Figure 4: Electron tomography (ET) analyses of mitochondria in spermatozoa and embryos.

(a,b) An ET slice image of mitochondria in an N2 spermatozoon (a). Mitochondria are enclosed by smooth and intact membranes (bracket in b). MO: membranous organelle. (c) An ET image of the cytoplasm of an N2 zygote. Maternal (arrows) and paternal (arrowheads) mitochondria are indicated. (d,e) An ET slice image of sperm mitochondria in an N2 zygote (d). Their mitochondrial membranes appear fuzzy (bracket) and have cracks (arrowheads) and their matrices contain electron dense spots (arrows) distinctive of paternal mitochondria (e). (f,g) 3D ET models of paternal mitochondria in an N2 spermatozoon (f) and an N2 zygote (g), generated from tomograms shown in b and e, respectively. Arrowheads in g mark punctures in the mitochondrial membranes seen as cracks in e. Mitochondrial membranes, cristae and speckles are coloured in red, green and blue, respectively. (h,i) Electron micrographs of fertilized eggs from the indicated crosses. Round mitochondria in the fzo-1(tm1133) egg contrast with elongated mitochondria in the N2 egg (arrows). Some fzo-1(tm1133) maternal mitochondria have a dark inclusion (red arrows). The dark spherical objects in h are lipid droplets. (j) Pie charts showing ratios of inclusion-positive to inclusion-negative maternal mitochondria in N2 (n=392) and fzo-1 (n=446) fertilized eggs. (k,l) Aberrant maternal mitochondria in fzo-1(tm1133) embryos with dark inclusions (arrowheads). Some of their cristae transform into long lamellae (arrows in l). (m,n) An ET slice image (m) of the cytoplasm of a fertilized fzo-1(tm1133) egg and 3D models of the mitochondria in m (n). Maternal (magenta arrowheads) and paternal (orange arrows) mitochondria in fzo-1(tm1133) eggs are easily differentiated by their distinctive features of electron-dense patches and cristae. One maternal mitochondrion is enclosed by an autophagosome (AuPh). A high magnification view of Auph and the maternal mitochondrion (green rectangle in m) is shown in the inset in n. The blue blobs in paternal mitochondria indicate the electron dense patches in m. The double membranes of the autophagosome are marked with arrowheads in the inset. Scale bars indicate 500 nm (a,c,d,h,i,k–m) or 100 nm (b,e and inset in n).