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Basal body

The basal body is a cylindrical organelle composed of nine triplet microtubules that forms the base of cilia and flagella in eukaryotic cells, serving as a template for axonemal assembly and anchoring these structures to the plasma membrane.^[1] Derived from the mother centriole within the centrosome, it transitions into a functional basal body during ciliogenesis to nucleate the growth of the microtubule-based axoneme, which exhibits a characteristic 9+0 or 9+2 arrangement depending on whether the cilium is sensory (non-motile) or motile.^[2] Structurally, the basal body features a proximal region of triplet microtubules for stability, a distal region often with doublet microtubules, and appendages such as distal appendages for membrane docking and subdistal appendages for anchoring to the cytoskeleton, along with associated proteins like pericentriolar material and rootlets.^[1] In motile cilia and flagella, the basal body organizes the axoneme's central pair and outer doublets, enabling intraflagellar transport (IFT) for protein delivery and dynein-driven bending motions essential for cell motility and fluid flow in tissues like the respiratory tract.^[2] For primary (sensory) cilia, it facilitates signal transduction in pathways such as Hedgehog signaling, allowing cells to sense environmental cues critical for development and homeostasis.^[1] Basal bodies exhibit multipotency in some organisms, capable of remodeling axonemes between motile and sensory forms, as seen in protozoan life cycles where IFT adjustments lead to structural reconfiguration over time.^[2] Dysfunction in basal body components, often due to mutations in genes encoding microtubule-associated proteins or IFT regulators, underlies ciliopathies—a group of disorders including polycystic kidney disease, Bardet-Biedl syndrome, and retinal degeneration—highlighting their role in human health.^[1] While the term "basal body" in eukaryotic contexts refers to this centriole-derived structure, in prokaryotes (bacteria), it denotes a distinct rotary motor embedded in the cell envelope that powers flagellar rotation via proton motive force, comprising ring complexes and a rod rather than microtubules.^[3] This prokaryotic basal body, varying between Gram-negative (with L, P, MS, and export rings) and Gram-positive species (simpler ring sets), evolved independently and shares no homology with its eukaryotic counterpart, underscoring convergent adaptations for motility across domains of life.^[3]

Definition and Overview

Definition

The basal body is a cylindrical protein complex found in eukaryotic cells, typically measuring approximately 200–250 nm in diameter and 400–500 nm in length, that serves as the primary anchoring structure for the axoneme of motile and non-motile cilia as well as flagella.^[4] This organelle provides structural stability by embedding into the plasma membrane and facilitating the extension of the axoneme outward from the cell surface. The term "basal body" was coined by Theodor Wilhelm Engelmann in 1880, based on observations using light microscopy to describe the basal structures supporting flagella in protozoa. As a microtubule-organizing center (MTOC), the basal body nucleates and precisely organizes the nine outer microtubule doublets characteristic of the axonemal structure, enabling coordinated beating in motile cilia or sensory functions in primary cilia.^[1] This nucleation occurs through the recruitment of γ-tubulin ring complexes to the basal body's microtubule triplets, which template the assembly of the axoneme's cytoskeleton.^[1] While structurally similar to centrioles, basal bodies represent a specialized modification of the mother centriole dedicated to ciliogenesis, rather than functioning as part of the centrosome for mitotic spindle organization.^[5] This transition from centriole to basal body involves the acquisition of distal and subdistal appendages that promote membrane docking and microtubule extension, distinguishing its role in post-mitotic cellular processes.^[5]

Historical Background

The basal body was first identified and named in 1880 by German physiologist Theodor Wilhelm Engelmann during his light microscopy observations of ciliated epithelial cells in protozoans and multiciliated tissues, where he described it as the granular base anchoring the ciliary apparatus. This initial recognition highlighted the structure's role in ciliary attachment, though resolution limits of light microscopy obscured finer details. Early 20th-century cytology introduced varied terminology for similar structures, such as "blepharoplast" for the flagellar base in plant gametes and algae, reflecting observations of de novo formation precursors rather than mature anchors.^[6] By the 1920s–1930s, terms like "basal granule" and "kinetosome" emerged in animal cell studies, evolving toward "basal body" as a unified descriptor for the ciliary base across eukaryotes, as consolidated in reviews by cytologists like E.B. Wilson.^[7] Advancements in electron microscopy during the 1950s provided the first ultrastructural insights, with Don W. Fawcett and Keith R. Porter's 1954 study of ciliated epithelia revealing the basal body's 9-triplet microtubule array (9+0 pattern) contrasting the axoneme's 9+2 doublet arrangement, establishing its role as a templating organelle. In the 1960s and 1970s, seminal works solidified the equivalence between basal bodies and centrioles, including Pierre Dustin's analyses of microtubule-based duplication and conversion in ciliated cells, which linked the structures through shared 9-triplet architecture and mitotic-ciliogenic transitions. Complementing this, Ellen R. Dirksen's 1971 electron microscopy of mouse oviduct ciliogenesis demonstrated procentriole budding from fibrogranular material, confirming centriole-to-basal body maturation in multiciliated epithelia.

