Ovaries of Acipenser baerii are of an alimentary type and probably are meroistic. They contain ovarian nests, individual follicles, inner germinal ovarian epithelium, and fat tissue. Nests comprise cystoblasts, germline cysts, numerous early previtellogenic oocytes, and somatic cells. Cysts are composed of cystocytes, which are connected by intercellular bridges and are in the pachytene stage of the first meiotic prophase. They contain bivalents, finely granular, medium electron dense material, and nucleoli in the nucleoplasm. Many cystocytes degenerate. Oocytes differ in size and structure. Most oocytes are in the pachytene and early diplotene stages and are referred to as the PACH oocytes. Oocytes in more advanced diplotene stage are referred to as the DIP oocytes. Nuclei in the PACH oocytes contain bivalents and irregularly shaped accumulation of DNA (DNA-body), most probably corresponding to the rDNA-body. The DNA-body is composed of loose, fine granular material, and comprises multiple nucleoli. At peripheries, it is fragmented into blocks that remain in contact with the inner nuclear membrane. In the ooplasm, there is the rough endoplasmic reticulum, Golgi complexes, free ribosomes, complexes of mitochondria with cement, fine fibrillar material containing granules, and lipid droplets. The organelles and material of nuclear origin form a distinct accumulation (a granular ooplasm) in the vicinity of the nucleus. Some of the PACH oocytes are surrounded by flat somatic cells. There are lampbrush chromosomes and multiple nucleoli present (early diplotene stage) in the nucleoplasm. These PACH oocytes and neighboring somatic cells have initiated the formation of ovarian follicles. The remaining PACH oocytes transform to the DIP oocytes. The DIP oocytes contain lampbrush chromosomes and a DNA-body is absent in nuclei. Multiple nucleoli are numerous in the nucleoplasm and granular ooplasm is present at the vegetal region of the oocyte.
It is a first report on the structure of germline cells in ovaries of albino sterlet
Acipenser ruthenus
L. 1758. Ovarian nests, follicles, and germinal epithelium have been examined in gynogenetic and control specimens of this species. The structure of oogonia (named the cystoblasts) and of germline cysts in the nests has been described in detail. Also, the asymmetry in the cytoplasm and early growth of cystocytes in the cysts and of early previtellogenic oocytes has been described. In the cytoplasm of cystoblasts and in all cystocytes, a precursor of granular cytoplasm (Balbiani cytoplasm) is present and defines future vegetal region in the oocytes. Interestingly, the nuclei in cystoblasts comprise a large dense body that contains deoxyribonucleic acid (DNA). The role of this body in formation of multiple nucleoli has been explained. During the zygotene and pachytene stages, massive extrachromosomal amplification of DNA begins in the nucleoplasm of all cystocytes. As a result of the accumulation of extra DNA, an irregularly shaped DNA-body is formed. Multiple nucleoli arise in this DNA-body and around fragments of dense bodies. The asymmetry of the early previtellogenic oocyte cytoplasm is well marked by the presence of the granular cytoplasm. Moreover, the cisternae of the rough endoplasmic reticulum, dictyosomes, mitochondria, complexes of mitochondria with cement, nuage accumulations, and lipid droplets are located in specific zones in the granular cytoplasm. The follicular epithelium is composed of two subpopulations of somatic follicular cells (FCs): the main body cells and future micropylar cells.
Specimens from two freshwater populations of the ninespine stickleback Pungitius pungitius in Poland showed morphologically differentiated sex chromosomes. A heteromorphic pair of chromosomes appeared only in male diploid cells. The Y chromosome was the largest chromosome in the P. pungitius karyotype.
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