The proximal centriole-like or PCL is an atypical type of

centriole In cell biology a centriole is a cylindrical organelle composed mainly of a protein called tubulin. Centrioles are found in most eukaryotic cells, but are not present in conifers (Pinophyta), flowering plants (angiosperms) and most fungi, and a ...

found in the

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

cells of insects. The PCL name is due to some similarity to the Proximal centriole found in

Vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

sperm and the hypothesis that the two structures are homologous. The PCL is an atypical type of centriole because it does not have

microtubules Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...

, a defining feature of centrioles. However, the PCL is a type of centriole for several reasons. (1) the PCL formation is dependent upon the same

genetic pathway A gene (or genetic) regulatory network (GRN) is a collection of molecular regulators that interact with each other and with other substances in the cell to govern the gene expression levels of mRNA and proteins which, in turn, determine the fun ...

that mediates the initiation of centriole formation. (2) The PCL is composed of centriolar proteins. (3) After

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

, the sperm PCL function like a centriole. The PCL recruits

pericentriolar material Pericentriolar material (PCM, sometimes also called pericent matrix) is a highly structured, dense mass of protein which makes up the part of the animal centrosome that surrounds the two centrioles. The PCM contains proteins responsible for microtub ...

(PCM) forming a

centrosome In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progres ...

that acts as a

microtubule-organizing center The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spindl ...

(MTOC). The PCL also serves as a platform to form a typical centriole in the

zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...

, as expected from a centriole. Also, the PCL is essential to form one of the two

spindle Spindle may refer to: Textiles and manufacturing * Spindle (textiles), a straight spike to spin fibers into yarn * Spindle (tool), a rotating axis of a machine tool Biology * Common spindle and other species of shrubs and trees in genus ''Euony ...

poles of the dividing zygote. The PCL was discovered in flies. However, it is also found in beetles, suggesting it is a common form of atypical centriole in insects. Another type of atypical type of

was discovered in human and bovine sperm. This is the distal centriole of the

spermatozoon A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, t ...

, which has atypical structure and composition. This spermatozoon distal centriole is composed of splayed microtubules surrounding previously undescribed rods of centriole luminal proteins, and it has only a subset of the protein found in a typical centriole. After fertilization, the atypical distal centriole that is attached to the sperm tail recruits

, forming a new centriole, and localizing to the

spindle pole In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a pr ...

during

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

. During sperm swimming, the atypical distal centriole attached to the sperm tail helps couple the tail movement with head movement. The atypical distal centriole forms a dynamic basal complex (DBC) together with other structures in the sperm neck (the proximal centriole and surrounding atypical pericentriolar matrix). The dynamic basal complex facilitates a cascade of internal sliding, coupling tail beating with head kinking. Altogether, the atypical distal centriole's properties suggest that it evolved into a transmission system that couples the sperm tail motors to the whole sperm, thereby enhancing sperm function.Khanal, S., M.R. Leung, A. Royfman, E.L. Fishman, B. Saltzman, H. Bloomfield-Gadelha, T. Zeev-Ben-Mordehai, and T. Avidor-Reiss. 2021. A dynamic basal complex modulates mammalian sperm movement. Nat Commun. 12:3808.. https://doi.org/10.1038/s41467-021-24011-0

References

{{Reflist Centrosome Insect biology