Structure and Components

Microtubule Arrangement

The basal body is characterized by a precise nine-triplet microtubule arrangement, forming a cylindrical barrel with ninefold radial symmetry. This structure consists of nine peripheral sets of three fused microtubules, known as triplets, each comprising an A-tubule (a complete microtubule with 13 protofilaments), a partial B-tubule (sharing a wall with the A-tubule and featuring 10 protofilaments), and a partial C-tubule (attached to the B-tubule with 10 protofilaments). The triplets are organized in a cartwheel-like pattern, where the A-tubules form the outer ring and the B- and C-tubules contribute to the inner linkages, providing structural rigidity and serving as a template for axonemal extension.^[8] High-resolution imaging via cryo-electron tomography has elucidated the ultrastructural details of this arrangement, revealing a cylindrical core approximately 260 nm in diameter and up to 600 nm in length, with the triplets spanning about 400 nm along the axis. The triplets exhibit an interspacing of roughly 40 nm between adjacent A-tubules, contributing to the overall geometric precision and stability of the structure, as confirmed in studies through the 2020s. Non-tubulin densities interconnect the triplets, forming a scaffold that maintains the barrel's integrity without additional central microtubules in the basal body itself.^[9]^[10] Associated appendages enhance the basal body's anchoring and compartmentalization functions. Basal feet, cone-shaped projections extending laterally from the subdistal region of the triplets, anchor the structure to the cytoskeleton, ensuring proper orientation and stability during ciliogenesis. Transition zone fibers, including Y-shaped links emanating from the distal triplets, seal the ciliary pocket at the base of the cilium, forming a diffusion barrier that regulates protein entry and maintains the organelle's composition. These appendages typically number up to nine subdistal and distal variants per basal body, varying in prominence based on cellular context.^[11] Structural variations in the microtubule arrangement occur depending on the type of cilium formed. In primary (non-motile) cilia, the basal body templates a 9+0 axoneme, lacking central microtubules and relying on doublet extensions from the A- and B-tubules for sensory functions. In contrast, motile cilia and flagella feature a 9+2 pattern, where the basal body supports the addition of a central microtubule pair to the nine doublets, enabling dynein-driven bending. Subdistal appendages, positioned below the distal ones, aid in microtubule anchoring and orientation in both types, while distal appendages facilitate membrane docking, with motile variants often showing more robust configurations for coordinated beating.^[12]^[13]

Protein Composition

The basal body consists of a complex array of proteins that form its cylindrical structure of nine microtubule triplets, with the cartwheel serving as the proximal scaffold for symmetry and microtubule organization. SAS-6 is a core cartwheel protein that oligomerizes to establish the nine-fold radial symmetry essential for basal body formation.^[14] SAS-4 (known as CPAP in humans) interacts with the cartwheel to recruit tubulin subunits, enabling microtubule nucleation and wall assembly around the cartwheel.^[15] These components ensure the precise templating of the triplet microtubules that define the basal body's architecture. Additional proteins stabilize the microtubule triplets and inner scaffold. POC1 acts as an inner junction protein, bridging and reinforcing connections between adjacent triplets to maintain structural integrity.^[16] FAM161A contributes to this inner scaffold alongside POC1, promoting overall triplet stability within the basal body lumen.^[16] Proteins associated with basal body appendages provide docking and anchoring functions. ODF2 localizes to basal feet, forming the foundational scaffold for these projections that orient the basal body.^[17] CEP164 is a key component of transition fibers (also called distal appendages), linking the basal body to the ciliary membrane during ciliogenesis.^[18] In subdistal appendages, ninein serves as a microtubule-anchoring protein, recruiting γ-tubulin complexes to stabilize attachments.^[19] δ- and ε-tubulins are specialized isoforms unique to the triplet microtubules of basal bodies and centrioles, where they incorporate into the C-tubule to enable complete triplet formation and stability, distinct from the doublets in axonemes.^[20] Recent cryo-EM studies from the 2020s have highlighted the role of CFAP family proteins in reinforcing inter-triplet linkages during basal body triplet assembly, providing molecular insights into their formation, including time-series reconstructions of the inner scaffold's helical structure.^[21] Proteomic analyses estimate that a mature basal body incorporates approximately 1,000–2,000 protein molecules, with tubulin isoforms accounting for about 50% of the mass due to their dominance in the microtubule scaffold.^[22]

Assembly and Formation

Centriole-Based Assembly

In uniciliated cells, such as those forming primary cilia, basal body assembly relies on the conversion of a pre-existing mature centriole, specifically the older mother centriole, into a basal body, with typically one such structure per cell. This process occurs primarily during the G0 or G1 phase of the cell cycle, when cells exit proliferation and enter quiescence, allowing the centrosome to migrate toward the plasma membrane. The mother centriole, distinguished by its age and prior acquisition of appendages during the previous cell cycle, serves as the template, ensuring asymmetric selection over the daughter centriole.^[5] The assembly proceeds in a series of coordinated steps beginning with docking of the mother centriole to the plasma membrane. Distal appendages on the mother centriole, which evolve into transition fibers, facilitate this anchoring by directly contacting and stabilizing the membrane, often aided by the recruitment of small Golgi-derived vesicles that form a ciliary cap. Following docking, intraflagellar transport (IFT) is initiated at the transition fibers, where IFT proteins and motors like kinesin-2 assemble to transport tubulin and other axonemal components, enabling the extension of the microtubule-based axoneme outward from the basal body. Maturation concludes with the addition of basal feet, formed from subdistal appendages, which provide additional anchorage and orient the cilium along the cell's polarity axis.^[23]^[4]^[5] Recent structural studies have detailed the progressive assembly of procentrioles, revealing stages including a "naked cartwheel" formation by SAS-6 and STIL, followed by a "bloom phase" of microtubule blade addition and an "elongation phase" with inner scaffold recruitment, critical for centriole maturation into basal bodies.^[21] Additionally, the ciliopathy protein HYLS1 has been identified as a key regulator that modulates the β-tubulin C-terminal tail to promote triplet microtubule assembly, ensuring centriole stability and length essential for basal body function.^[24] Molecular regulation ensures precise timing and fidelity of this centriole-based pathway. Polo-like kinase 1 (PLK1) and Aurora A kinase play key roles in centriole maturation and the transition to basal body formation, with PLK1 promoting disengagement and appendage assembly post-mitosis, while Aurora A influences capping protein removal to permit axoneme elongation. The cartwheel structure, essential for the ninefold microtubule symmetry, is scaffolded by SAS-6 oligomerization, indirectly regulated through upstream kinases like PLK4 during earlier biogenesis but stabilized in the basal body context. Centriolar satellites, including PCM1-containing complexes, facilitate protein recruitment to the mother centriole, enhancing vesicle docking and IFT initiation.^[25]^[5]^[26]

