Cilia

Cilia are short, hair-like projections extending from the surface of eukaryotic cells. They are composed of microtubules and are covered by the cell membrane. Cilia function in movement (either of the cell itself or of fluids and particles across the cell surface) and sensory reception.

Types of Cilia:

Structure:

• Core (axoneme): Made of microtubules in a “9 + 2” arrangement in motile cilia:
  • 9 doublet microtubules in a ring
  • 2 single microtubules in the center
• Basal body: Anchors the cilium to the cell and is derived from a centriole
• Covered in plasma membrane

Primary (non-motile) cilia typically have a “9 + 0” arrangement (no central pair).

Function:

Motile Cilia:

• Beat in coordinated waves to:
  • Move mucus and debris in the respiratory tract
  • Propel egg cells in the fallopian tubes
  • Circulate cerebrospinal fluid in the brain ventricles

Primary Cilia:

• Act as mechanical and chemical sensors
• Involved in cell signaling pathways (e.g., Hedgehog signaling)
• Help regulate cell growth, polarity, and differentiation

Cilia vs. Flagella:

Clinical Relevance:

• Primary ciliary dyskinesia (PCD): Genetic disorder where cilia are immotile or dysfunctional → leads to respiratory infections, infertility, and situs inversus.
• Polycystic kidney disease (PKD): Linked to defects in primary cilia in kidney cells.
• Bardet-Biedl syndrome and Joubert syndrome: Ciliopathies involving defective sensory cilia.

Summary Table: