Microvilli Cilia and Flagella

Many cells have surface extensions called microvilli, cilia, and flagella. These aid in absorption, movement, and sensory processes.

Microvilli

Microvilli9 (MY-cro-VIL-eye; singular, microvillus) are extensions of the plasma membrane that serve primarily to increase a cell's surface area (figs. 3.10 and 3.11a-b). They are best developed in cells specialized for absorption, such as the epithelial cells of the intestines and kidney tubules. They give such cells 15 to 40 times as much absorptive surface area as they would have if their apical surfaces were flat. On many cells, microvilli are little more than tiny bumps on the plasma membrane. On cells of the micro = small + villi = hairs

Table 3.2 Functions of the Glycocalyx

Protection

Immunity to Infection

Defense Against Cancer

Transplant Compatibility

Cell Adhesion Fertilization

Embryonic Development

Cushions the plasma membrane and protects it from physical and chemical injury

Enables the immune system to recognize and selectively attack foreign organisms Changes in the glycocalyx of cancerous cells enable the immune system to recognize and destroy them Forms the basis for compatibility of blood transfusions, tissue grafts, and organ transplants Binds cells together so that tissues do not fall apart Enables sperm to recognize and bind to eggs

Guides embryonic cells to their destinations in the body taste buds and inner ear, they are well developed but serve sensory rather than absorptive functions.

Individual microvilli cannot be distinguished very well with the light microscope because they are only 1 to 2 ^m long. On some cells, they are very dense and appear as a fringe called the brush border at the apical cell surface. With the scanning electron microscope, they resemble a deep-pile carpet. With the transmission electron microscope, microvilli typically look like finger-shaped projections of the cell surface. They show little internal structure, but some have a bundle of stiff filaments of a protein called actin. Actin filaments attach to the inside of the plasma membrane at the tip of the microvillus, and at its base they extend a little way into the cell and anchor the microvillus to a protein mesh called the terminal web. When tugged by another protein in the cytoplasm, actin can shorten a microvillus to "milk" its absorbed contents downward into the cell.

Cilia

Cilia (SIL-ee-uh; singular, cilium10) (figs. 3.11c-e and 3.12) are hairlike processes about 7 to 10 ^m long. Nearly every human cell has a single, nonmotile primary cilium a few micrometers long. Its function in many cases is still a mystery, but some of them are sensory. In the inner ear, they play a role in the sense of balance; in the retina of the eye, they are highly elaborate and form the light-absorbing part of the receptor cells; and they are thought to monitor fluid flow through the kidney tubules. In some cases they open calcium gates in the plasma membrane. Sensory cells in the nose have multiple nonmotile cilia which bind odor molecules.

Motile cilia are less widespread, occurring mainly in the respiratory tract and the uterine (fallopian) tubes. There may be 50 to 200 of these cilia on the surface of one cell. Cilia beat in waves that sweep across the surface of an epithelium, always in the same direction (fig. 3.13). Each cilium bends stiffly forward and produces a power stroke that pushes along the mucus or other matter. Shortly after a cilium begins its power stroke, the one just ahead of it begins, and the next and the next—collec-tively producing a wavelike motion. After a cilium completes its power stroke, it is pulled limply back by a recovery stroke that restores it to the upright position, ready to flex again.

_Think About It_

How would the movement of mucus in the respiratory tract be affected if cilia were equally stiff on both their power and recovery strokes?

10cilium = eyelash

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

3. Cellular Form and Function

Text

© The McGraw-H Companies, 2003

Function Glycocalyx Cell Membrane

Glycocalyx

- Plasma membrane

Actin microfilaments

Actin microfilaments

Cytosol

Microvillus Actin

Cilia

Microvilli

Cilia

Microvilli

Sil Flagella

Peripheral microtubules

Central microtubules

Dynein arms

Peripheral microtubules

Central microtubules

Dynein arms

Shaft of cilium i

Dynein arms Central microtubules Peripheral microtubules s

Axoneme membrane

s membrane

Third microtubule of basal body terminates at cell surface

Plasma

Microtubules And Microfilaments

Figure 3.11 Microvilli and Cilia. (a and fa) Electron micrograph and diagram of microvilli in cross section. (c and d) Electron micrograph and diagram of cilia in cross section. (e) Three-dimensional structure of a cilium at the point where it meets the cell surface.

Third microtubule of basal body terminates at cell surface

Plasma

Figure 3.11 Microvilli and Cilia. (a and fa) Electron micrograph and diagram of microvilli in cross section. (c and d) Electron micrograph and diagram of cilia in cross section. (e) Three-dimensional structure of a cilium at the point where it meets the cell surface.

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 3 Cellular Form and Function 105

Cilia could not beat freely if they were embedded in sticky mucus (see insight 3.2). Instead, they beat within a saline (saltwater) layer at the cell surface. Chloride pumps in the apical plasma membrane produce this layer by pumping Cl_ into the extracellular fluid. Sodium ions follow by electrical attraction and water follows by osmosis. Mucus essentially floats on the surface of this layer and is pushed along by the tips of the cilia.

