Cilia and Flagella

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These whiplike appendages extend from the surface of many types of eukaryotic cells.

Function

Cilia and flagella move liquid past the surface of the cell.

Structure

Both cilia and flagella consist of:

This electron micrograph (courtesy of Peter Satir) shows a cilium in cross section.

Each cilium (and flagellum) grows out from, and remains attached to, a basal body embedded in the cytoplasm. Basal bodies are identical to centrioles and are, in fact, produced by them. For example, one of the centrioles in developing sperm cells — after it has completed its role in the distribution of chromosomes during meiosis — becomes a basal body and produces the flagellum.

The Sliding-Filament Model of Bending

Motion of cilia and flagella is created by the microtubules sliding past one another. This requires:

Dynein powers the sliding of the microtubules against one another — first on one side, then on the other.

(The bending of cilia (and flagella) has many parallels to the contraction of skeletal muscle fibers.)
Link to discussion of the sliding-filament model of skeletal muscle.

Testing the Model

Other Parallels

There are other parallels between the sliding filaments of skeletal muscle and the sliding microtubules of cilia.

The Primary Cilium

Motile, "9+2", cilia are found only on certain cells in the vertebrate body, e.g., the epithelia lining the airways.

But almost every cell in mammals has — or had — a single primary cilium. The primary cilium grows out of the older of the two centrioles that the cell inherited following mitosis [Link].

The primary cilium does not beat because it lacks the central pair of microtubules; that is, it is "9+0".

Where functions have been identified, they all involve sensory reception.

Some examples:

Mechanoreceptors

A primary cilium extends from the apical surface of the epithelial cells lining the kidney tubules and monitors the flow of fluid through the tubules. Inherited defects in the formation of these cilia cause polycystic kidney disease.

Chemoreceptors

We detect odors by receptors on the primary cilium of olfactory neurons. [Link]

Many types of cells detect extracellular signaling molecules, e.g., nutrients, growth factors, hormones, with receptors localized on their primary cilium. These signals may be transduced into the nucleus where they alter gene expression.

Photoreceptors

The outer segment of the rods in the vertebrate retina is also derived from a primary cilium. [View]

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18 October 2013