02679nas a2200241 4500008004100000022002500041245011900066210006900185260001600254300000900270520193000279653001602209653002402225653001602249653001202265653002002277653001602297100001802313700001602331700002202347700002102369856004702390 2024 eng d a0302-766X, 1432-087800aOrganization of the stalk system on electrocytes in mormyrid weakly electric fish Campylomormyrus compressirostris0 aOrganization of the stalk system on electrocytes in mormyrid wea cDec-20-2024 a1-173 a

The adult electric organ in weakly electric mormyrid fish consists of action-potential-generating electrocytes, structurally and functionally modified skeletal muscle cells. The electrocytes have a disc-shaped portion and, on one of its sides, numerous thin processes, termed stalklets. These unite to stalks leading to a single main stalk that carries the innervation site. Here, we describe the 3-dimensional layout of the stalklet/stalk system in adult Campylomormyrus compressirostris by differential interference contrast microscopy and confocal fluorescence microscopy. Using antibodies against Na+/K+-ATPase α-subunit and plasma membrane Ca2+-ATPase, we show that these ion pumps are differentially distributed over the stalklet/stalk system, with plasma membrane Ca2+-ATPase being enriched on the stalklet membrane. Stalklets are distributed and organized in a quite uniform pattern on the posterior face of the electrocyte disc and fuse to terminal stalks. The latter then unite in a mostly dichotomic mode to stalks of increasing thickness, with the main stalk measuring about 100 µm in diameter. We further analyse the structural organization of stalklets and stalks, with a characteristic cytoskeletal system of bundled actin filaments in the centre and nuclei in subsurface position. These results suggest that the stalklet/stalk system is adapted in its structural layout to generate an action potential highly synchronized over the entire disc-portion of the electrocyte, accounting for the short electric organ discharge in this species. Our results suggest that actin-related proteins overexpressed in electrocytes, as shown previously by transcriptome analysis, may be involved in the organization of the unique F-actin system in stalklets and stalks.

10aCa2+-ATPase10aConfocal microscopy10aelectrocyte10aF-actin10aPlasma membrane10aSodium pump1 aBaumann, Otto1 aCheng, Feng1 aKirschbaum, Frank1 aTiedemann, Ralph uhttps://doi.org/10.1007/s00441-024-03938-y03020nas a2200217 4500008004100000245015300041210006900194260001700263520226100280653001702541653001602558653001802574653001502592653001902607100001902626700002202645700002102667700002602688700001602714856007202730 2024 eng d00aA new type of spermiogenesis in teleost fish: formation of the aflagellate sperm in Campylomormyrus compressirostris (Osteoglossomorpha: Mormyridae)0 anew type of spermiogenesis in teleost fish formation of the afla c29 June 20243 a

Osteoglossomorpha, the bony tongue fishes, show great variation in morphology, behavioural strategies, reproductive biology and gamete ultrastructure. The order Osteoglossiformes is the only vertebrate taxon, in which four types of sperm (monoflagellate, biflagellate and aflagellate aquasperm and the complex introsperm) have been described. It is also the only vertebrate lineage in which aflagellate spermatozoa exist. The aim of this study was to analyse the structure of the testis and the process of spermiogenesis in the mormyrid Campylomormyrus compressirostris during the breeding season using light and electron microscopy (transmission and scanning). Males of this species have a single testis of the anastomosing tubular type. The tubules of the anterior part of the testis contain cysts with developing germ cells, and this region is much wider than the posterior part, which consists of efferent ducts filled with sperm cells. The cysts are filled with single or mitotic spermatogonia, primary and secondary spermatocytes and early spermatids. At the stage of spermatids with fine granular chromatin, the cysts rupture and successive stages of spermatid differentiation take place in the testicular lumen; we therefore characterise this process as ‘extracystic spermiogenesis’. Sperm development in C. compressirostris is extremely simple and involves chromatin condensation in the central region of the nucleus, a slight decrease in nuclear volume, the appearance of numerous vesicles in the cytoplasm that form a tubular-vesicular system at the base of the nucleus. Both centrioles and mitochondria are translocated to the peripheral region of the midpiece, which forms the opposite pole to the nucleus. There are many differences between the types of spermiogenesis described so far in teleosts and that found in C. compressirostris, including the loss of flagellum formation. This unique type of spermiogenesis is restricted to species of the families Mormyridae and Gymnarchidae, all of which possess aflagellate spermatozoa. Our data demonstrate that the spermatid differentiation and existence of the aflagellate spermatozoon are a unique phenomena not only among teleosts but also in the whole vertebrate lineage.

10aelephantfish10apermatozoon10aSertoli cells10aspermatids10aTubular testis1 aDymek, Anna, M1 aKirschbaum, Frank1 aTiedemann, Ralph1 aSiemiƄski, Krzysztof1 aPecio, Anna uhttps://www.sciencedirect.com/science/article/pii/S094420062400045X