|MadSci Network: Cell Biology|
Dear Jude, Your question concerns a very interesting issue in biology and evolution, namely, how cells differ from one another. Cells have evolved specialized shapes that help them to accomplish their functions more efficiently, thereby improving chances for survival over evolutionary time. Just as giraffes with long necks were favored by natural selection (the cornerstone of Darwinís theory of evolution) in dry times when the only leaves to be eaten were on trees, so too has evolution shaped cells, some of which carry out amazingly specialized functions, and whose shapes can take on fantastically elaborate forms to help in those functions. Letís use a familiar multicellular organism like man as an example, although we could use a monkey, a shrimp, a tree, or seaweed just as well. The 200 or so different types of cells in your body actually differ from one another in just a few ways, one of the most important of which is shape. The genes inside the nucleus of a muscle cell are exactly the same as those in a nerve cell, and those are the same as in a cell that makes bone. With only a few exceptions (mainly having to do with changes in a few genes required for specific immune recognition by certain white blood cells, or in egg or sperm cells which have only half of the set of genes), the genes are identical in ALL your cells. In fact, the genes that are actually switched on are mostly identical from cell to cell. All the basic biochemistry needed for energy production, for the synthesis of proteins, and for moving things around inside your cells is the same from one cell type to another. Biologists call these "housekeeping genes," genes needed to keep the cellís basic functions rolling along, whatever the specific cell type. Itís when you come to issues of specialization that things get really interesting, both in terms of gene switching and in cell shape. Here are two extreme examples. In your body, you have some neurons, single nerve cells, with microscopic projections like little wires, called axons, that are over 1 meter long. A single cell can have an axon that goes from your spinal cord all the way down your leg. This speeds up the travel of a nerve impulse by avoiding the need to get handed along from cell to cell, like the pails in a bucket brigade, instead performing the equivalent of sending it through a hose. At the other size extreme of your bodyís cells you have red blood cells, that are only about 7 micrometers across. By being small, they can get through all the capillaries, the microscopic blood vessels that supply your tissues, without getting stuck. Red cells are also in the shape of a disk thatís a little concave on both faces to get the most surface area for oxygen and carbon dioxide exchange; and by packing millions of them into every cubic millimeter of blood, you get a tremendous surface area for gas exchange per unit volume. Red cells are also unique among your bodyís cells because they spit out their nuclei as they mature to give them the greatest carrying capacity for hemoglobin, the red, gas-carrying protein molecule. In the cochlea of your inner ear you have cells that have little hairs projecting off them in what is called the organ of Corti. This is where sound vibrations that are transmitted through your ear drum, and then through the tiny bones on your middle ear, get changed into nerve signals. The hairs vibrate in response to the sound waves, and they act as transducers that change the pressure oscillations in the fluid into nerve impulses. And the light-sensitive cells of your retina also have shapes that enable them to carry out their task with efficiency. In the cone cells (the color-sensitive cells), for example, there are stacks and stacks of membranes that carry a light-sensitive pigment and a single nerve connection at the base of the cell. This arrangement gives each cell a very high sensitivity to light but confines it to a tiny area of the retina, allowing you to resolve visually objects that are close together. In the lens, on the other hand, you have layers of transparent cells that look for all the world like lasagna noodles, kind of flat rectangles but with curly, interdigitated edges. They are all stacked together in bundles so that the whole lens can change shape when muscles attached to it contract to change your eyes' focus. In the small intestine, you have specialized absorptive cells lining the inner surface to take up nutrients out of the food youíve eaten. The upper surface of each cell is covered with hundreds of microvilli, micrometer-sized hairs that are covered with transporter molecules that take in the good stuff and leave behind most of what you donít need. In other words, Iím sure you get the idea that cell shapes are intimately connected with the many specialized functions they carry out. Even single-celled organisms have highly specialized shapes that help them carry out their life cycles in the most efficient and successful manner, some with cilia for swimming around, some with little exoskeletons for mechanical rigidity, some with sticky surfaces for getting a hold on their favorite place to abide, and so on. For more on the cells in your body, any histology text or atlas will have good diagrams and explanations of cell functions, and how they are integrated into specialized tissues. Thanks for your question. I hope this answers it in a way that makes you want to keep learning about cells and what they do. Paul Odgren Dept. of Cell Biology Univ. of Massachusetts Medical School
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