The world is filled with extraordinary details of plants and animals that function largely unseen and unknown to most of us. We take for granted what goes on inside our bodies in very general terms, and are not surprised that birds fly, crickets chirp, and flowers bloom in season. But when we peer through a microscope at the workings of tiny plants and animals, it is difficult to comprehend the precision and order that appears beneath the lens.
I remember the first thing I saw under a microscope. Long ago a kindly high school teacher invited me, a mere sixth-grader passing by, into his laboratory to look at a one-celled animal he identified as Vorticella, the name of a creature I had never heard about, much less seen. Rhythmically beating hairs around its bell-shaped body produced a little vortex in the surrounding water. I could scarcely believe what I saw. How was it possible so tiny a creature could exercise such flawlessly coordinated motion?
Many years later, as a scientist studying a wide variety of related life forms (such as Paramecium, known to every beginning biology student), I remained in awe of those little beating hairs, or cilia, and the manner in which supposedly simple creatures or individual cells in larger animals controlled their beating.
What is a single cilium? A standard dictionary definition reads, "A microscopic hairlike process extending from the surface of a cell. Capable of rhythmical motion, it acts in unison with other such structures to bring about the movement of the cell or of the surrounding medium." To be sure cilia move a one-celled Paramecium forward at a good clip, and by creating a vortex in front of a stalked Vorticella they move the surrounding medium to bring food to this stationary animal. The real mystery, of course, is cellular control of such rhythmical motion.
Years ago meticulous laboratory techniques for staining and observing the network of interconnecting threads beneath a Paramecium's "skin" revealed a system of fibrils that reminded researchers of a nervous system in a far larger animal. It was believed a series of impulses proceeds through this fibrillar system in a definite sequence, thereby controlling the rhythmic flexing of many thousands of individual cilia in several hundred separate rows, although not all at once, so waves of contraction are seen to pass over the cell's surface. If portions of the fibrillar system are severed, as in the ciliate Euplotes, coordination of cirri, or fused cilia, is lost.
Then the electron microscope came along and revealed a system of hitherto unimagined complexity beneath the surface of "ordinary" ciliates like Tetrahymena. There is an entire network of microtubules running beneath the pellicle ("skin") of this protist: transverse tubules, longitudinal tubules, and those running behind and underneath each cilium. No one could find evidence they conducted stimulating impulses, but they did appear to be stabilizing anchors for beating cilia in typical ciliates.
Nevertheless studies confirmed that coordination through fibrils or microtubules probably does occur in Euplotes and other highly specialized hypotrich ciliates like Stylonychia, all possessing leg-like cirri with which to scamper around like tiny mice. All one has to do to demonstrate this is with a bit of micro-surgery make an incision between clusters of cirri, severing any connections. At once those cirri at one end of the animal try to go one way, those at the opposite end the other way, and the result is a microscopic version of Dr. Doolittle's push-me-pull-you. But you can't do this with Paramecium and the vast majority of other ciliates that have orderly rows of identical cilia anchored in place. There the rhythm of adjacent cilia appears due to the retarding effect of the surrounding water upon the close crowding of one cilium to another. One cilium is stimulated by the preceding one after a delay calculated in milliseconds that is caused by the water's interference, and when this occurs in sequence down a line of thousands of cilia, waves of contraction are clearly visible.
When you look at rows of beating cilia, it is easy to be reminded of Roman or Phoenician galleys, except those early ships were propelled by banks of oars beating in unison, usually synchronized by the sound of a drum or gong. Animals using cilia do a better, smoother, and more complex, job. The way I explained it to zoology students was to imagine a field of grain waving in the wind. From above, you see spaced, rhythmic waves parading across the wheat field, yet you know the plants aren't going anywhere. Now suppose there was no wind, but the wheat plants were bending and beating of their own accord: they would produce a stiff breeze overhead! And if the field itself wasn't firmly anchored, it might take off and move of its own accord across the landscapean acre of wheat flying off into the distance!
The analogy is this: each wheat plant would be a giant cilium, its anchoring roots underground perhaps linking up with the roots of others (the fibrillar system). Alternating stiff bending and relaxed recovery of wheat stalks would result in undulating waves. In biology we call such movement (in tissues or on the surface of Paramecium) "metachronal rhythm," and all that means is a rhythm that "changes time" to produce waves. It is a beautiful sight under a microscope to an observer who realizes this is an action originally derived from simple creatures that came to live together in the dawn of life.
It is theorized that during very early times in the earth's history, slender, highly motile, free-living cells we now call spirochetes (many modern species of which are disease-causing parasites and not at all nice) somehow took up mutualistic co-existence with larger cells of a very different sort, then remained sticking out of the host's surface like wriggling hair. The benefits of this arrangement are not hard to imagine, for both partners would be aided by moving to new areas where food might be more plentiful and conditions more favorable. Over millions of years the union became permanent, and today there is an endless array of cells possessing either flagella (long, whiplike hairs), or short coordinated cilia. All these propelling structures have a common internal organization of nine fibers surrounding two central fibers, the same sort of arrangement found in free-living spirochetes. Depending upon how some of the fibers contract, the cilium or flagellum bends one way or another, either in a rigid power stroke, or a more relaxed recovery stroke.
