Effects of Simulated Microgravity In Vitro on Human Metaphase II Oocytes: An Electron Microscopy-Based Study

Abstract

The Gravity Force to which living beings are subjected on Earth rules the functionality of most biological processes in many tissues. It has been reported that a situation of Microgravity (such as that occurring in space) causes negative effects on living beings. Astronauts returning from space shuttle missions or from the International Space Station have been diagnosed with various health problems, such as bone demineralization, muscle atrophy, cardiovascular deconditioning, and vestibular and sensory imbalance, including impaired visual acuity, altered metabolic and nutritional status, and immune system dysregulation. Microgravity has profound effects also on reproductive functions. Female astronauts, in fact, suppress their cycles during space travels, and effects at the cellular level in the early embryo development and on female gamete maturation have also been observed. The opportunities to use space flights to study the effects of gravity variations are limited because of the high costs and lack of repeatability of the experiments. For these reasons, the use of microgravity simulators for studying, at the cellular level, the effects, such as those, obtained during/after a spatial trip, are developed to confirm that these models can be used in the study of body responses under conditions different from those found in a unitary Gravity environment (1 g). In view of this, this study aimed to investigate in vitro the effects of simulated microgravity on the ultrastructural features of human metaphase II oocytes using a Random Positioning Machine (RPM). We demonstrated for the first time, by Transmission Electron Microscopy analysis, that microgravity might compromise oocyte quality by affecting not only the localization of mitochondria and cortical granules due to a possible alteration of the cytoskeleton but also the function of mitochondria and endoplasmic reticulum since in RPM oocytes we observed a switch in the morphology of smooth endoplasmic reticulum (SER) and associated mitochondria from mitochondria-SER aggregates to mitochondria–vesicle complexes. We concluded that microgravity might negatively affect oocyte quality by interfering in vitro with the normal sequence of morphodynamic events essential for acquiring and maintaining a proper competence to fertilization in human oocytes.

The Oxidative Phosphorylation and Cytoskeleton Proteins of Mouse Ovaries after 96 Hours of Hindlimb Suspension

“…However, on the other hand, the development of oxidative stress occurs when there is an imbalance in the production of ROS and the antioxidant system [28] and, moreover, can lead to abnormal mitochondrial fission [29]. According to electron microscopy data, when cells were exposed to simulated microgravity, the fissions took place, but the number of mitochondria did not change [7]. This can be achieved by increasing the content of other respiratory chain complexes.…”

“…Cultivation of mouse oocytes for 6 h in a random positioning machine leads to reorganization of the cytoskeleton and redistribution of actin from the cortical layer to the center of the cell and tubulin from the center to the cortical cytoskeleton [6]. With longer (24 h) cultivation of human oocytes, it was shown by transmission electron microscopy that the perivitelline space and the thickness of the zona pellucida asymmetrically increase, resulting in the redistribution of cortical granules, the fusion of mitochondria, and a change in the morphology of the smooth endoplasmic reticulum [7].…”

Receptors Implicated in Microgravity-Induced Bone Loss

“…At the cellular level, changes in fluid distribution may lead to cytoskeletal filament reorganizations [77,181,200]. Since the cytoskeleton is linked to cell polarity, cell adhesions, and signaling associated with cell-cell and cell-matrix adhesions, as well as from ligand receptors, such nanoscale alterations could represent major players in the changes that trigger microgravity-induced bone loss [77,[201][202][203].…”

Heavy Metals in Follicular Fluid Affect the Ultrastructure of the Human Mature Cumulus-Oocyte Complex

“…Oocytes obtained from ovarian stimulation were inseminated to produce a maximum of three embryos (Table 1) and then transferred into the uterus. Supernumerary oocytes (1-3/woman), corresponding with respective aspired FF, surrounded by intact CCs with homogeneous cytoplasm, were subjected to ultrastructural analyses according to Miglietta et al (2023) [19].…”