs in lecithotrophic and matrotrophic nutrition in poeciliid fishes

We used histological techniques to describe the morphology of the yolk and pericardial sacs in developing embryos of the lecithotrophic species Girardinus creolus, Gambusia puncticulata, Limia vittata, and Quintana atrizona, in comparison with the extreme matrotrophic Heterandria formosa. In lecithotrophic species, the yolk sac was enlarged and lasted until the final stages of development, while in H. formosa it was completely absorbed soon after fertilization. Lecithotrophic poeciliids showed a pericardial sac with a single layer of blood vessels covering the dorsal surface of the cephalic region only, while H. formosa showed a more complex largely vascularized pericardial sac covering the entire dorsal surface, except the caudal region. In advanced gestation of G. creolus, a vascular plexus of the yolk sac reaches the pharyngeal region, behind the gills, suggesting that the pharynx may play a role in embryonic nutrition in lecithotrophic species. These morphological evidences suggest that matrotrophy derives from lecithotrophy.

Structure of the gonoduct of the viviparous teleost Cnesterodon decemmaculatus (Jenyns, 1842) (Poeciliidae)

During embryogenesis, teleost females do not develop Müllerian ducts, which form the oviducts in all other vertebrates. Thus, when they reach sexual maturity they do not have oviducts. In viviparous teleosts, the lack of oviducts means that the development of the embryos occurs as an intraovarian gestation, unique among vertebrates. The ovary is an unpaired hollow organ whose cavity is continuous with the caudal portion, the gonoduct, characterized by the absence of germinal cells, which opens to the exterior at the gonopore. The gonoduct attains essential function as a barrier between the germinal region of the ovary and the exterior during all reproductive stages. This study describes the functional morphology of the gonoduct in the viviparous teleost Cnesterodon decemmaculatus during non-gestation (previtellogenesis and vitellogenesis) and gestation. The ovaries were processed using histological techniques and stained with hematoxylin-eosin, and periodic acid Schiff. The gonoduct has two regions: cephalic and caudal, and is formed by three histological layers, which are, from inside to the periphery: (a) tunica mucosa; (b) tunica muscularis; and (c) tunica serosa. In the cephalic region there are mucosal folds extending into the lumen and forming a structure similar to a cervix. The histology of the gonoduct indicates essential functions, that is, (a) the control of the luminal diameter in the limit to the germinal region of the ovary by the presence of a cervix; (b) during insemination the gonoduct receives the spermatozoa, may store and transport them to the germinal region; (c) the presence of melano-macrophage centers indicates support of immunological processes, especially during gestation when these centers increase in size; (d) production of exocrine secretions; and (e) it is the birth canal, internally lined by an ciliated epithelium and surrounded by smooth musclesboth tissues supposedly supporting the birth process.

Structures Associated with Oogenesis and Embryonic Development during Intraovarian Gestation in Viviparous Teleosts (Poeciliidae)

Viviparity in teleosts involves, invariably, the ovary in a gestational role. This type of viviparity is due to the combination of unique aspects, different from those found in the rest of vertebrates. These aspects are: The ovary has a saccular structure; the germinal epithelium lines the ovarian lumen; the absence of oviducts; and the intraovarian insemination, fertilization, and gestation. The communication of the germinal zone of the ovary to the exterior is via the caudal zone of the ovary—the gonoduct. The germinal epithelium is composed of oogonia and oocytes scattered individually or in cell nests among somatic epithelial cells. In the ovarian stroma the follicles are included which are formed by the oocyte, which is surrounded by follicular cells and the vascularized theca. The oogenesis comprises three stages: chromatin-nucleolus, previtellogenesis, and vitellogenesis. There is no ovulation, as the oocyte is retained in the follicle. During the insemination, the spermatozoa enter into the ovarian lumen and the intrafollicular fertilization occurs, followed by intrafollicular gestation. The intraovarian gestation of poeciliids involves morphological characteristics associated with the intrafollicular embryogenesis and types of nutrition, such as lecithotrophy and matrotrophy. In lecithotrophy, the nutrients come from the yolk reserves stored during oogenesis, whereas in matrotrophy the nutrients are provided by supplies from maternal tissues to the embryo during gestation. The maternal–embryonic metabolic interchanges converge through the development of the association of maternal and embryonic blood vessels, establishing a follicular placenta.

