Cell: The Unit of Life - Cilia | Types of cilia | Structure and Function of cilia | Muhammad Asghar

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cilia.
The term ‘cilia’ is a Latin term meaning ”eyelash“  indicating the tiny eyelash-like appearance of the structure.

However we can define cilia as,

Cilia are membrane-bound, tiny, slender, hair-like structures mostly present on the surface of protozoans and some animal cells,
that provides locomotion and movement.

Cilia are most prominent in protozoans of the phylum Ciliophora which are characterized by the presence of cilia.

Cilia are different from flagella which are mostly longer and fewer in number on the cell. Cilia also differ from flagella in other aspects like composition, movement, and functions.

Cilia are present only in eukaryotic cells and cannot be found on prokaryotes like bacteria. Instead, bacteria contain other structures called pili that perform similar functions to the cilia.
On the cell surface, cilia can occur either in short transverse rows in the form of a membrane or in groups to form cirri.

The movement of cilia mostly occurs in a rhythmic manner, and individual cilium does not move independently.


Types of cilia
1. Primary cilia
Primary cilia are solitary, nonmotile cilia found in most mammalian cells that are projected from the apical surface of polarized and differentiated cells.
Primary cilia are specialized cellular organelles like other cell organelles like mitochondria, endoplasmic reticulum and Golgi apparatus.
Primary cilia are differentiated from other types of cilia in the presence of a 9+0 arrangement of microtubules in the axoneme.
These lack the central singlet of microtubules which are responsible for the motility of the cilia. The cilia are anchored to the cell by means of basal body nucleated by the centriole.
Primary cilia are found in different cells in the mammalian body like stem cells, epithelial, endothelial, connective tissue, and muscle cells.
Cilia that are not associated with motility are often involved in sensory functions. These cilia act as antennae that receive signals from the environment which are then transfered to the cell.
The ciliary membrane contains various receptors, channels, and signaling proteins that are involved in the process.
Aberrant form and function of primary cilia might result in disorders known as ciliopathies. These disorders have a wide range of clinical manifestations ranging from Bardet-Biedl syndrome or oral-facial-digital syndrome.

2. Motile cilia
Motile cilia or moving cilia are cilia that are primarily involved in the movement of the organisms or different substances through a passage.
These are typically found on the specialized epithelial lining of the airways, paranasal sinuses, oviduct, and the ventricular system of the brain.
Motile cilia occur in large numbers and move in a coordinated beating exhibiting pendulous, unciform, infundibuliform, or undulant movement.
Motile cilia are the only cilia that are found in ciliates that use them for locomotion or to move liquid through their surface.
Motile cilia consist of a 9+2 structure with nine peripheral microtubule doublets and two centrally located singlet microtubules.
The doublet microtubules consist of a complete A tubule with 13 protofilaments and an incomplete B tubule with 10 protofilaments.
The doublets are linked together by nexin bridges responsible for the bending motions of the cilia. The doublets are connected to the central apparatus or two singlets by the means of radial spikes.

Structure of Cilium.

Cilia are distinguished into two types; motile cilia and nonmotile cilia, based on the patterns of microtubules present in the axonemes of the cilia. The overall basic structure of both the cilia is the same, except the axoneme.
The following are the parts of cilia observed in the ultrastructure;

1. Ciliary membrane.
The ciliary membrane is the outer covering of the cilia that surrounds the internal axoneme and core of the cilia.

2. Ciliary matrix.
The space within the ciliary membrane formed of a watery matrix is the ciliary matrix. The matrix consists of embedded microtubules forming the axoneme of the cilium.

3. Axoneme.
The most important structure of the cilia is the basic microtubular structure called an axoneme.
Axoneme forms an axial structure within the cilia which is responsible for the motility of the cilia.
The axoneme of the cilia is about 0.2-10 µm in diameter, and the length ranges from a few microns to 1-2 mm.
Motile cilia contain an axoneme composed of a 9+2 arrangement of the microtubules. The microtubules are arranged with nine doublets surrounding a central pair of singlet microtubules.