De Novo Formation

In multiciliated tissues, such as the airway epithelium, hundreds of basal bodies assemble de novo to support the formation of numerous motile cilia, a process primarily mediated by specialized electron-dense structures known as deuterosomes. These structures enable the rapid, acentriolar production of centrioles that differentiate into basal bodies, distinct from the more limited centriole duplication in proliferating cells. Deuterosomes arise transiently during differentiation of multiciliated cells (MCCs), such as those in the respiratory tract and brain ependyma, where they nucleate the majority (80-90%) of new centrioles required for ciliogenesis.^[27]^[28] The assembly process unfolds in sequential steps. First, deuterosomes form from fibrogranular material in the cytoplasm, driven by the self-assembly of Deup1 protein into stable macromolecular condensates that constitute the deuterosome core; Deup1, a paralog of Cep63, is specifically expressed in differentiating MCCs and is indispensable for this nucleation.^[29]^[30] Second, procentriolar organization centers (POCs) emerge on the deuterosome surface, where procentrioles bud off in a templated manner; this step relies on the kinase Plk4 (the vertebrate homolog of ZYG-1 in C. elegans), which recruits core centriole components like Sas6 to initiate assembly, producing up to hundreds of immature procentrioles per deuterosome.^[30]^[27] Finally, the procentrioles mature into full-length centrioles, detach from the deuterosomes, and migrate to the apical cell membrane, where they dock via their distal appendages to anchor cilia.^[28] This pathway operates independently of parental centrioles, as demonstrated in cells depleted of pre-existing centrioles, confirming its de novo nature.^[30] In situ cryo-electron tomography has recently visualized the progressive biogenesis of basal bodies in mouse ependymal cells, identifying six stages from deuterosome-dependent procentriole formation to mature multicilia, highlighting roles for microtubule-inner proteins like CEP41 in stabilizing triplet microtubules during multiciliogenesis.^[31] Key regulators ensure fidelity in deuterosome-mediated amplification. Deup1 not only scaffolds deuterosome formation but also recruits Plk4 and Cep152 to POCs, facilitating procentriole generation; its absence severely impairs deuterosome biogenesis but does not halt all amplification.^[29]^[28] The monocentriolar pathway, utilizing the single pre-existing centriole as a template, acts as a compensatory mechanism, amplifying a smaller subset of centrioles when deuterosomes are compromised.^[28] In the 2020s, studies using genetic knockouts revealed that Cep63 and Cep152 contribute to deuterosome nucleation and compensation; in Deup1-deficient MCCs, Cep63 enhances Cep152 recruitment to the parental centriole, sustaining centriole numbers through an adaptive boost to the monocentriolar route, with reductions of only 10-22% in amplification efficiency.^[32] This de novo formation contrasts with centriole-based assembly by enabling explosive, non-templated production tailored to MCC demands.^[30]

Functions

Role in Ciliogenesis

The basal body serves as the foundational platform for ciliogenesis, orchestrating the assembly of the ciliary axoneme by providing structural templates and recruiting essential transport machinery. Derived from the mother centriole, it migrates to the apical plasma membrane in quiescent cells, where it docks and initiates microtubule extension to form the cilium. This process ensures the precise organization of the 9+0 or 9+2 microtubule architecture characteristic of primary or motile cilia, respectively.^[33]^[34] Central to ciliogenesis is the basal body's role in microtubule nucleation. At its proximal end, γ-tubulin ring complexes (γ-TuRCs) associate with the basal body to nucleate and organize the axonemal microtubules, templating the extension of doublet microtubules from the nine triplet microtubules of the basal body itself. This nucleation is facilitated by proteins such as γ-tubulin and ε-tubulin, which integrate into the structure to stabilize the emerging axoneme. The basal body's triplet arrangement provides a scaffold that dictates the radial symmetry of the cilium, ensuring faithful replication of the microtubule pattern during assembly.^[35]^[33] Following nucleation, the basal body docks to the plasma membrane via its distal appendages, forming the transition zone that seals the ciliary compartment and regulates protein entry. Proteins like Cep164, ODF2, and Cep83 mediate this docking, while the transition zone—assembled with components such as Cep290 and MKS proteins—acts as a ciliary gate. Extension then proceeds through intraflagellar transport (IFT), where anterograde motors like kinesin-2 transport tubulin dimers and other building blocks along the axoneme to the distal tip for microtubule polymerization. This IFT-dependent elongation, powered by dynein-2 for retrograde transport, balances assembly and turnover to achieve proper ciliary length.^[34]^[36] The basal body also contributes to ciliary maintenance, particularly in sensory functions. In primary cilia, it integrates the Hedgehog signaling pathway by localizing receptors like Smoothened and Patched within the cilium, enabling signal transduction for developmental patterning. Maintenance relies on continuous IFT cycles and pericentriolar satellites, which deliver regulatory proteins such as IFT88 and PCM1 to sustain axonemal integrity. For motile cilia, the basal body coordinates the attachment of dynein arms and radial spokes, generating the 9+2 axoneme's bending waves through ATP-dependent sliding of microtubules. This organization ensures synchronized beating, as seen in epithelial cells where basal body orientation aligns ciliary motion for fluid propulsion.^[37]^[38]

