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A centrosome consists of two barrel-shaped centrioles embedded in a matrix of proteins known as the pericentriolar material (PCM). The PCM serves as a platform for protein complexes that regulate organelle trafficking, protein degradation and spindle assembly. Perhaps most important for cell division, the PCM concentrates tubulin and serves as the primary organizing centre for microtubules in metazoan somatic cells. Thus, similar to other well-described organelles, such as the nucleus and mitochondria, the cell has compartmentalized a multitude of vital biochemical reactions in the PCM. However, unlike these other organelles, the PCM is not membrane bound, but rather a dynamic collection of protein complexes and nucleic acids that constitute the organelle's interior and determine its boundary. How is the complex biochemical machinery necessary for the myriad centrosome functions concentrated and maintained in the PCM? Recent advances in proteomics and RNAi screening have unveiled most of the key PCM components and hinted at their molecular interactions ( table 1). Now we must understand how the interactions between these molecules contribute to the mesoscale organization and the assembly of the centrosome. Among outstanding questions are the intrinsic mechanisms that determine PCM shape and size, and how it functions as a biochemical reaction hub.

Centrosomes are the main microtubule-organizing centre of animal cells and are important for many critical cellular and developmental processes from cell polarization to cell division. At the core of the centrosome are centrioles, which recruit pericentriolar material to form the centrosome and act as basal bodies to nucleate formation of cilia and flagella. Defects in centriole structure, function and number are associated with a variety of human diseases, including cancer, brain diseases and ciliopathies. In this review, we discuss recent advances in our understanding of how new centrioles are assembled and how centriole number is controlled. We propose a general model for centriole duplication control in which cooperative binding of duplication factors defines a centriole ‘origin of duplication’ that initiates duplication, and passage through mitosis effects changes that license the centriole for a new round of duplication in the next cell cycle. We also focus on variations on the general theme in which many centrioles are created in a single cell cycle, including the specialized structures associated with these variations, the deuterosome in animal cells and the blepharoplast in lower plant cells.

Centrioles are among the largest protein-based structures found in most cell types, measuring approximately 250 nm in diameter and approximately 500 nm long in vertebrate cells. Here, we briefly review ultrastructural observations about centrioles and associated structures. At the core of most centrioles is a microtubule scaffold formed from a radial array of nine triplet microtubules. Beyond the microtubule triplets of the centriole, we discuss the critically important cartwheel structure and the more enigmatic luminal density, both found on the inside of the centriole. Finally, we discuss the connectors between centrioles, and the distal and subdistal appendages outside of the microtubule scaffold that reflect centriole age and impart special functions to the centriole. Most of the work we review has been done with electron microscopy or electron tomography of resin-embedded samples, but we also highlight recent work performed with cryoelectron microscopy, cryotomography and subvolume averaging. Significant opportunities remain in the description of centriolar structure, both in mapping of component proteins within the structure and in determining the effect of mutations on components that contribute to the structure and function of the centriole.

The cartwheel is a subcentriolar structure consisting of a central hub and nine radially arranged spokes, located at the proximal end of the centriole. It appears at the initial stage of the centriole assembly process as the first ninefold symmetrical structure. The cartwheel was first described more than 50 years ago, but it is only recently that its pivotal role in establishing the ninefold symmetry of the centriole was demonstrated. Significant progress has since been made in understanding its fine structure and assembly mechanism. Most importantly, the central part of the cartwheel, from which the ninefold symmetry originates, is shown to form by self-association of nine dimers of the protein SAS-6. This finding, together with emerging data on other components of the cartwheel, has opened new avenues in centrosome biology.

The cartwheel is a subcentriolar structure consisting of a central hub and nine radially arranged spokes, located at the proximal end of the centriole. It appears at the initial stage of the centriole assembly process as the first ninefold symmetrical structure. The cartwheel was first described more than 50 years ago, but it is only recently that its pivotal role in establishing the ninefold symmetry of the centriole was demonstrated. Significant progress has since been made in understanding its fine structure and assembly mechanism. Most importantly, the central part of the cartwheel, from which the ninefold symmetry originates, is shown to form by self-association of nine dimers of the protein SAS-6. This finding, together with emerging data on other components of the cartwheel, has opened new avenues in centrosome biology.

A centrosome consists of two barrel-shaped centrioles embedded in a matrix of proteins known as the pericentriolar material (PCM). The PCM serves as a platform for protein complexes that regulate organelle trafficking, protein degradation and spindle assembly. Perhaps most important for cell division, the PCM concentrates tubulin and serves as the primary organizing centre for microtubules in metazoan somatic cells. Thus, similar to other welldescribed organelles, such as the nucleus and mitochondria, the cell has compartmentalized a multitude of vital biochemical reactions in the PCM. However, unlike these other organelles, the PCM is not membrane bound, but rather a dynamic collection of protein complexes and nucleic acids that constitute the organelle's interior and determine its boundary. How is the complex biochemical machinery necessary for the myriad centrosome functions concentrated and maintained in the PCM? Recent advances in proteomics and RNAi screening have unveiled most of the key PCM components and hinted at their molecular interactions (table 1). Now we must understand how the interactions between these molecules contribute to the mesoscale organization and the assembly of the centrosome. Among outstanding questions are the intrinsic mechanisms that determine PCM shape and size, and how it functions as a biochemical reaction hub.

Centrioles are among the largest protein-based structures found in most cell types, measuring approximately 250 nm in diameter and approximately 500 nm long in vertebrate cells. Here, we briefly review ultrastructural observations about centrioles and associated structures. At the core of most centrioles is a microtubule scaffold formed from a radial array of nine triplet microtubules. Beyond the microtubule triplets of the centriole, we discuss the critically important cartwheel structure and the more enigmatic luminal density, both found on the inside of the centriole. Finally, we discuss the connectors between centrioles, and the distal and subdistal appendages outside of the microtubule scaffold that reflect centriole age and impart special functions to the centriole. Most of the work we review has been done with electron microscopy or electron tomography of resin-embedded samples, but we also highlight recent work performed with cryoelectron microscopy, cryotomography and subvolume averaging. Significant opportunities remain in the description of centriolar structure, both in mapping of component proteins within the structure and in determining the effect of mutations on components that contribute to the structure and function of the centriole.

Centrosomes are the main microtubule-organizing centre of animal cells and are important for many critical cellular and developmental processes from cell polarization to cell division. At the core of the centrosome are centrioles, which recruit pericentriolar material to form the centrosome and act as basal bodies to nucleate formation of cilia and flagella. Defects in centriole structure, function and number are associated with a variety of human diseases, including cancer, brain diseases and ciliopathies. In this review, we discuss recent advances in our understanding of how new centrioles are assembled and how centriole number is controlled. We propose a general model for centriole duplication control in which cooperative binding of duplication factors defines a centriole 'origin of duplication' that initiates duplication, and passage through mitosis effects changes that license the centriole for a new round of duplication in the next cell cycle. We also focus on variations on the general theme in which many centrioles are created in a single cell cycle, including the specialized structures associated with these variations, the deuterosome in animal cells and the blepharoplast in lower plant cells.