Bookshelf

(A) Schematic of the first cycle of centrosome duplication that immediately follows fertilization. Figure and electron micrographs courtesy of Amy Maddox and Thomas Müller-Reichert, respectively. Centrosome duplication consists of alternating cycles of new centriole assembly and splitting of the centriole pairs. A pair of centrioles (grey) enters the egg with the sperm during fertilization. The sperm centrioles acquire pericentriolar material (PCM; orange) in the egg and begin to nucleate microtubule asters. New daughter centrioles (green) assemble adjacent to each of the sperm centrioles so that by metaphase each centrosome contains two full-length centrioles, one inherited from the sperm and one that formed in the embryo cytoplasm. As the embryo enters mitosis, the amount of PCM around the centrioles and the number of microtubules nucleated by the centrosomes increases in a process called centrosome maturation. The centriole pairs split in late anaphase/telophase and the mitotic PCM breaks down, so that each daughter cell inherits a pair of small centrosomes each containing a single centriole. (B) Two images of prometaphase/metaphase centrioles with singlet microtubules in either cross section (arrows) or a longitudinal orientation (arrowhead) are shown. Bar is 250 nm.