Role in Mitosis and Polarity

In animal cells, basal bodies function as centrioles during mitosis, serving as core components of the centrosome to organize microtubule-based spindle poles. The pericentriolar material (PCM) surrounds the centrioles and nucleates microtubules essential for spindle assembly, ensuring proper chromosome segregation.^[39] PLK4, a key kinase, regulates centriole duplication to maintain two centrosomes per cell, preventing monopolar spindles that could lead to aneuploidy.^[40] In the absence of PLK4, spindle formation fails in early mouse embryos, highlighting its critical role beyond duplication in microtubule organization.^[40] Basal bodies contribute to cellular polarity by facilitating asymmetric inheritance during stem cell divisions, where the mother or daughter centriole is preferentially retained in the self-renewing daughter cell depending on the organism or cell type, contributing to unequal distribution of fate determinants and centrosomal components to maintain stemness.^[41] In planar cell polarity (PCP) pathways, basal body orientation aligns cellular structures along a tissue plane, integrating with core PCP proteins like Dishevelled to establish directional cues in epithelial sheets.^[42] During cell migration, such as in wound healing or embryonic development, basal bodies reposition toward the leading edge to direct microtubule dynamics and stabilize protrusions. The mother centriole's appendages, including those involving Cenexin, modulate centrosome positioning and microtubule anchorage, promoting persistent migration.^[43] This repositioning integrates with polarity signals to orient the centrosome ahead of the nucleus.^[44] In acentriolar cells like mammalian oocytes, analogous microtubule-organizing centers (MTOCs) perform similar functions without basal bodies, self-organizing to form acentrosomal spindles for chromosome alignment and segregation. These MTOCs recruit PCM-like components to nucleate microtubules, mimicking centriole-based poles and ensuring bipolarity through fragmentation and clustering.^[45] This adaptation underscores the conserved role of MTOCs in mitosis despite structural differences.^[46]

Relation to Centrioles

Structural Similarities

The basal body and centriole exhibit profound architectural homology, underscoring their shared evolutionary lineage as core microtubule-organizing organelles in eukaryotes. Both structures feature a canonical nine-triplet microtubule wall arranged in a 9+0 configuration, forming a cylindrical barrel that provides mechanical stability and templating capacity.^[47] This triplet organization consists of A, B, and C subfibers within each microtubule, with the A- and B-tubules complete across all nine triplets and the C-tubule incomplete or absent in the outermost one.^[48] Central to this symmetry is the cartwheel structure at the proximal end, a nine-spoked hub-and-rim assembly that dictates the radial organization during biogenesis. The SAS-6 protein oligomerizes to form the cartwheel's central hub, with spokes extending to anchor each triplet's A-tubule, a mechanism conserved in both centrioles and basal bodies.^[49]^[50] In vertebrates, these organelles display nearly identical dimensions, with a length of approximately 500 nm and a diameter of about 200 nm, facilitating their interchangeable roles in cellular architecture.^[48]^[47] The appendages further highlight this parallelism: distal and subdistal appendages on mature centrioles structurally and functionally mirror the transition fibers and basal feet of basal bodies, both comprising nine radiating elements that link the organelle to the plasma membrane or pericentriolar material.^[51] Exceptions to this homology include the absence of procentriole buds on basal bodies during interphase, a duplication hallmark of centrioles, and the complete lack of centriole- or basal body-based structures in higher plants, which instead utilize acentriolar MTOCs for microtubule organization.^[47]^[52]

Functional Interconversion

In quiescent (G0) cells, the mother centriole undergoes a functional conversion to a basal body, enabling ciliogenesis. This process involves the disassembly of the pericentriolar material (PCM), which reduces microtubule-organizing activity and allows the centriole to migrate toward the plasma membrane. Concurrently, intraflagellar transport (IFT) proteins are recruited to the distal end of the mother centriole, facilitating the assembly of the ciliary axoneme. Docking to the membrane occurs via distal appendages, a step regulated by the BBSome complex, which mediates the trafficking of ciliary cargo proteins through interactions with Rab8 GTPase.^[34]^[53] The reversion from basal body to centriole occurs upon ciliary resorption, typically at the onset of S-phase during cell cycle re-entry. This transition is marked by the re-recruitment of PCM components to restore centrosomal function for mitosis. Key regulators include Aurora A kinase, which, activated by HEF1, promotes tubulin deacetylation via HDAC6 to disassemble the cilium, and Polo-like kinase 1 (Plk1), which facilitates centriole disengagement and PCM maturation.^[34] Molecular switches underpin these state changes: pericentrin, a core PCM scaffold protein, is depleted from basal bodies to suppress centrosomal activity, while outer dense fiber protein 2 (ODF2) is incorporated into the appendages of the mother centriole to promote membrane anchoring and ciliogenesis.^[54] Recent studies from the 2020s highlight epigenetic mechanisms, such as histone modifications (e.g., H3K27me3 and H3K4me3), that stabilize these functional states by regulating gene expression during cell cycle transitions and ciliogenesis, as revealed through analyses of chromatin dynamics.^[55]

