Early stages of ciliogenesis in the respiratory epithelium of the nasal cavity of rabbit embryos

Centriole amplification by mother and daughter centrioles differs in multiciliated cells

The semi-conservative centrosome duplication in cycling cells gives rise to a centrosome composed of a mother and a newly formed daughter centriole. Both centrioles are regarded as equivalent in their ability to form new centrioles and their symmetric duplication is crucial for cell division homeostasis. Multiciliated cells do not use the archetypal duplication program and instead form more than a hundred centrioles that are required for the growth of motile cilia and the efficient propelling of physiological fluids. The majority of these new centrioles are thought to appear de novo, that is, independently from the centrosome, around electron-dense structures called deuterosomes. Their origin remains unknown. Using live imaging combined with correlative super-resolution light and electron microscopy, we show that all new centrioles derive from the pre-existing progenitor cell centrosome through multiple rounds of procentriole seeding. Moreover, we establish that only the daughter centrosomal centriole contributes to deuterosome formation, and thus to over ninety per cent of the final centriole population. This unexpected centriolar asymmetry grants new perspectives when studying cilia-related diseases and pathological centriole amplification observed in cycling cells and associated with microcephaly and cancer.

Emerging roles for tubulin folding cofactors at the centrosome

Despite its fundamental role in centrosome biology, procentriole formation, both in the canonical and in the de novo replication pathways, remains poorly understood, and the molecular components that are involved in human cells are not well established. We found that one of the tubulin cofactors, TBCD, is localized at centrosomes and the midbody, and is required for spindle organization, cell abscission, centriole formation and ciliogenesis. Our studies have established a molecular link between the centriole and the midbody, demonstrating that this cofactor is also necessary for microtubule retraction during cell abscission. TBCD is the first centriolar protein identified that plays a role in the assembly of both "centriolar rosettes" during early ciliogenesis, and at the procentriole budding site by S/G(2), a discovery that directly implicates tubulin cofactors in the cell division, cell migration and cell signaling research fields.

Centrioles to basal bodies in the spermiogenesis of Mastotermes darwiniensis (Insecta, Isoptera)

In addition to their role in centrosome organization, the centrioles have another distinct function as basal bodies for the formation of cilia and flagella. Centriole duplication has been reported to require two alternate assembly pathways: template or de novo. Since spermiogenesis in the termite Mastotermes darwiniensis lead to the formation of multiflagellate sperm, this process represents a useful model system in which to follow basal body formation and flagella assembly. We present evidence of a possible de novo pathway for basal body formation in the differentiating germ cell. This cell also contains typical centrosomal proteins, such as centrosomin, pericentrin-like protein, gamma-tubulin, that undergo redistribution as spermatid differentiation proceeds. The spermatid centrioles are long structures formed by nine doublet rather than triplet microtubules provided with short projections extending towards the surrounding cytoplasm and with links between doublets. The sperm basal bodies are aligned in parallel beneath the nucleus. They consist of long regions close to the nucleus showing nine doublets in a cartwheel array devoid of any projections; on the contrary, the short region close to the plasma membrane, where the sperm flagella emerge, is characterized by projections similar to those observed in the centrioles linking the basal body to the plasma membrane. It is hypothesized that this appearance is in connection with the centriole elongation and further with the flagellar axonemal organization. Microtubule doublets of sperm flagellar axonemes are provided with outer dynein arms, while inner arms are rarely visible.

Adult ependymal cells are postmitotic and are derived from radial glial cells during embryogenesis

Ependymal cells on the walls of brain ventricles play essential roles in the transport of CSF and in brain homeostasis. It has been suggested that ependymal cells also function as stem cells. However, the proliferative capacity of mature ependymal cells remains controversial, and the developmental origin of these cells is not known. Using confocal or electron microscopy (EM) of adult mice that received bromodeoxyuridine (BrdU) or [3H]thymidine for several weeks, we found no evidence that ependymal cells proliferate. In contrast, ependymal cells were labeled by BrdU administration during embryonic development. The majority of them are born between embryonic day 14 (E14) and E16. Interestingly, we found that the maturation of ependymal cells and the formation of cilia occur significantly later, during the first postnatal week. We analyzed the early postnatal ventricular zone at the EM and found a subpopulation of radial glia in various stages of transformation into ependymal cells. These cells often had deuterosomes. To directly test whether radial glia give rise to ependymal cells, we used a Cre-lox recombination strategy to genetically tag radial glia in the neonatal brain and follow their progeny. We found that some radial glia in the lateral ventricular wall transform to give rise to mature ependymal cells. This work identifies the time of birth and early stages in the maturation of ependymal cells and demonstrates that these cells are derived from radial glia. Our results indicate that ependymal cells are born in the embryonic and early postnatal brain and that they do not divide after differentiation. The postmitotic nature of ependymal cells strongly suggests that these cells do not function as neural stem cells in the adult.

