The formation of both germline cysts and the germinal epithelium is described during the ovary development in Cyprinus carpio. As in the undifferentiated gonad of mammals, cords of PGCs become oogonia when they are surrounded by somatic cells. Ovarian differentiation is triggered when oogonia proliferate and enter meiosis, becoming oocytes. Proliferation of single oogonium results in clusters of interconnected oocytes, the germline cysts, that are encompassed by somatic prefollicle cells and form cell nests. Both PGCs and cell nests are delimited by a basement membrane. Ovarian follicles originate from the germline cysts, about the time of meiotic arrest, as prefollicle cells surround oocytes, individualizing them. They synthesize a basement membrane and an oocyte forms a follicle. With the formation of the stroma, unspecialized mesenchymal cells differentiate, and encompass each follicle, forming the theca. The follicle, basement membrane, and theca constitute the follicle complex. Along the ventral region of the differentiating ovary, the epithelium invaginates to form the ovigerous lamellae whose developing surface epithelium, the germinal epithelium, is composed of epithelial cells, germline cysts with oogonia, oocytes, and developing follicles. The germinal epithelium rests upon a basement membrane. The follicles complexes are connected to the germinal epithelium by a shared portion of basement membrane. In the differentiated ovary, germ cell proliferation in the epithelium forms nests in which there are the germline cysts. Germline cysts, groups of cells that form from a single founder cell and are joined by intercellular bridges, are conserved throughout the vertebrates, as is the germinal epithelium. Anat Rec 293:1581–1606, 2010.

Teleostei present great plasticity regarding sex change. During sex reversal, the whole gonad including the germinal epithelium undergoes significant changes, remodeling, and neoformation. However, there is no information on the changes that occur within the interstitial compartment. Considering the lack of information, especially on the role played by metalloproteinases (MMPs) in fish gonadal remodeling, the aim of this study was to evaluate the action of MMPs on gonads of sex reversed females of Synbranchus marmoratus, a fresh water protogynic diandric fish. Gonads were processed for light microscopy and blood samples were used for the determination of plasma sex steroid levels. During sex reversal, degeneration of the ovaries occurred and were gradually replaced by the germinal tissue of the male. The action of the MMPs induces significant changes in the interstitial compartment, allowing the reorganization of germinal epithelium. Leydig cells also showed an important role in female to male reversion. The gonadal transition coincides with changes in circulating sex steroid levels throughout sex reversion. The action of the MMPs, in the gonadal remodeling, especially on the basement membrane, is essential for the establishment of a new functional germinal epithelium.

Testis differentiation from representatives of the Otophysi (Cyprinus carpio), Percomorpha (Amatitlania nigrofasciata), and Atherinomorpha (Poecilia reticulata) was comparatively described. In the undifferentiated gonad of C. carpio, the primordial germ cells (PGCs) are scattered throughout the gonads while in A. nigrofasciata and P. reticulata the PGCs are restricted to the ventral periphery. In the dorsal region of the developing gonads, with the exception of C. carpio, somatic cell rearrangements result in the differentiation of the sperm duct. Pre‐Sertoli cells wrap around single spermatogonia forming cysts that proliferate forming acinar‐clusters. In C. carpio and A. nigrofasciata, the cysts in each acinar‐cluster move away from each other, creating a central lumen. In C. carpio, the acinar‐clusters then fuse to each other forming tubules that become lined by the germinal epithelium. Subsequently, the tubules anastomose dorsally and create the sperm duct. In A. nigrofasciata, the acinar‐clusters elongate, forming lobules that individually connect to the sperm duct. These are lined by the germinal epithelium. In P. reticulata, the spermatogonial cysts remain in the acinar‐cluster organization. Subsequently, developing ducts connect each cluster to the sperm duct and lobules subsequently develop. In the differentiated testis of C. carpio and A. nigrofasciata, spermatogonia are distributed along the lengths of the anastomosing tubules or lobules, respectively. However, in P. reticulata, the spermatogonia remain restricted to the terminal end of the lobules. Considering testis ontogeny, the spermatogonial acinar‐cluster is the adult characteristic of more derived taxa that approximate the early gonad developmental stages of the basal taxa. Anat Rec, 297:1137–1162, 2014.