Cytoplasmic lattices are not linked to mouse 2-cell embryos developmental arrest

Abstract

Cytoplasmic lattices are important regulators of oocyte maturation. They store components of the protein synthesis machinery including ribosomes and, among others, they are involved in the regulation of microtubule dynamics in both mouse and human. Cytoplasmic lattices undergo dramatic reorganizations at crucial stages of oocyte maturation, where they are abundantly present in the cytoplasm of developmentally competent oocytes named SN (Surrounded Nucleolus) while they are rare in the cytoplasm of 2-cell stage-arresting NSN (Not Surrounded Nucleolus) oocytes, suggestive of a requirement of cytoplasmic lattices for development past the 2-cell stage. Here, to elucidate this requirement, 2-cell mouse embryos derived from SN and NSN oocytes were analyzed by transmission electron microscopy. Contrary to what had been proposed hitherto, cytoplasmic lattices are present in 2-cell embryos derived not only from SN, but also from NSN oocytes, irrespective of the embryo production system (intra cytoplasmic sperm injection, parthenogenesis). Hence our conclusion that cytoplasmic lattices do not count among the factor(s) responsible for the embryo arrest at this crucial stage of development.

Figure 1.

Representative TEM images of a) parthenogenetic NSN-derived 2-cell embryo and b) parthenogenetic SN-derived 2-cell embryo; c) NSN-derived 2-cell embryo obtained by ICSI; d) SN-derived 2-cell embryo obtained by ICSI. Arrows point to cytoplasmic lattices.

Figure 2.

Integrated density evaluation (mean value ± SD in arbitrary units, a.u.) of detectable CPLs in cytoplasmic representative areas (see materials and methods) of PN and PS (Parthenogenetic activated surrounded (S) and not-surrounded (N) nucleolus oocytes) and ICSI S and ICSI N (Intracytoplasmic Sperm Injected SN and NSN oocytes).

Figure 3.

Representative TEM images of a) parthenogenetic NSN-derived 2-cell embryo with vesicular mitochondria (black arrows and black asterisk); b) SN-derived 2-cell embryo obtained by ICSI showing non-defective mitochondria (white asterisk); a’ and b’ magnification of a vesicular and normal mitochondria respectively.