Brush Cells Trachea

Figure 3.12 Cilia of the Trachea. Several nonciliated, mucus-secreting goblet cells are visible among the ciliated cells. The goblet cells have short microvilli on their surface.

Insight 3.2 Clinical Application

Cystic Fibrosis

The significance of chloride pumps becomes especially evident in cystic fibrosis (CF), a hereditary disease especially affecting white children of European descent. CF is usually caused by a defect in which cells make chloride pumps but fail to install them in the plasma membrane. Consequently, there is an inadequate saline layer on the cell surface and the mucus is dehydrated and overly sticky. This thick mucus plugs the ducts of the pancreas and prevents it from secreting digestive enzymes into the small intestine, so digestion and nutrition are compromised. In the respiratory tract, the mucus clogs the cilia and prevents them from beating freely. The respiratory tract becomes congested with thick mucus, often leading to chronic infection and pulmonary collapse. The mean life expectancy of people with CF is about 30 years.

The structural basis for ciliary movement is a core called the axoneme11 (ACK-so-neem), which consists of an array of thin protein cylinders called microtubules. There are two central microtubules surrounded by a ring of nine microtubule pairs—an arrangement called the 9 + 2 structure (see fig. 3.11 d). The central microtubules stop at the cell surface, but the peripheral microtubules continue a short distance into the cell as part of a basal body that anchors the cilium. In each pair of peripheral micro-tubules, one tubule has two little dynein (DINE-een) arms. Dynein,12 a motor protein, uses energy from ATP to "crawl" up the adjacent pair of microtubules. When micro-tubules on the front of the cilium crawl up the micro-tubules behind them, the cilium bends toward the front.

Flagella

A flagellum13 (fla-JEL-um) is a whiplike structure much longer than a cilium, but with an identical axoneme. The only functional flagellum in humans is the tail of a sperm cell.

Figure 3.12 Cilia of the Trachea. Several nonciliated, mucus-secreting goblet cells are visible among the ciliated cells. The goblet cells have short microvilli on their surface.

" axo = axis + neme = thread l2dyn = power, energy + in = protein l3flagellum = whip

Moving Mucus
Figure 3.13 Ciliary Action. (a) Cilia of an epithelium moving mucus along a surface layer of saline. (b) Power and recovery strokes of a cilium.

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

106 Part One Organization of the Body

Before You Go On

Answer the following questions to test your understanding of the preceding section:

6. How does the structure of a plasma membrane depend on the amphiphilic nature of phospholipids?

7. Distinguish between integral and peripheral proteins.

8. Explain the differences between a receptor, pump, and cell-adhesion molecule.

9. How does a gate differ from other channel proteins? What three factors open and close membrane gates?

10. What roles do cAMP, adenylate cyclase, and kinases play in cellular function?

11. Identify several reasons why the glycocalyx is important to human survival.

12. How do microvilli and cilia differ in structure and function?

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Responses

  • annie johnson
    What three factors open and close membrane gates?
    8 years ago
  • iacopo
    What is the differences between a receptor, pump, and celladhesion molecule.?
    8 years ago
  • james rockwood
    How many microtubules are found in each cilium cylinder?
    8 years ago
  • AATIFA MICHAEL
    Do microvilli have a chloride pump?
    8 years ago
  • ARNALDO
    What is the difference between microvilli, cilia, and flagella?
    8 years ago
  • Theobald
    How does the structure of a plasma membrane depend on the amphiphilic nature of phospholipids?
    8 years ago
  • bernd
    How do cilia and flagella help with adhesion & adsorption?
    8 years ago
  • sara
    How does cilia affect the surface area of a cell?
    8 years ago
  • Dieter Sankt
    Are sperm flagellum like microvilli or cilia?
    7 years ago
  • fre-qalsi saare
    Does an apical surface of a cell always have microvilli?
    7 years ago
  • rosalia
    What is the purpose for microvilli Do they perform the same way as cilia?
    6 years ago
  • thomas
    Why do the microvilli have such an effect on the surface area?
    6 years ago
  • yerusalem
    Which sense is NOT dependent on cilia or microvilli?
    5 years ago
  • natsnet
    Does dynien anchor the cilium to the plasma membrane?
    5 years ago
  • Eunice
    What senses are dependant on cilia or microvilli?
    4 years ago
  • Carola Paju
    How can individual microvilli be distinguished?
    4 years ago
  • salvia
    Which one is short cilia,flagella or microvilli?
    4 years ago
  • chanelle
    What lines the respiratory tract cilia, flagella, microvilli, or goblet cells?
    4 years ago
  • arabella
    Do villi and microvilli aid in movement?
    3 years ago
  • awet
    How can individual microvilli be distingushed?
    3 years ago
  • Laila
    What vein in the arm is blood drawn frowhere in the body are microvilli and cilia common?
    3 years ago
  • minna
    How to distinguish cilia from microvili under light microscope?
    1 year ago
  • rose
    How to view individual microvilli in a microscope?
    8 days ago

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