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Size: The sizes of human cells vary considerably.
Very few of them are visible to the unaided eye
or with a magnifying glass. The largest cells, the
ova (egg cells), become just barely visible at a
diameter of 0.15 mm (150 µm) or more. (1 µm =
1 micrometre = 1 thousandth of a millimetre).
Many of the larger neural (nerve) cells have a
diameter of between 0.12 and 0.2 mm = 120 to
200 µm. Many neural cells have extensions
(processes) up to one metre or more in length,
but their diameter is only a few micrometres. The
sizes of most other human cells vary between
5 µm and 20 µm, depending on the tissue type.
Even so, some neural cells and some giant cells in
bone marrow can measure up to 80 µm or more,
several times the average cell size. Measuring
4 to 5 µm, the glial cells (Latin glia = glue) are,
along with sperm cells, among the smallest
human cells. The diameter of the “heads” of
sperm cells is between 3 and 5 µm. With their
average diameter of 7.5 µm, the red blood corpuscles
also count among the smallest cells. The
median cell size is between 30 and 50 µm.
Shape: Because of their special functions the
shapes of most cells differ widely. They are eminently
suited to their locality. Cells occurring in
covering layers, like an epithelium, can be cubical,
flat, or prismatic, and are as densely packed as
their shapes allow. Many cells are spherical, or
spindle shaped like those in the smooth muscle
tissue. Nerve and connective tissue cells can have
long, branching processes.
Multitasking: Even though the basic plan of all
cells is the same, they may differ significantly
according to their function. Every kind of cell is
specialised to perform certain tasks in the organism.
The red blood cells (erythrocytes) transport
oxygen, the nerve cells carry information, glandular
cells secrete specific substances, muscle
cells are responsible for body movements, and
sex cells serve reproduction. The various functions
of a cell are all coded for by way of specific
genetic information, stored in certain sections of
the DNA (desoxyribonucleic acid) of the cell itself.
Pre-programmed here are processes like cell division
and the synthesis of all the necessary proteins.
These two processes are essential prerequisites
for the development of a multi-cellular
organism from a fertilised ovum. It is remarkable
that such diversified cells as are found in the
brain, lungs, muscles, or liver, all develop from
undifferentiated precursors. Cells with different
functions also have different “life expectancies”.
Some cells last for only a few hours or days (eg
intestinal epithelial cells), and others, like neural
cells, live as long as the organism itself.
Number: The estimated total number of cells
constituting the human body is an astonishing
100 million million (10 14 )! It is practically impossible
to visualise a number like 10 14 . To count up
to this number, counting non-stop at the rate of
one per second, day and night, would take three
million years. The number of red corpuscles in
the blood, the most numerous of all the cell
types, is 25 x 10 12 = 25 million million. But, unlike
all other cells, they have neither a nucleus nor
cellular organelles. The total number of brain
cells is about hundred thousand million.
Taking the average size of a cell as 40 µm, all the
cells of a human body, placed side by side, would
form a chain reaching 100 times around the
equator.
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