MadSci Network: Cell Biology

Re: What is the importance of cell shape???

Date: Tue Feb 24 14:52:57 1998
Posted By: Paul Odgren, Instructor, Cell Biology, University of Massachusetts Medical School (Dept. of Cell Biology)
Area of science: Cell Biology
ID: 887377971.Cb

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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