Sardet C , Prodon F , Dumollard R , Chang P , Chênevert J .

FIG. 1. Views of the egg cortex. Additional material can be found on our Web site: http://www.obs-vlfr.fr/biomarcell.html. Eggs and isolated cortices of sea urchin (A–H), ascidian (I–M), and amphibian (N). The ER is colorized orange/red in D, E, K, L, M, and N; ribosomes on the ER are colorized yellow in K, L, and N; the CGs are colorized ocre in D, E, and F; the MFs are colorized green in K, L, and N; the MTs are colorized dark blue in K; the cytoplasmic face of the PM is light brown in K, L, and N; and coated plaques and vesicles are colorized light blue in L and N. (A) Living eggs of the sea urchin Paracentrotus lividus display a band of pigmented vesicles (b), one of the earliest described cortical markers of animal (a)–vegetal (v) polarity (bright field optics, blue filter). (B) Isolated cortices attached to a polylysine coated surface. The band of pigmented vesicles (b) is retained in cortices. The cortex in the lower right corner has also retained cortical granules (dark field optics, blue filter). (C) Isolated cortices (cortical hulls) suspended in homogenization milieu (dark field optics). (D) An egg (top left) and an isolated cortex (bottom right) side by side. In the isolated cortex, the cER network (er) and cortical granules (cg) remain attached to the plasma membrane (pm) which is covered by microvilli (mv) (electron micrograph (EM) thin section). (E) Oblique tangential section through an isolated cortex showing the cER network (er) surrounding cortical granules (cg). Microvilli (mv) and plasma membrane (pm) are on top (EM, thin section). (F) External face of the egg plasma membrane (pm) studded with short micropapillae. A lawn of cortical granules is situated beneath (EM, freeze fracture replica). (G and H) The same isolated egg cortex. The cortical granules (cg) visualized in G are surrounded by cER (er) labeled with the lipophilic dye DiI(C16)3 in H (DIC optics in G and fluorescence microscopy in H). (I) A fertilized egg of the ascidian Phallusia mammillata. The animal (a)–vegetal (v) polarity is evident by the concentration of actin microfilaments (labeled with fluorescent Phalloidin) which forms near the vegetal pole (v) 5 min after fertilization (confocal microscopy). (J) Cortex isolated from the vegetal (v) region of an egg 5 min after fertilization (such as the egg in I). The cER network labeled with a lipophilic dye (red) and MF network labeled with fluorescent phalloidin (green) are concentrated in the vegetal (v) pole (confocal microscopy). (K) Fragment of isolated cortex from an unfertilized egg showing the networks of cER (er), microfilaments (mf), and microtubules (mt) (EM, fast freeze/deep etch replica). (L) Detail of an isolated cortex like that shown in K. The cER (er) is studded with ribosomes and particles. Microfilaments (mf) and coated plaques (cp) are visible as well as the cytoplasmic face of the plasma membrane (EM, fast freeze/deep etch replica). (M) Vegetal cortical region of an egg showing the cER (er) and one mitochondrion (mi) (EM, thin section). (N) Fragment of a Xenopus oocyte cortex isolated during vitellogenesis. A large number of coated plaques and vesicles (cp) are present on the underside of the plasma membrane (EM, fast freeze/deep etch replica).

FIG. 2. Transformation of the cortex during activation. This schematic representation is inspired from the work done on sea urchins and Xenopus eggs where major exocytotic and endocytic events take place after the egg is fertilized. (A) Unfertilized egg cortex phenotype. The plasma membrane (grey) has a cell surface coat (black) and its cytoplasmic face comprises coated plaques (blue), short microfilaments (green), cortical vesicles (cortical granules: ocre, acidic/pigmented vesicles: blue) and an attached network of cortical endoplasmic reticulum (red) studded with ribosomes (yellow). (B) At fertilization, the calcium wave triggers waves of cortical reorganizations. Cortical granules fuse with the plasma membrane (I left), exocytosing structural proteins and enzymes which form the fertilization membrane. After the passage of the calcium wave, endocytic activity is stimulated, the endoplasmic reticulum is fragmented (II), and microfilaments polymerize and bundle in elongating microvilli and underneath the plasma membrane (II middle, and III right). (C) Fertilized egg cortex phenotype. Microvilli have elongated and bundles of microfilaments are abundant underneath the plasma membrane. Acidic/pigmented vesicles have moved closer to the plasma membrane.