Feminization, altered gonadal development, and liver damage in least killifish (Heterandria formosa) exposed to sublethal concentrations of 17α-ethinylestradiol

The widespread use of the synthetic estrogen 17 α-ethinylestradiol (EE₂) has resulted in elevated levels in aquatic environments, where it is known to act as an endocrine disrupting chemical affecting fish and other aquatic organisms. Examining changes in the structure of the fish' gonads and liver has proven to be an effective approach for assessing these impacts. While changes have been reported for various fish species, it is not clear whether impacts are equally severe in live-bearing fishes. The present study looked at gonadal and liver development in EE₂-exposed least killifish (Heterandria formosa), a live-bearing Poeciliid. Exposures to 0, 5, or 25 ng/L EE₂ began within six days of birth and continued until fish became sexually mature 12-23 weeks later. Exposure to 5 ng/L EE₂ resulted in severe intersex in fish with external male characteristics, a slowdown of spermatogenesis in these intersex fish and a slowdown of oogenesis in the female fish. Moreover, these fish had a variety of liver injuries. Fish exposed to 25 ng/L EE₂ exhibited intersex but at a lower frequency than occurred at 5 ng/L. In contrast, liver damage and slowdown of both oogenesis and spermatogenesis exhibited the typical dose-dependence. These findings illustrate the importance of including histological analyses when assessing endocrine disruption in fish, demonstrate that the live-bearing mode of reproduction appears to provide limited protection from the effects of waterborne EE₂, and provide further evidence that EE₂ has multiple impacts on fish health and reproduction that are severe enough to potentially affect fish populations.

A hypothesis on paradoxical privileged portal vein metastasis of hepatocellular carcinoma. Can organ evolution shed light on patterns of human pathology, and vice versa?

Unlike other carcinomas, hepatocellular carcinoma (HCC) metastasizes to distant organs relatively rarely. In contrast, it routinely metastasizes to liver vasculature/liver, affecting portal veins 3-10 times more often than hepatic veins. This portal metastatic predominance is traditionally rationalized within the model of a reverse portal flow, due to accompanying liver cirrhosis. However, this intuitive model is not coherent with facts: 1) reverse portal flow occurs in fewer than 10% of cirrhotic patients, while portal metastasis occurs in 30-100% of HCC cases, and 2) portal vein prevalence of HCC metastasis is also characteristic of HCC in non-cirrhotic livers. Therefore, we must assume that the route for HCC metastatic dissemination is the same as for other carcinomas: systemic dissemination via the draining vessel, i.e., via the hepatic vein. In this light, portal prevalence versus hepatic vein of HCC metastasis appears as a puzzling pattern, particularly in cases when portal HCC metastases have appeared as the sole manifestation of HCC. Considering that other GI carcinomas (colorectal, pancreatic, gastric and small bowel) invariably disseminate via portal vein, but very rarely form portal metastasis, portal prevalence of HCC metastasis appears as a paradox. However, nature does not contradict itself; it is rather our wrong assumptions that create paradoxes. The 'portal paradox' becomes a logical event within the hypothesis that the formation of the unique portal venous system preceded the appearance of liver in evolution of chordates. The analysis suggests that the appearance of the portal venous system, supplying hormones and growth factors of pancreatic family, which includes insulin, glucagon, somatostatin, and pancreatic polypeptide (HGFPF) to midgut diverticulum in the early evolution of chordates (in an Amphioxus-like ancestral animal), promoted differentiation of enterocytes into hepatocytes and their further evolution to the liver of vertebrates. These promotional-dependent interactions are conserved in the vertebrate lineage. I hypothesize that selective homing and proliferation of malignant hepatocytes (i.e., HCC cells) in the portal vein environment are due to a uniquely high concentration of HGFPF in portal blood. HGFPF are also necessary for liver function and renewal and are significantly extracted by hepatocytes from passing blood, creating a concentration gradient of HGFPF between the portal blood and hepatic vein outflow, making post-liver vasculature and remote organs less favorable spaces for HCC growth. It also suggested that the portal vein environment (i.e., HGFPF) promotes the differentiation of more aggressive HCC clones from already-seeded portal metastases, explaining the worse outcome of HCC with the portal metastatic pattern. The analysis also offers new hypothesis on the phylogenetic origin of the hepatic diverticulum of cephalochordates, with certain implications for the modeling of the chordate phylogeny.

Mechanisms of reproductive allocation as drivers of developmental plasticity in reptiles

Developmental plasticity in offspring phenotype occurs as a result of the environmental conditions embryos experience during development. The nutritional environment provided to a fetus is an important source of developmental plasticity. Reptiles are a particularly interesting system to study this plasticity because of their varied routes of maternal nutrient allocation to reproduction. Most reptiles provide their offspring with all or most of the nutrients they require in egg yolk (lecithotrophy) while viviparous reptiles also provide their offspring with nutrients via a placenta (placentotrophy). We review the ways in which both lecithotrophy and placentotrophy can lead to differences in the nutrients embryonic reptiles receive, and discuss how these differences lead to developmental plasticity in offspring phenotype. We finish by reviewing the ecological and conservation consequences of nutritional-driven developmental plasticity in reptiles. If nutritional-driven developmental plasticity has fitness consequences, then understanding the basis of this plasticity has exciting potential to identify how reptile recruitment is affected by environmental changes in food supply. Such knowledge is critical to our ability to protect taxa threatened by environmental change.