Associated Diseases

Ciliopathies Overview

Ciliopathies represent a diverse group of genetic disorders arising from mutations in genes that encode proteins essential for the structure and function of cilia, including those associated with basal bodies and centrioles, leading to dysfunctional ciliary assembly or signaling.^[56] These conditions often manifest as multisystemic diseases affecting organs reliant on ciliary motility or sensory functions, with an estimated prevalence of approximately 1 in 1,000 births worldwide.^[57] Ciliopathies are broadly classified into motile ciliopathies, which impair the beating of motile cilia and include disorders such as primary ciliary dyskinesia characterized by respiratory and fertility issues; sensory ciliopathies, which disrupt non-motile primary cilia involved in signal transduction, exemplified by retinal degenerations like those in Leber congenital amaurosis; and syndromic ciliopathies, which involve multiple organ systems and encompass conditions such as polycystic kidney disease with renal cyst formation.^[58]^[59]^[60] The underlying mechanisms typically involve disruptions in intraflagellar transport (IFT), which is critical for ciliary protein delivery and maintenance; failures in basal body docking to the plasma membrane, preventing proper cilium initiation; or defects in appendage structures, resulting in absent or shortened cilia that compromise cellular signaling and tissue homeostasis.^[59] Most ciliopathies follow an autosomal recessive inheritance pattern, though rare dominant or X-linked forms exist, and over 200 genes have been implicated as of 2025, including members of the TTLL family that regulate tubulin polyglutamylation essential for ciliary stability. Recent studies as of 2025 have identified additional genes, such as CEP76, further expanding the genetic basis of these disorders.^[61]^[62]

Specific Disorders

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy caused by mutations in any of at least 21 BBS genes (BBS1-BBS21), which encode components of the BBSome complex and associated chaperonins essential for intraflagellar transport (IFT).^[63] The BBSome functions as a cargo adaptor that interacts with the IFT machinery at the basal body to facilitate the entry and trafficking of membrane proteins into the cilium, and disruptions in this process lead to defective ciliogenesis and signaling.^[64] Clinically, BBS manifests with progressive retinal dystrophy leading to vision loss, central obesity, postaxial polydactyly, renal abnormalities, and cognitive impairment, often resulting from impaired hedgehog and other ciliary signaling pathways.^[65] Joubert syndrome is a genetically heterogeneous disorder primarily affecting the central nervous system, with mutations in genes such as ARL13B and INPP5E disrupting ciliary function and planar cell polarity. ARL13B, a small GTPase localized to the ciliary membrane, regulates ciliary architecture and sonic hedgehog signaling, while INPP5E, a phosphoinositide phosphatase enriched in the axoneme and basal body, maintains ciliary lipid composition to support protein trafficking and polarity. These mutations impair basal body docking and orientation, leading to the characteristic molar tooth sign on brain MRI due to cerebellar vermis hypoplasia and deepened interpeduncular fossa, along with progressive ataxia, hypotonia, developmental delays, and episodic breathing abnormalities in infancy. Primary ciliary dyskinesia (PCD) arises from defects in the motility of respiratory, fallopian tube, and sperm flagella, often due to mutations in genes like DNAH5 and DNAI1, which encode heavy and intermediate chains of the outer and inner dynein arms responsible for axonemal bending.^[66] These dynein arm components assemble at the basal body during ciliogenesis and are transported via IFT to their axonemal positions; mutations prevent proper arm formation or attachment, resulting in immotile or dyskinetic cilia anchored to dysfunctional basal bodies. Key symptoms include chronic respiratory infections from impaired mucociliary clearance, situs inversus in approximately 50% of cases due to randomized nodal cilia motility during embryogenesis, and male infertility from asthenozoospermia.^[67] Oral-facial-digital syndrome type 1 (OFD1), an X-linked dominant ciliopathy lethal in males, stems from mutations in the OFD1 gene, which encodes a centrosomal protein critical for centriole elongation and basal body docking to the plasma membrane during ciliogenesis.^[68] OFD1 localizes to centriolar satellites and the distal appendages of the basal body, where it regulates microtubule organization and IFT initiation; pathogenic variants cause mislocalization of basal bodies, leading to shortened or absent primary cilia and disrupted planar polarity. Affected females exhibit malformations of the face (e.g., clefts, hypertelorism), oral cavity (e.g., frenula, lobed tongue), digits (e.g., syndactyly, brachydactyly), and central nervous system (e.g., polycystic kidneys, corpus callosum agenesis). Post-2013 studies using CRISPR-Cas9 models in human cells and zebrafish have confirmed the causality of OFD1 variants by recapitulating basal body docking defects and ciliopathy phenotypes upon targeted disruption.

