A novel approach for 3D reconstruction of mice full-grown oocytes by time-of-flight secondary ion mass spectrometry.

Currently two techniques exist for 3D reconstruction of biological samples by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The first, based on microtomy and combining of successive section images, is successfully applied for tissues, while the second, based on sputter depth profiling, is widely used for cells. In the present work, we report the first successful adaptation of sectioning technique for ToF-SIMS 3D imaging of a single cell-fully grown mouse germinal vesicle (GV) oocyte. In addition, microtomy was combined with sputter depth profiling of individual flat sections for three-dimensional reconstruction of intracellular organelles. GV oocyte sectioning allowed us to obtain molecule-specific 3D maps free from artifacts associated with surface topography and uneven etching depth. Sputter depth profiling of individual flat slices revealed fine structure of specific organelles inside the oocyte. Different oocyte organelles (cytoplasm, germinal vesicle, membranes, cumulus cells) were presented on the ion images. Atypical nucleoli referred to as "nucleolus-like body" (NLB) was detected inside the germinal vesicle in PO and CN ions generated by nucleic acids and proteins respectively. Significant difference in PO intensity in the NLB central area and NLB border was found. This difference appears as a bright halo around the center area. The NLB size calculated for PO and CN ion images is 12.9 ± 0.2 μm and 11.9 ± 0.2 μm respectively, which suggests that bright halo of PO ions is a chromatin compaction on the NLB surface. Areas of approximately 1.0-2.5 μm size inside nucleoplasm with increased PO and CN signal were registered in germinal vesicle. Observed compartments have different sizes and shapes, and they are likely attributed to chromocenters or chromosomes.