Cilia 101: Structure, Types, and Functions

Table of Contents

• Cilia Introduction
• Structure of Cilia
• Cilia formation mechanism/ Ciliogenesis
• Types of Cilia
• Functions of Cilia
• Examples of Cilia

Cilia

Cilia, resembling tiny hair-like structures, are present on the surfaces of eukaryotic cells and serve as a means of locomotion for various protozoans and animals.

• The term 'cilia' is derived from Latin, signifying eyelash, reflecting the structure's small, eyelash-like appearance.
• Cilia are most prominently found in protozoans belonging to the Ciliophora phylum, distinguishing this group of microorganisms.
• In complex animals like vertebrates, ciliated cells are distributed across different tissues, each with distinct functions.
• Differing from flagella, cilia are shorter, more numerous, and vary in composition, movement, and functions.
• Exclusive to eukaryotic cells, cilia are absent in prokaryotes like bacteria, where pili perform similar functions.
• On the cell surface, cilia may arrange in short transverse rows forming a membrane or cluster together to form cirri.
• Ciliary movement typically occurs in a rhythmic manner, with individual cilia operating cohesively rather than independently.
• The primary function of cilia is movement across liquid surfaces, although they can also act as structures for mechanoreception and feeding in certain cases.
• The roles of cilia differ in unicellular and multicellular organisms; in humans, epithelial cilia move various substances through lumens.
• Some cells produce immobile cytoplasmic protrusions called stereocilia, which structurally differ from true cilia known as kinocilia.
• The structure and composition of cilia can be easily studied by scraping the pharyngeal epithelium of a frog with a spatula and observing it under a microscope.

Structure of Cilia

Cilia possess a membrane-bound, microtubule-containing structure derived from centrioles, projecting into the extracellular space. This structural framework is both resilient and dynamic, featuring specific mechanisms regulating composition and function. Two primary types of cilia exist: motile and nonmotile, distinguished by the arrangement of microtubules in their axonemes, with the overall basic structure being identical, except for the axoneme.

Examining the ultrastructure of cilia reveals distinct components:

1. Ciliary Membrane:

• The outer covering of cilia, the ciliary membrane, envelops the internal axoneme and core.
• It is continuous with the cell membrane but differs in composition, being approximately 9.5 nm thick with fewer proteins than the cell membrane.
• Unique proteins in the ciliary membrane prevent the loss of ATP and maintain specific ion concentrations essential for ciliary movement.
• Somatic cilia possess a cilia necklace, a region within the membrane acting as a selective barrier during the passage of particles.

2. Ciliary Matrix:

• The watery matrix within the ciliary membrane is the ciliary matrix, comprising embedded microtubules forming the axoneme.

3. Axoneme:

• The pivotal microtubular structure within cilia is the axoneme, responsible for their motility.
• Ranging from 0.2 to 10 µm in diameter, with lengths varying from a few microns to 1-2 mm.
• Motile cilia feature a 9+2 arrangement of microtubules, with nine doublets surrounding a central pair of singlet microtubules.
• Axoneme microtubules polymerize from αβ-tubulin heterodimers, with a fast polymerizing end at the ciliary tip.
• Outer and inner dynein arms, radial spikes, and central-pair projections are present in the axoneme, each hosting numerous proteins responsible for cilia assembly and function.

Cilia formation mechanism/ Ciliogenesis

• Ciliogenesis, the process of cilia formation within a cell, unfolds through multiple stages.
• The biogenesis of a cilium is a meticulously orchestrated and regulated procedure, involving various organelles, cellular mechanisms, and signaling pathways.
• Cilia formation initiates post the mitotic cycle of cell division, allowing free centrioles to undergo axoneme nucleation.
• Throughout this process, the centriole acquires distinct distal and subdistal appendages. Interacting with a post-Golgi vesicle, distal appendages flatten the ciliary extension of the centriole and fuse it with the cell membrane.
• The positioning and orientation of the developing cilia hinge on the original arrangement of basal bodies and centrioles.
• The final stage of cilia formation entails the construction of the axoneme through molecular motors and associated proteins.
• Tubulin protein subunits are assembled at the distal growing ends during this step, known as the intracellular route.
• In the extracellular route, the basal body is present at the apical membrane before axoneme growth, with protein accumulation ensuing.
• Cilia stabilization relies on post-translational modification of tubulin proteins through acetylation and detyrosination.
• Proteins essential for cilia formation are produced in the cytoplasm and transported to the cilia through intraflagellar transport.
• Continuous addition of new tubulin proteins to the cilium occurs even after completion, but the cilium's length remains constant as older tubulin degrades.
• Each step of ciliogenesis is meticulously regulated by diverse mechanisms, and any alterations in the system can impact the structure and motility of cilia.

