Pre-meiotic transformation of germplasm-related structures during male gamete differentiation in Xenopus laevis

SummaryTo highlight the ultrastructural features of transformation occurring with germplasm-related structures (GPRS), the spermatogenic cells of Xenopus laevis were studied by transmission electron microscopy and quantitative analysis. It was determined that in spermatogonia and spermatocytes, the compact germinal granules underwent fragmentation into particles comparable with inter-mitochondrial cement (IMC). Fragments of IMC agglutinated some cell mitochondria and resulted in the creation of mitochondrial clusters. Clustered mitochondria responded with loss of their membranes that occurred by the twisting of membranous protrusions around themselves until multi-layered membranes were formed. The mitochondrial affinity of multi-layered membranes was proven by an immunopositive test for mitochondrial dihydrolipoamide acetyltransferase. As a consequence of mitochondrial membrane twisting, the naked mitochondrial cores appeared and presumably underwent dispersion, which is the terminal stage of GPRS transformation. As no GPRS were observed in spermatids and sperm, it was assumed that these structures are functionally assigned to early stages of meiotic differentiation.

Premeiotic transformation of germ plasm-related structures during the sea urchin spermatogenesis

SummaryThe germ plasm-related structures (GPRS) and the transformation that occurs to them during the spermatogenesis of the sea urchin Anthocidaris crassispina were studied by electron microscopy and morphometry. The GPRS were observed in spermatogonia and spermatocytes, but not in spermatids and sperm, which suggests an important role for these structures during the onset of meiosis. It was proposed that the germinal granules are fragmented into the compact electron-dense nuage, and fragments of the latter penetrate into the periphery of the compact electron-lucent nuage. The process of nuage integration is completed with the formation of the combined nuage, which aggregates some mitochondria into clusters. Once formed, the mitochondrial clusters undergo dissemination and assume the appearance of the dispersed nuage with mitochondrial derivatives, which in turn develops into the scattered nuage. The scattered nuage, which presumably presents the composite mixture saturated with mitochondrial matrix, terminates the GPRS transformation.

Translational control in early development: CPEB, P-bodies and germinal granules

Selective protein synthesis in oocytes, eggs and early embryos of many organisms drives several critical aspects of early development, including meiotic maturation and entry into mitosis, establishment of embryonic axes and cell fate determination. mRNA-binding proteins which (usually) recognize 3′-UTR (untranslated region) elements in target mRNAs influence the recruitment of the small ribosomal subunit to the 5′ cap. Probably the best studied such protein is CPEB (cytoplasmic polyadenylation element-binding protein), which represses translation in the oocyte in a cap-dependent manner, and activates translation in the meiotically maturing egg, via cytoplasmic polyadenylation. Co-immunoprecipitation and gel-filtration assays revealed that CPEB in Xenopus oocytes is in a very large RNP (ribonucleoprotein) complex and interacts with other RNA-binding proteins including Xp54 RNA helicase, Pat1, RAP55 (RNA-associated protein 55) and FRGY2 (frog germ cell-specific Y-box protein 2), as well as the eIF4E (eukaryotic initiation factor 4E)-binding protein 4E-T (eIF4E-transporter) and an ovary-specific eIF4E1b, which binds the cap weakly. Functional tests which implicate 4E-T and eIF4E1b in translational repression in oocytes led us to propose a model for the specific inhibition of translation of a target mRNA by a weak cap-binding protein. The components of the CPEB RNP complex are common to P-bodies (processing bodies), neuronal granules and germinal granules, suggesting that a highly conserved ‘masking’ complex operates in early development, neurons and somatic cells.