Cell Biology of Normal and Abnormal Ciliogenesis in the Ciliated Epithelium

Ciliogenesis is divided into four stages: (1) generation of centrioles, (2) migration of duplicated centrioles, (3) formation of the basal body-associated structures, and (4) elongation of cilia. The ultrastructural profile of ciliogenesis is fundamentally the same among various kinds of animal species. In acentriolar centriologenesis, centrioles are generated around deuterosomes by the use of fibrous granules. Components of the centriolar precursor structures, and genes that regulate the differentiation of ciliated cells, have been revealed. Ciliary abnormalities are classified into two categories: specific congenital defects of ciliary structure and acquired nonspecific anomalies of the ciliary apparatus. When ciliogenesis is disturbed, various nonspecific ciliary abnormalities develop in the cell. Inhibition of centriole migration results in the development of intracytoplasmic axonemes, cilia within periciliary sheaths, and intracellular ciliated vacuoles. Swollen cilia and the bulging type of compound cilia are formed during ciliary budding and elongation. Primary cilia can also develop from one of a pair of centrioles. They lack dynein arms and are immobile, but work as a mechanosensor and play a role during morphogenesis of the kidney. Abnormal function or structure of primary cilia results in the development of polycystic kidney disease. The axonemes of primary cilia or monocilia in the embryonic node cells are associated with dynein arms and move vortically. They have a role in determining the left-right (L-R) asymmetry of the fetus. This review also discusses the ciliogenesis of a primary cilium in the cell.

Centriolar satellites: molecular characterization, ATP-dependent movement toward centrioles and possible involvement in ciliogenesis

We identified Xenopus pericentriolar material-1 (PCM-1), which had been reported to constitute pericentriolar material, cloned its cDNA, and generated a specific pAb against this molecule. Immunolabeling revealed that PCM-1 was not a pericentriolar material protein, but a specific component of centriolar satellites, morphologically characterized as electron-dense granules, approximately 70-100 nm in diameter, scattered around centrosomes. Using a GFP fusion protein with PCM-1, we found that PCM-1-containing centriolar satellites moved along microtubules toward their minus ends, i.e., toward centrosomes, in live cells, as well as in vitro reconstituted asters. These findings defined centriolar satellites at the molecular level, and explained their pericentriolar localization. Next, to understand the relationship between centriolar satellites and centriolar replication, we examined the expression and subcellular localization of PCM-1 in ciliated epithelial cells during ciliogenesis. When ciliogenesis was induced in mouse nasal respiratory epithelial cells, PCM-1 immunofluorescence was markedly elevated at the apical cytoplasm. At the electron microscopic level, anti-PCM-1 pAb exclusively labeled fibrous granules, but not deuterosomes, both of which have been suggested to play central roles in centriolar replication in ciliogenesis. These findings suggested that centriolar satellites and fibrous granules are identical novel nonmembranous organelles containing PCM-1, which may play some important role(s) in centriolar replication.

Ultrastructural observation on 'transitional tubules' in human oviductal ciliogenic cells

In the human oviduct epithelium during ciliogenesis, short tubular structures were found in the transitional zone between the basal body and cilium. The tubules, termed transitional tubules from their location, were 34-36 nm in diameter and 0.13 +/- 0.06 micron in length; the number around a basal body was variable, but usually 4-6. The cytoplasmic leaflets of the tubule membranes were coated by electron-dense material and appeared to be connected to alar sheets. The transitional tubules existed transiently during ciliogenesis. The exact role of transitional tubules is unknown, but considering their location, they may fix the basal body in the apical cytoplasm during ciliary elongation and/or may be related to formation of alar sheets.