Evolutionary Perspectives

In Eukaryotic Lineages

The basal body exhibits remarkable structural conservation across diverse eukaryotic lineages that retain these organelles, characterized by a canonical nine-fold symmetrical array of microtubule triplets arranged in a cylindrical barrel. This 9-triplet architecture is universally observed in opisthokonts, encompassing animals, fungi, and their closest unicellular relatives, as well as in excavates, one of the most basal eukaryotic supergroups. In excavates, such as certain flagellated protists, the basal bodies typically support biflagellated cells where one basal body nucleates a motile cilium and the other facilitates feeding-related functions, underscoring their ancient role in motility and organization.^[69]^[69] Within the archaeplastida supergroup, basal bodies are retained in photosynthetic lineages like green algae (e.g., Chlamydomonas reinhardtii), where they function in flagellar assembly and undergo dynamic reorganization during semi-open mitosis by internalizing and associating with spindle poles. However, this structure has been secondarily lost in higher plants, such as angiosperms and gymnosperms, likely as an adaptation to a sessile lifestyle and simplified microtubule cytoskeleton, with mitosis relying instead on diffuse microtubule-organizing centers anchored by plant-specific γ-tubulin ring complex factors. This loss represents an evolutionary simplification unique to land plant lineages within archaeplastida, while red algae lack flagella and associated basal bodies, glaucophytes retain them in motile forms such as Cyanophora.^[70]^[71] Structural variations from the canonical 9-triplet form occur in specific eukaryotic groups, often linked to specialized functions. In kinetoplastids, such as trypanosomes (Trypanosoma brucei), the basal bodies maintain the 9-triplet core but feature modifications including a probasal body that assembles posteriorly to the mature one, along with a flagella connector and the tripartite attachment complex (TAC), which physically links the basal bodies to the mitochondrial kinetoplast DNA for coordinated segregation during cell division. These adaptations enable the parasite's unique developmental cycle in the tsetse fly vector, where new flagellum assembly initiates with basal body duplication and rotation. In some insects, such as Drosophila, basal bodies in sperm flagella exhibit the standard 9 triplets, but somatic centrioles (pre-basal bodies) consist of 9 doublets, reflecting context-dependent maturation and contributing to the diversity of microtubule blade compositions across insect tissues.^[72]^[73] Basal bodies are widely regarded as an ancestral feature of eukaryotes, predating the last eukaryotic common ancestor (LECA), estimated to have existed around 1.8 billion years ago during the Proterozoic eon. Phylogenetic reconstructions indicate that LECA possessed centriole-like basal bodies capable of nucleating a motile 9+2 cilium with dynein arms and a central microtubule pair, likely derived from an early cytoskeletal innovation in a biflagellated ancestor. This supports the view that basal body-based ciliogenesis was integral to the emergence of eukaryotic motility and sensory capabilities.^[74]^[75] Recent phylogenomic studies from the 2020s have reinforced the deep conservation of basal body structures within opisthokonts. For instance, cryo-electron tomography of flagella in choanoflagellates (Salpingoeca rosetta), the closest unicellular relatives to animals, reveals basal bodies with a 9+2 axoneme, axonemal dyneins, radial spokes, and a central pair complex that closely mirror those in metazoan sperm flagella, including fixed central pair orientation and narrow spoke heads. These shared features, present in the urchoanozoan ancestor, highlight the evolutionary continuity within the opisthokont clade, which unites choanoflagellates, animals, and fungi, and predate the advent of multicellularity.^[76]^[76]

Comparisons to Prokaryotic Structures

The basal body of prokaryotic flagella functions as a rotary motor anchored in the bacterial cell envelope, comprising a series of protein rings that drive filament rotation for locomotion. Key components include the MS ring embedded in the inner membrane, formed by approximately 34 subunits of the FliF protein with an outer diameter of about 30 nm; the P ring associated with the peptidoglycan layer; the L ring in the outer membrane; and the cytoplasmic C ring. Unlike eukaryotic basal bodies, these structures are entirely protein-based, lacking any microtubule organization.^[77]^[78]^[79] This prokaryotic basal body shares significant homology with the type III secretion system (T3SS), a protein export apparatus in Gram-negative bacteria, particularly in the core export machinery that translocates flagellar components through the membranes. Structural and sequence similarities indicate a common evolutionary origin, with evidence suggesting that non-flagellar T3SSs evolved from the flagellar system through genetic modifications, including deletions and recruitments of components from other cellular structures. The shared rotary motor elements, such as the transmembrane export apparatus, further highlight this ancestry, enabling both systems to function as sophisticated nanomachines for protein translocation.^[80]^[81] Despite these homologies, prokaryotic and eukaryotic basal bodies exhibit fundamental differences in motility mechanisms and architecture. Bacterial flagella achieve propulsion through extracellular rotation of the rigid helical filament, powered by the MotA/MotB stator complex that harnesses the proton motive force to torque the rotor at up to 100,000 revolutions per minute. In contrast, eukaryotic flagella and cilia generate propulsive bending waves via intracellular dynein motors that induce sliding between microtubule doublets, without rotation or a proton-driven stator. Prokaryotic basal bodies also lack the characteristic 9-triplet microtubule motif of eukaryotic versions, underscoring their distinct cytoskeletal foundations.^[82]^[83] Evolutionary perspectives propose that the shared T3SS-like export apparatus in prokaryotic flagella represents an ancestral rotary system that influenced eukaryotic basal body development, potentially through endosymbiotic gene transfers from bacterial ancestors. Although direct endosymbiotic links to injectisomes remain speculative, comparative genomics reveals conserved motifs in secretion and motor proteins across domains, supporting divergence from a common prokaryotic progenitor during eukaryogenesis. Recent structural studies reinforce this by highlighting modular adaptations in T3SS components that parallel basal body assembly pathways.^[84]^[85]^[86]

