290 3-DIMENSIONAL VISUALIZATION OF BOVINE OOCYTE FERTILIZATION BY CONFOCAL LASER SCANNING MICROSCOPY

Abstract

Cattle can serve as a model organism to resolve central questions in mammalian reproduction that cannot be clarified in the mouse model due to notable species-specific peculiarities of early mouse embryogenesis. As part of a project on structural, molecular, and functional deficiencies of bovine oocytes, we started to systematically investigate fertilization and the onset of embryo development in vitro by 3-dimensional multicolor fluorescence microscopy. We are using 3D visualisation as key approach to clarify the multiple parallel and sequential processes and events of fertilization as well as to identify and classify errors and failures. Moreover, we aim to gain insights into the mechanisms of aberrations by linking processes at the cellular and the molecular level. We studied class I and II oocytes collected from slaughterhouse ovaries and matured for 23 h in vitro. Oocytes were fixed at different times from 4 to 12 h postinsemination with formaldehyde in a microtubule-stabilising buffer containing taxol in such a way that the 3-dimensional cell architecture was maintained, and were stained for DNA, microtubules, and f-actin microfilaments. In addition, serine 10-phosphorylated histone H3 was used as a marker for chromosome condensation and the spindle midbody. For 3-dimensional imaging of the oocytes in toto, confocal serial sections were captured at 1-µm intervals using a 40× objective (NA = 1.3). For imaging details, we used a high spatial sampling density (pixel size: 50 × 50 nm, z-step size: 200 nm) close to the Nyquist criterion and image restoration by maximum likelihood estimation (MLE) deconvolution. A series of more than 500 three-dimensional snapshots of fertilized oocytes at different points in time gives a first detailed view on the spatial and temporal course of the sperm entry, the formation of the paternal pronucleus and the sperm aster, completion of oocyte meiosis and the formation of the maternal pronucleus, as well as dynamic changes of the cytoskeleton. Moreover, we can document a spectrum of abnormalities including spontaneous parthenogenetic oocyte activation, polyspermy, and aberrations of meiosis I and II. The latter include irregular spindle formation and chromosome segregation, the occurrence of chromatin bridges and abnormal spindle positioning and rotation (e.g. leading to nonextrusion of a first or a second polar body or the extrusion of two second polar bodies). Our microscopic investigation in the bovine system contributes to unraveling the origins of irregular cleavage, aneuploidy, and mosaicism in mammals. Three-dimensional high-speed microscopy of oocytes and zygotes in affordable timeframes could be of great value in improving the differential diagnosis of oocyte and sperm dysfunction, as well as in identifying and dissecting problems, limitations, and potential risks of reproductive technologies (ART).This work is supported by the Deutsche Forschungsgemeinschaft (DFG FOR 1041).