The beating movement of a single cilium is exquisitely effective. The power stroke consists of beating stiffly in one direction, while during the recovery stroke the cilium is pulled back in floppy fashion close to the cell surface, thereby offering little resistance. Once in position, it straightens out and the stiff power stroke begins again.
Flagella may extend to the rear of a cell and push it forward by snakelike wriggling, or stick out in front and draw it along. Those protists with flagella at the anterior of a cell, such as many phytoflagellates like Euglena, wave in complex fashion, some of them creating negative and positive pressures resembling a ship's propellerall this with a single flexible "hair."
We humans possess both flagella and cilia. Each sperm cell is propelled by a trailing flagellum that accelerates the little torpedo forward in its quest to fertilize an egg. Once discharged from an ovary, an egg is carried downward in cilia-coated Fallopian tubes where fertilization takes placethat is, if a healthy and persistent sperm cell is capable of swimming far enough upstream against the current created by millions of beating cilia.
Both men and women (and all other mammals) have cilia in two other places in their bodies. One location is somewhat familiar: cilia coat the trachea and bronchial tubes leading down into the lungs. If we breathe in sooty air for a while (never a good idea), we tend to expel sooty sputum for a day or two after. Unsightly though this may be, it is a good thing, because it means our lungs and the tubes leading to them are getting rid of potentially harmful particles that should not be allowed to reside far down in such delicate and life-sustaining tissue.
The other array of cilia is so hidden, so little known to almost everyone other than physicians and zoologists, it comes as a surprise to learn that cilia exist and function busily inside our brain and spinal cord. When a human being is an early tiny embryo, long before the fetal stage, its nervous system is a simple hollow tube, little more than an in-folding of ciliated embryonic skin, or epithelium. And the embryo retains this basic pattern into adulthood, for the brain is hollow, and so is the spinal cord. The empty spaces in the brain are called ventricles (not to be confused with chambers in the heart having the same name), and the hollow running down the spinal cord is called a cerebro-spinal canal. Fluid filling these spaces is kept in slow motion by the beating of cilia lining both ventricles and canal. This watery, clear fluid is also found on the outside of brain and spinal cord and is called, simply enough, cerebro-spinal fluid. Sometimes physicians tap a bit of this fluid from between the vertebrae of the spinal column to look for blood cells resulting from injury or for dangerous disease-causing organisms.
As I study members of the animal kingdom, I find cilia everywhere. Earthworms and small aquatic worms in our local ponds have kidney-like organs that contain beating cilia needed to expel nitrogen wastes. Tiny microscopic rotifersharmless, cosmopolitan, and the delight of microscopists everywhereswim by means of two cilia-bearing projections held out in front. If you study one species of rotifer, Asplanchna, under a microscope, you should be able to see its flame cells, which are tiny chambers containing clusters of cilia that appear to flicker like flames. Their activity creates pressure that causes waste fluids to be expelled to the outside through an excretory canal.
Clams, oysters, and mussels feed and breathe only because of wide, curtain-like gills that are densely covered with cilia. By their coordinated beating action, water is drawn in through one siphon of a bivalve, passed along its gills where oxygen and food are removed, then dissolved wastes are expelled through another siphon formed from the soft tissues of these shelled molluscs.
In fact, it would be a lot easier to name the very few groups of animals, such as nematodes, that don't have cilia somewhere in their bodies, or at least in some stage of their development. Lots of marine creatures have ciliated larvae, while other animals that are quite largecomb jellies, for exampleswim exclusively by means of cilia.
Ciliary (and flagellary) activity is a perfect solution to a multitude of problems, whether allowing a small animal to get from here to there, or to bring food to a creature without it having to go anywhere, or to carry down an egg preparatory to fertilization and starting a new life, to get rid of soot from delicate lungs, or to freshen the innermost recesses of our brains. And we needn't stop with the animal kingdom, for the swimming sperm of mosses and ferns propel themselves in much the same way with flagella, as do a host of microscopic planktonic plants in both sea and pond.
I think of an old maxim, "natura maxime miranda in minimis," or "nature is greatest in little things." If you can figure out a better way than using cilia and flagella to precisely meet life's continuing demands by overcoming countless microscopic challenges in the living world, you'll be superior to a billion years of biological evolutionary trial, error, and accomplishment. But you won't succeed.
© 1999 William H. Amos
Bill Amos, a retired biologist and frequent contributor to Micscape, is an active microscopist and author. He lives in northern Vermont's forested hill country colloquially known as the Northeast Kingdom, and takes delight in studying the several ponds on his land.
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Published in the June 1999 edition of Micscape Magazine.
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