Histochemical Characterization of Oocytes in the Pink Cuskeel (Genypterus blacodes)

In the present study we histochemically and lectinhistochemically characterized the growing oocytes of the pink cuskeel (Genypterus blacodes). We used histochemical methods for the localization and characterization of glycoconjugates (GCs) and lectin histochemical techniques for the identification of specific sugar residues. We analyzed presence and distribution of GCs in the different structures of the growing follicles (cortical alveoli, globules, yolk granules and zona radiata). During the initial stage of vitellogenesis, the oocytes presented small yolk granules composed of GCs that gradually increased during exogenous vitellogenesis. These GCs contained moderate quantities of α-D-mannose, D-glucose, N-acetylglucosamine and N-acetyl-neuraminic acid. The cortical alveoli contained both neutral and carboxylated GCs, and lectin techniques detected N-acetylgalactosamine, galactose and L-fucose. The zona radiata showed a strong positive reaction to PAS and it reacted weakly with more specific techniques, such as KOH/PA*S and PA/Bh/KOH/PAS. This structure showed GCs with oxidizable vicinal diols, O-acyl sugars and sialic acid residues with different substitution types and presented N-acetylgalactosamine and L-fucose specific residues. The oocytes follicular envelope evidenced neutral and acidic non-sulfated GCs and high concentrations of α-D-mannose, D-glucose, galactose and N-acetylgalactosamine. The intergranular cytoplasmic GCs were mainly rich in α-D-mannose, D-glucose, N-acetylgalactosamine, N-acetylglucosamine and N-acetyl-neuraminic acid. These results enhance the comprehension of the structure and functionality of the pink cuskeel ovarian follicles, and provide a useful tool for the study of this tissue in other teleost species.

Evidence of maternal copper and cadmium transfer in two live-bearing fish species

We studied maternal transfer of an essential metal (copper) and a non-essential one (cadmium) in the live-bearing fishes Heterandria formosa and Gambusia affinis. The goals of this study were: (1) to determine whether metals are transferred from exposed females to their developing offspring; (2) to determine if this transfer differs between two fish species that differ in their degree of maternal provisioning during development; (3) to determine the duration of maternal metal transfer once females are no longer exposed; and (4) to determine whether copper and cadmium are transferred equivalently. We exposed gravid females to background levels (control) or 0.15 µM of metal for 10 days, and then transferred them to clean water. We allowed females to give birth to up to three broods, and then quantified metal levels in offspring born at least 3 days after the transfer. We detected maternal metal transfer for both metals and in both species. Offspring metal levels decreased as females spent more time in clean water. Similarly, metal levels were lower in later broods than in earlier ones. Maternal metal transfer was higher in H. formosa than in G. affinis. Our results constitute the first report of maternal metal transfer in live-bearing fishes, and show that developing embryos acquire both essential and non-essential metals from their mothers in both species. This shows that metal toxicity may be an issue for live-bearing fish in clean environments when the previous generation has encountered metal pollution.

Gonad morphology, gametogenesis, and reproductive modes in fishes of the tribe Starksiini (Teleostei, Blenniiformes)

A comparative study of the reproductive organs in 17 of the 30 species of the tribe Starksiini (Labrisomidae, Blenniiformes) and related labrisomids reveals the major traits of gamete form and production and likely reproductive modes. The testes are of the lobular type and have a testicular gland and sperm ducts. Isodiametric sperm (aquasperm) with a globular head or anisodiametric sperm (introsperm) with an elongate head, or both, were observed in the studied species. Both types have either one or two flagella in the sperm tail. Ovaries of the Starksiini are bilobed and exhibit synchronous or asynchronous egg production. Although viviparity or "ovoviviparity" reportedly characterizes the group, our study revealed evidence of both internal and external fertilization and three modes of reproduction. External fertilization or ovuliparity is suggested for the Starksia atlantica and S. lepicoelia species complexes by the presence in males of a short genital papilla that is not reinforced through adhesion with the first anal-fin spine and by the absence of sperm within the ovaries. Internal fertilization and zygoparity is indicated for most species by the presence of an intromittent papilla in males that is adhered to the first anal-fin spine, "nests" of sperm within the ovaries, absence of embryos within the ovarian lamellae and usually thick egg envelopes bearing dense covers of adhesive filaments. Internal fertilization and embryoparity is indicated for starksia fulva and Xenomedea rhodopyga by an intromittent papilla that is adhered to the first anal-fin spine of males, anisodiametric sperm in males, delicate egg envelopes without adhesive filaments and developing embryos within follicular envelopes or within the follicle in females. Although many of these features are seen in the internally fertilizing clinid blennies, starksiins differ in retaining the testicular gland typical of labrisomids and in lacking sperm packaging typical of other internally fertilizing teleosts.