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Although the term is sometimes used synonymously with basal body (e.g., Wilson, 3), the blepharoplast of plant cells is not one basal body but the precursor to ...
[7]
Cell Motility: III The Basal Apparatus - SpringerLink
As Fawcett (1961) has pointed out, the body at the base of the flagellum or cilium has been given many different names, not all of which are strictly homologous ...
[8]
The ABCs of Centriole Architecture: The Form and Function of Triplet ...
These triplets consist of three linked microtubules, named the A-, B-, and C-tubules (Fig. 1).
[9]
Three-dimensional structure of basal body triplet revealed by ...
Dec 13, 2011 · Here, we report the structure of the basal body triplet at 33 Å resolution obtained by electron cryo-tomography and 3D subtomogram averaging.
[10]
Centriole and transition zone structures in photoreceptor cilia ...
Jan 5, 2024 · Key to their functions are structures at their base: the basal body, the transition zone, the “Y-shaped links,” and the “ciliary necklace.”
[11]
A primer on the mouse basal body - PMC - PubMed Central - NIH
Apr 25, 2016 · The basal body is a highly organized structure essential for the formation of cilia. Basal bodies dock to a cellular membrane through their distal appendages.Missing: Theodor Wilhelm coined
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The Mechanics of the Primary Cilium: An Intricate Structure with ...
Therefore motile cilia are commonly referred to as 9+2 cilia in contrast to the primary cilium's 9+0 arrangement. In addition, the doublets of the 9+2 ...Missing: variations | Show results with:variations
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Human basal body basics | Cilia - BioMed Central
Mar 14, 2016 · Distal and subdistal appendages are located at the distal end of BB, where they play indispensable roles in cilium formation and function.
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An interaction network of inner centriole proteins organised by ...
Nov 14, 2024 · Recent findings suggest that Tetrahymena POC1 plays a role in promoting the integrity of centriole triplet MTs as a junction protein that links ...
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High-resolution characterization of centriole distal appendage ...
Mar 1, 2019 · Cep164, a novel centriole appendage ... Two appendages homologous between basal bodies and centrioles are formed using distinct Odf2 domains.
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Hierarchical assembly of centriole subdistal appendages via ...
Apr 19, 2017 · Ninein acts as a microtubule-anchoring protein that recruits microtubule nucleation protein complex γ-tubulin ring complex via its N terminus ...
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Centriole assembly at a glance - Company of Biologists journals
Feb 20, 2019 · Centriole assembly is completed by the formation of sub-distal and distal appendages during the cell cycle after the birth of the procentriole, ...
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Deuterosome Mediated Centriole Biogenesis - PMC - NIH
Multi-ciliated cells represent an interesting variation of centriole duplication in that these cells generate greater than 100 centrioles, which form the basal ...
[21]
Regulation of cilia abundance in multiciliated cells - eLife
Apr 26, 2019 · It was recently proposed that deuterosomes are nucleated from the original PC, suggesting that the de novo centriole assembly pathway is ...Regulation Of Cilia... · Modulating Plk4 Expression... · Materials And Methods
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Biophysical and biochemical properties of Deup1 self-assemblies
Mar 3, 2021 · Deuterosome protein 1 (Deup1) self-assembles into stable condensates that can be the structural core of the deuterosome during multiciliogenesis.
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Parental centrioles are dispensable for deuterosome formation and function during basal body amplification | EMBO reports
### Summary of Deuterosome Formation and Related Processes
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Massive centriole production can occur in the absence of ...
Jun 2, 2020 · It has been proposed that CEP63 could compensate for the loss of DEUP1 by enhancing the recruitment of CEP152 to the parent centrioles to ...
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https://pmc.ncbi.nlm.nih.gov/articles/PMC7938805/
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https://www.embopress.org/doi/full/10.15252/embr.201846735
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Ciliary proteins link basal body polarization to planar cell ... - Nature
Dec 9, 2007 · Ciliary proteins link basal body polarization to planar cell polarity regulation. Chonnettia Jones,; Venus C Roper,; Isabelle Foucher ...
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Atypical function of a centrosomal module in WNT signalling drives ...
May 29, 2019 · Centrosomes have well-described functions in directional cell migration (e.g., wound healing), so here we investigated centrosome function ...
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https://doi.org/10.1186/s13630-016-0030-8
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Basal bodies and centrioles: Their function and structure
Basal bodies and centrioles have several appendages that are associated with the microtubules. The basal body undergoes a transition to double microtubules.
[35]
SAS-6 is a Cartwheel Protein that Establishes the 9-Fold Symmetry ...
Dec 18, 2007 · SAS-6 has been suggested to function in the formation of a centriolar precursor, a central tube that then assembles nine-singlet microtubules on its surface.
[36]
Self-assembling SAS-6 Multimer Is a Core Centriole Building Block
The conserved centriole protein, SAS-6, is a cartwheel component that functions early in centriole formation. Here, combining biochemistry and electron ...
[37]
Two appendages homologous between basal bodies and centrioles ...
Both ciliary basal bodies and centrioles display two kinds of appendage-like structures: transition fibers (TFs) and basal feet (BF) in ciliary basal bodies ...<|control11|><|separator|>
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Dividing without centrioles: innovative plant microtubule organizing ...
γ-Tubulin in basal land plants: characterization, localization and implication in the evolution of acentriolar microtubule organizing centers. ,. The Plant ...<|control11|><|separator|>
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Ciliopathies: an expanding disease spectrum - PMC - PubMed Central
Ciliopathies comprise a group of disorders associated with genetic mutations encoding defective proteins, which result in either abnormal formation or function ...
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Ciliopathies
Prevalence: between 1 in 500 and 1 in 1000 – approx 12.5 million worldwide, 70,000+ in UK ... Incidence: 1 in 20,000 live births; In utero: sometimes fatal; ...
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Nov 11, 2024 · Ciliopathies can be broadly classified as due to motile or primary cilia malfunction. See individual articles for in-depth discussions. motile ...
