Cilia

Cilia are slender, hair-like projections from the surface of eukaryotic cells, enclosed by the plasma membrane, and absent in bacteria and archaea. They play vital roles in locomotion, fluid movement, and sensory functions across organisms. Cilia are short, slender, hair-like projections arising from the surface of eukaryotic cells, composed of microtubules and covered by the plasma membrane. They are specialised for locomotion, fluid movement, and sensory detection. In some cells, cilia are motile and beat rhythmically, while in others they act as sensory organelles.

1.0Types of Cilia

Motile Cilia 

• Feature nine peripheral microtubule doublets surrounding two central microtubules in the axoneme.
• Present in multiple copies per cell (e.g., ~200 in respiratory epithelium).
• Facilitate coordinated wave-like beating to move fluids (e.g., mucus in the respiratory tract), propel eggs in the oviduct, and circulate cerebrospinal fluid in brain ventricles.

Non-motile (Primary) Cilia

• Have nine peripheral doublets without central microtubules.
• Common in most human cells (except blood cells), functioning as sensory organelles or cellular “antennae” for signal transduction, mechanosensation, and developmental pathways.

2.0Cilia Structure

• Axoneme
  The core microtubule structure of cilia, known as the axoneme, exhibits a “9+2” pattern in motile cilia and “9+0” in primary cilia. This structure provides scaffold and motility via dynein arms and associated proteins.
• Basal Body & Transition Zone
  Cilia originate from a basal body—derived from the mother centriole—with nine microtubule triplets. The transition zone (ciliary gate) regulates protein entry into the cilium.
• Intraflagellar Transport (IFT)
  IFT is essential for ciliogenesis and cilium maintenance. It involves bidirectional movement of protein complexes along the axoneme: anterograde (kinesin-2 powered) and retrograde (dynein-2 powered) transport.
• Additional Structures
  Ciliary rootlets (protein-based cytoskeletal structures) extend from the basal body into the cell and provide structural support.

3.0Cilia Functions

• Locomotion and Fluid Movement
  In unicellular organisms like Paramecium, motile cilia enable swimming and feeding. In humans, respiratory cilia clear mucus and debris; oviductal cilia transport ova; ependymal cilia circulate cerebrospinal fluid.
• Sensory Reception & Signal Transduction
  Primary cilia detect mechanical and chemical signals, regulating key pathways like Hedgehog signalling and contributing to organ development. They also function in sensory cells, such as olfactory neurons and photoreceptors.

4.0Ciliopathies (Cilia‑Related Disorders)

Defects in cilia structure or function lead to ciliopathies—genetic disorders affecting motile and/or primary cilia. Examples include primary ciliary dyskinesia, polycystic kidney disease, retinal degeneration, congenital heart defects, and mitral valve prolapse.

5.0Ciliated Cells in Animals and Humans

• Respiratory epithelium: Cells have motile cilia to clear mucus and debris.
• Oviduct epithelium: Cilia help transport ova.
• Ependymal cells: Line the brain ventricles and circulate cerebrospinal fluid.
• Kidney tubular cells: Contain primary cilia that act as mechanosensors.
• Photoreceptor cells: Non-motile cilia are involved in vision.