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In this study, we evaluated the morphology and morphometry of the layers that make up the follicular complex surrounding mature oocytes in the six fish species Auchenipterichthys longimanus, Ageneiosus ucayalensis, Hypophthalmus marginatus, Baryancistrus xanthellus , Panaqolus tankei and Peckoltia oligospila , belonging to the order Siluriformes, which inhabit the Amazon basin. On the basis of the morphology and thickness of the layers of the follicular complex, the species were divided into two groups: 1- A. longimanus, A. Ucayalensis and H. marginatus and 2 - B. xanthellus , P. tankei and P. oligospila. The total thickness of the layers that make up the follicular complex showed a difference between type III and IV oocytes for all species of each group. Differences in the theca layer, follicular cells and zona radiata between species and between groups were submitted to statistical analysis. Morphologically, group 1 showed columnar follicular cells and thin zona radiata . Meanwhile, group 2 displayed a layer of cuboidal-shaped follicular cells layer and thicker zona radiata. These differences may be related to the environment and reproductive behaviors, as group 1 migrates without parental care and has eggs that are generally smaller and abundant. While group 2, represented by loricariidae, inhabit lotic environments, have reproductive tactics of parental care and eggs that are generally large and in small numbers. Therefore, we can infer that the follicular complex in mature oocytes can predict the reproductive tactics of the species., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2023 Published by Elsevier Ltd.)

Oogenesis in fish is a process that involves cell proliferation, growth and maturation. In rivers blocked by hydroelectric dams, these factors can be altered and affect the reproduction mechanism of the species. Regarding this scenario, the present study aimed to describe oogenesis in two cichlids native to the Amazon basin, Geophagus argyrostictus and G. altifrons, during pre- and post-dam periods of the Xingu River. Females of both Geophagus species were captured and biometric measurements were taken. Afterwards, the fish were euthanized, and the gonads were removed and subjected to histological processing for light microscopy, scanning electron microscopy and immunoperoxidase staining. Oocyte diameter and zona radiata thickness were also measured. The ovaries appeared in pairs, where according to their shape, size, vascularization and cell organization, five stages were described for both species. In the post-dam period, there was a reduction in the mean diameter of the oocytes, especially type IV, and a decrease in the thickness of the zona radiata, mainly, in G. argyrostictus. In both species, there was a greater presence of oocyte atresia in the post-dam period. PCNA immunoreactivity was more intense in type I and II oocytes, while vitellogenin immunoreactivity occurred in the cytoplasm and follicular cells of oocytes III and IV. These data suggest that because of changes in the level/flow of the river, the gonads of these two species adjust to the new environment, with a decrease in mean diameter and zona radiata thickness of the oocytes, which can interfere with the reproduction of the animals., Competing Interests: Declaration of Competing Interest The authors declare that there are no conflicts of interest for the publication of this study., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The present study was conducted to assess the testicular structure and germ cell ultrastructure of Hypophthalmus marginatus during spermatogenesis. Semen and sections of the mid-region of the testis were collected, processed, and analyzed using optical and electron microscopy. Macroscopically, the testes of H. marginatus were filiform, and the testicular parenchyma was composed of spermatogenic cells that proliferated, organized within spermatic cysts. During spermiogenesis, spermatids had no nuclear rotation. The proximal centriole was perpendicular to the distal centriole, characteristic of type III spermiogenesis. Spermatozoa were released into the lumen of the seminiferous tubules and had an ovoid head without an acrosome, condensed nucleus, and shallow nuclear fossa. The midpiece was short, with a single long flagellum. The flagellum had the typical axoneme structure, with nine pairs of peripheral and a central pair of microtubules. The thin end piece comprised only peripheral microtubules. Spermatogenesis in H. marginatus features filiform testes, cystic spermatogenesis, and type III spermiogenesis., (Copyright © 2019 Elsevier B.V. All rights reserved.)

This contribution describes the growth of oocytes, addressing the formation of structures that compose the follicular complex, as well as the remodeling of the extracellular matrix, specifically laminin, fibronectin and type IV collagen during gonadal maturation. Thirty-seven females of the Acari zebra fish, Hypancistrus zebra were captured and the ovaries were submitted to histological processing for light and electron microscopy and immunohistochemistry techniques. Oogonia and four stages (I - IV) of oocytes were distinguished, and structures such as the postovulatory follicle and atretic oocytes (initial and advanced atresia) were observed. The follicular complex consists of the mature oocyte, zona radiata (Zr1, Zr2 and Zr3), follicular cells, basement membrane and theca. During oocyte growth, proteins of the extracellular matrix showed different intensities of staining. Based on these observations, a model of oocyte growth is proposed to define specific characteristics of the oocyte and the remodeling of the extracellular matrix in the ovary of H. zebra. This model of oocyte growth can be extended to other species of ornamental fishes. This study contributes data for induced fertilization and eventual conservation of this species.