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Genes and molecular pathways underpinning ciliopathies - PMC
In summary, basal body-associated defects can compromise cilium formation or function, resulting in diverse ciliopathies that are often characterized by ...
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Ciliopathy - an overview | ScienceDirect Topics
The ciliopathies include autosomal recessive and dominant polycystic kidney disease, Bardet-Biedl syndrome, Meckel syndrome (MKS), Joubert's syndrome, ...
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Ciliopathies: Genetics in Pediatric Medicine - PMC - PubMed Central
The prevalence of BBS varies markedly between populations—from 1:160,000 in northern Europe to 1:13,500 births in isolated communities in Kuwait. Cardinal BBS ...
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Molecular diagnoses in the congenital malformations caused by ...
It is estimated that around 1000 genes contribute to ciliogenesis and cilium function,12–15 and ciliopathies are highly genetically heterogeneous.16 17 ...<|separator|>
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Jun 20, 2012 · Bardet–Biedl syndrome (BBS) is a rare autosomal recessive ciliopathy characterised by retinal dystrophy, obesity, post-axial polydactyly, renal dysfunction, ...
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Intraflagellar transport protein RABL5/IFT22 recruits the BBSome to ...
Feb 4, 2020 · Our data propose that nucleotide-dependent recruitment of the BBSome to the basal body by IFT22 regulates BBSome entry into cilia.
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Clinical and genetic aspects of primary ciliary dyskinesia/Kartagener ...
Apr 25, 2009 · Axonemal dynein intermediate-chain gene (DNAI1) mutations result in situs inversus and primary ciliary dyskinesia (Kartagener syndrome). Am ...
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Picking up speed: advances in the genetics of primary ciliary ...
Nov 5, 2013 · Primary ciliary dyskinesia (PCD) is a rare disease of children and was the first human disease linked to cilia dysfunction. Nearly four decades ...
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NEK9 regulates primary cilia formation by acting as a selective ...
Jun 2, 2021 · Centriolar oral-facial-digital syndrome 1 (OFD1) is essential for ciliogenesis; it promotes the docking of the basal body to the plasma ...Missing: mislocalization | Show results with:mislocalization
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Evolution of centriole assembly - ScienceDirect.com
May 18, 2020 · In this Primer, we discuss the evolution of the structure and function of centrioles and basal bodies, a specific set of cytoskeletal components ...
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The Centrosome and the Primary Cilium: The Yin and Yang ... - MDPI
Even in higher plants, which lack basal bodies and centrosomes (presumably, due to a secondary loss) (Figure 1E), prior to nuclear envelope breakdown, MTs ...<|control11|><|separator|>
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Timing and original features of flagellum assembly in trypanosomes ...
Apr 5, 2020 · The flagellum is a cylindrical microtubule-based structure composed of the basal body made of nine microtubule triplets followed by the ...<|control11|><|separator|>
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Drosophila melanogaster as a model for basal body research - Cilia
Jul 5, 2016 · The fruit fly has various types of basal bodies and cilia, which are needed for sensory neuron and sperm function.
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Stepwise evolution of the centriole-assembly pathway
May 1, 2010 · ... centrosome, and/or as a basal body, tethered to the membrane. This suggests that they were present in the last eukaryotic common ancestor (LECA) ...
[58]
The last eukaryotic common ancestor (LECA): Acquisition of ... - PNAS
Aug 29, 2006 · A “parabasal body” (= golgi) is often also associated. Many forms of motility including cytoskeletal-based ingestion originated from the ...
[59]
Three-dimensional flagella structures from animals' closest ... - eLife
Nov 17, 2022 · This is a thorough, beautiful and compelling study of the flagellar structure of the choanoflagellate S. rosetta as reconstituted by cryo-electron tomography.
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Molecular structure of the intact bacterial flagellar basal body - PMC
The basal body of the bacterial flagellar motor is broadly composed of four structures; LP-ring, rod, MS-ring, and C-ring. The MS-ring functions as a structural ...
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Structure of the molecular bushing of the bacterial flagellar motor
Jul 22, 2021 · The basal body consists of the C ring, MS ring, LP ring and rod (Fig. 1). The MS-C ring acts as a rotor of the flagellar motor. The MS ring ...
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Native flagellar MS ring is formed by 34 subunits with 23-fold and 11 ...
So, the MS ring structure in the native flagellar basal body is composed of 34 FliF subunits. We also analyzed the symmetry of the C ring of the native ...
[63]
Type III secretion systems and bacterial flagella: Insights into their ...
Type III secretion systems and bacterial flagella are broadly compared at the level of their genetic structure, morphology, regulation, and function.
[64]
Stepwise formation of the bacterial flagellar system - PNAS
Most significantly, there are well established sequence and structural homologies between bacterial flagella and the type III secretion system (TTSS) ...
[65]
The evolution of eukaryotic cilia and flagella as motile and sensory ...
The microtubule rootlet structures that stabilize basal bodies in the cytoplasm do vary phylogenetically and therefore the nature of those that might have been ...<|control11|><|separator|>
[66]
Molecular Architecture of the Bacterial Flagellar Motor in Cells
Cryo-electron microscopy (cryo-EM) studies have provided the most detailed structures of the purified basal body, which contains the MS ring, the C ring, the ...
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Type III secretion systems: the bacterial flagellum and the injectisome
At their core, they share a type III secretion system (T3SS), a transmembrane export complex that forms the extracellular appendages, the flagellar filament and ...Table 1 · 3. Injectisome-T3ss Derived... · Box 1. T3ss Chaperones
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Assembly, Functions and Evolution of Archaella, Flagella and Cilia
Mar 19, 2018 · Localization of intraflagellar transport protein IFT52 identifies basal body transitional fibers as the docking site for IFT particles. Curr ...
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Type III secretion systems: the bacterial flagellum and the injectisome
Oct 5, 2015 · At their core, they share a type III secretion system (T3SS), a transmembrane export complex that forms the extracellular appendages, the ...The flagellum and the... · Structural and functional... · Type III secretion export is a...