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. Author manuscript; available in PMC: 2015 Aug 1.
Published in final edited form as: Nat Chem. 2014 Dec 15;7(2):130–139. doi: 10.1038/nchem.2133

Figure 3. Zinc fixation enables ultrastructural identification of zinc-enriched cortical compartments by Scanning Transmission Electron Microscopy with Energy Dispersive Spectroscopy (STEM-EDS).

Figure 3

(a) Zinc-fixation schematic. Eggs were fixed and treated with NaHS to form ZnS. Following ethanol dehydration and resin embedding, eggs were used intact for X-ray fluorescence (XFM) tomography or sectioned prior to STEM-EDS or XFM Bionanoprobe analysis.

(b) Diagram of STEM microscope with dual EDS detectors for zinc mapping.49

(c) Z-contrast image of a 200 nm section of a resin-embedded MII egg following zinc fixation. Vesicles, zona pellucida (ZP), plasma membrane (PM) and ooplasm indicated. Bright and dark areas indicate regions with high and low molecular weight content respectively. Bright signal is concentrated in cortical compartments. Scale bar = 0.5 µm.

(d) Histogram of diameters of cortical compartments in STEM-EDS samples (20 nm bins). Distribution centers on a diameter of ~260 nm. Data from 23 Zn-enriched compartments from 8 eggs.

(e) EDS spectra of bright compartment (blue) and cytoplasm (red). Zinc signal is enriched in the compartment relative to the cytoplasm.

(f) Z-contrast, Zn and S EDS maps of a cortical region in a MII egg. Overlay demonstrates that Zn-rich regions correspond to areas high in S and electron density (Z-contrast).