Figure 1.

Centriole structures at key stages of the cell cycle. (A) Centrosome structure in S-phase cells. Each centrosome is composed of two centrioles, surrounded by pericentriolar material (gray cloud). The mother centriole is vertical and has appendages (black triangles) on its distal end. The procentriole is at a right angle and has a cartwheel structure within its proximal end. (B) Centriole structure in procentriole at G₂–M. The centriole has elongated to its full length. Cross sections through the centriole are shown at right. A-, B-, and C-tubules are labeled. (C) Mother centriole at the base of a cilium. The centriole has appendages and lacks a cartwheel. Cross sections through the centriole are shown at right. A-, B-, and C-tubules are labeled.