A: Scientists are not exactly sure how cells establish their
different functions, but they think that DNA (deoxyribonucleic acid), the
hereditary material carried by the chromosomes, controls differentiation
partly by directing the production of certain enzymes. As these enzymes
appear in a cell, their chemical reactions cause the cell to become specialized.
The variations between cells reflect the tasks of different
cells. Some of the cells nearest to the "basic" design include the hexagonal
liver cells, which perform many complex chemical reactions. Other simple
cells are those that provide support and lining in many tissues. Often
these column-shaped cells produce the sticky substance, mucus, and are
edged with minute hair-like projections (cilia), which can move substances
along. The fat cells found beneath the skin and around many organs are
simple cells whose cytoplasm is packed with globules of fat. The fat is
used to provide insulation and energy. Red blood cells, which carry oxygen
and carbon dioxide within the circulation, are unusual in having no nuclei
in their mature form. In contrast, many of the white blood cells, part
of the body's defense system against disease, have very large nuclei.
Three of the most specialized sorts of cells are muscle
cells, nerve cells, and reproductive cells. Muscle cells are greatly elongated
and have the power of contraction made possible by special proteins that
can slide over one another.
Because muscle contraction is energy-intensive, muscle cells
have huge numbers of mitochondria.
Nerve cells are also elongated but have specialized membranes
for transmitting the electrical impulses of nerve messages. Each nerve
cell ends in a cell body bearing projections that lie close to similar
projections on adjacent nerve cells.
Messages "jump the gaps" with the aid of chemicals made
in the nerve cells.
Reproductive cells (sperm from the male and eggs from the
female) are unique in containing only half the usual number of chromosomes.