Centrioles

CENTRIOLES, small round or oval structures that apparently represent the most important component of the cell center (centrosome). The size and shape of centrioles can vary within significant limits. There are indications that their shape changes with age. Most often, the presence of two centrioles in a cell is described, but in some cases their number can be significantly larger. Centrioles are connected to each other by a protoplasmic bridge—the centrodesmus—and are the most essential part of the centrosome. It is precisely around them that radial figures (the astrosphere) arise in the protoplasm during the stage of cell division. Judging by their ability to stain intensely and stably with certain special dyes, centrioles possess a high degree of density.—The question of how centrioles appear in the cell is very important. The concept existing at the end of the 19th century that centrioles reproduce by division occurring in the telophase of karyokinesis was shaken by the experiments of Wilson, who showed that centrioles can arise anew in the protoplasm under the influence of certain external stimuli (introducing a salt crystal into an egg). There is, however, an opinion that in this case a pre-existing structure merely becomes visible.—The position of centrioles in the cell is highly variable; most often they are located near the nucleus, sometimes indenting its membrane; in multinucleated cells they are found between the nuclei; in a number of cases, especially in protozoa, an intranuclear position of centrioles has been described. The function of centrioles is in many respects unclear; it must be noted that they play an essential role in the process of cell division, being the center toward which the chromosomes move. However, centrioles are not an indispensable component of karyokinesis, since in a number of cases (in all higher plants) they

A

In A—salamander leukocyte: a—centriole; B—various forms of centrioles (centrodesmuses) of rabbit leukocytes. are not described at all. The significance of centrioles as the kinetic center of the cell in processes such as ciliary and flagellar movement is not definitively established. The so-called Henneguy-Lenhossék theory (1898) views the basal bodies of animal cells and the blepharoplasts of plants as modified centrioles. The development of the sperm tail from the centriole appears particularly demonstrative. Cases have been described (in protozoa) where a basal body equipped with a flagellum simultaneously performs the functions of a centriole in karyokinesis (see figure). However, the entire question as a whole cannot be considered definitively resolved (see also Karyokinesis, Cell).

S. Zalkind.