Long-term mitochondria-endoplasmic reticulum complexes in the oocytes and early embryos of a matrotrophic pseudoscorpion Chelifer cancroides

Mitochondria are considered key organelles for proper oocyte growth, maturation, fertilization, and embryo development. During oogenesis, they have been found to be highly dynamic organelles that interact with other cellular components. In this study, we analyzed the morphology and behavior of mitochondria in the oocytes and early embryos of the cosmopolitan pseudoscorpion Chelifer cancroides. Analyses were carried out using light, confocal, and transmission electron microscopy. Our results show that, in the early stages of oocyte growth, mitochondria gather close to the germinal vesicle within an organelle assemblage termed the Balbiani body (Bb). In the Bb, mitochondria increase in number and display a high membrane potential. In advanced previtellogenesis, when the Bb disperses, the mitochondria gradually populate the entire ooplasm and join the endoplasmic reticulum to form mitochondria-endoplasmic reticulum (mt-ER) complexes. Within these complexes, mitochondria significantly change morphology and reduce activity. The mt-ER complexes persist throughout oocyte growth until early embryogenesis. During cleavage, they are unevenly segregated to the micromeres that form the embryo body. Our results suggest that in Chelifer, the Balbiani body plays a role in mitochondrial multiplication. We also discuss the role of the mt-ER complexes and propose that they promote low activity of mitochondria to protect mtDNA and supply the embryo with substantial organelles for the early stages of development.