Types of Cilia

1. Primary Cilia:

• Primary cilia, singular and nonmotile, extend from the apical surface of polarized and differentiated mammalian cells, resembling other specialized cellular organelles.
• Differentiated by a 9+0 arrangement of microtubules in the axoneme, primary cilia lack the central singlet responsible for motility, anchoring themselves to the cell via a basal body nucleated by the centriole.
• Discovered by Zimmerman in 1898, Sergei Sorokin named them in 1968. Various hypotheses propose their origin, either vestigial, essential for cell cycle control, or as sensory organelles.
• Present in diverse mammalian cells, such as stem cells, epithelial, endothelial, connective tissue, and muscle cells, primary cilia often function in sensory roles, acting as antennae for signal reception and transduction.
• The ciliary membrane harbors receptors, channels, and signaling proteins, initiating signaling cascades that vary across different cell types.
• Aberrations in primary cilia form and function may lead to ciliopathies, encompassing disorders like Bardet-Biedl syndrome and oral-facial-digital syndrome.

2. Motile Cilia:

• Motile cilia, responsible for moving organisms or substances through passages, predominantly reside in specialized epithelial linings such as airways, paranasal sinuses, oviduct, and the brain's ventricular system.
• Exhibiting coordinated beating with pendulous, unciform, infundibuliform, or undulant movements, motile cilia have a 9+2 structure with peripheral doublets and central singlet microtubules.
• Nexin bridges connect doublet microtubules, influencing cilia bending motions, while radial spikes link doublets to the central apparatus or two singlets.
• Motor cilia movement is regulated by periciliary fluid levels, with the ciliary membrane containing sodium channels acting as fluid level sensors.

3. Nodal Cilia:

• Nodal cilia, motile with a 9+0 microtubule arrangement, are present in embryos during early development, structurally resembling primary cilia but containing dynein arms for movement.
• Exhibiting clockwise movement, nodal cilia move extraembryonic fluid through the nodal surface, crucial for left-right orientation determination and embryonic fluid dynamics.
• Occurring on cells in the node or embryonic organizer during the gastrula stage, nodal cilia are often surrounded by primary cilia involved in sensory responses around the embryo.

Functions of Cilia

• In the protozoan Phylum Ciliates, cilia serve as the primary organs of movement, responding to environmental changes and facilitating the movement of extracellular fluid across the organism's surface.
• Rhythmic cilia movement is utilized by certain organisms to clear unwanted substances, microorganisms, and mucus, contributing to disease prevention.
• Motile cilia on various human epithelial surfaces are instrumental in processes such as the movement of essential substances in the oviduct and the removal of dust particles in the respiratory tract.
• Diverse types of cilia play vital roles in regulating the cell cycle and organ development.
• Nonmotile or primary cilia feature unique proteins and receptors on their surface, functioning as chemoreceptors. For instance, primary cilia in the kidney are involved in sensing urine flow, while those in the retina contribute to photoreception.
• Cilia in pathogenic organisms act as crucial virulence factors, facilitating colonization on different surfaces.
• Some animals associate cilia with the secretion of vesicular ectosomes.
• Cilia play a pivotal role in signal transduction pathways, regulating intracellular Ca2+ levels and planar cell polarity.

Examples of Cilia

1. Cilia in ciliated epithelium

• Epithelial cells in various parts of the human body form cilia, resulting in ciliated epithelium.
• This type of epithelium is commonly found in areas with frequent contact with the external environment.
• In the respiratory tract, the ciliated epithelium plays a crucial role in sweeping dust particles, mucus, and trapped particles, including bacteria, out of the body.
• Ciliated epithelium in the retina and kidney functions as sensory structures, involved in sensory reception.
• Within the oviduct, cilia facilitate the movement of the ovum from the ovaries to the fallopian tubes, contributing to the process of fertilization.

2. Cilia in Paramecium

• Cilia serve as crucial cell organelles in Paramecium, playing a pivotal role in both locomotion through water and the ingestion of food into the cytosome.
• Distributed throughout the organism's body, most cilia are dedicated to facilitating locomotion, with caudal cilia in the gullet being notably longer and nonmotile.
• Additionally, cilia participate in the initial stages of the mating reaction, contributing to the process of conjugation.
• The sweeping action of cilia directs prey organisms, along with water, into the oral groove, aiding in the feeding process.
• Structurally, cilia in Paramecium resemble those in other eukaryotes, featuring a basal body, axoneme, and ciliary membrane.