272 7.3 Model Organisms
DNA damage or by shortening of cellular structures called “telomeres,” which are repeating
DNA sequences that cap the end of chromosomes (see Chapter 2). Telomeres normally get
shorter with each subsequent cell division such that at a critical telomere length cell death is
triggered by the complex biochemical and cellular process of apoptosis. However, immortal
cells can continue undergoing cell division and be grown under cultured in vitro conditions
for prolonged periods. This makes them invaluable for studying a variety of cell processes in
complex animal cells, especially human cells.
Cancer cells are natural examples of immortal cells but can also be prepared using bio
chemical methods. Common immortalized cell lines include the Chinese hamster ovary,
human embryonic kidney, Jurkat (T lymphocyte, a cell type used in the immune response),
and 3T3 (mouse fibroblasts from connective tissue) cells. However, the oldest and most com
monly utilized human cell strain is the HeLa cell. These are epithelial cervical cells that were
originally cultured from a cancerous cervical tumor of a patient named Henrietta Lacks in
1951. She ultimately died as a result of this cancer but left a substantial scientific research
legacy in these cells. Although there are potential limitations to their use in having undergone
potentially several mutations from the original normal cell source, they are still invaluable to
biomedical research utilizing biophysical techniques, especially those that use fluorescence
microscopy.
7.3.3 MODEL PLANTS
Traditionally, plants have received less historical interest as the focus of biophysical
investigations compared to animal studies, due in part to the lower relevance to human bio
medicine. However, global issues relating to food and energy (see Chapter 9) have focused
recent research efforts in this direction in particular. Many biophysical techniques have
been applied to monitoring the development of complex plant tissues, especially involving
advanced light microscopy techniques such as light sheet microscopy (see Chapter 4), which
has been used to study the development of plant roots from the level of a few cells up to com
plex multicellular tissue.
The most popular model plant organism is Arabidopsis thaliana, also known commonly
as mouse ear cress. It is a relatively small plant with a short generation time and thus easy to
cultivate and has been characterized extensively genetically and biochemically. It was the first
plant to have its full genome sequenced.
7.3.4 MODEL ANIMALS
Two key model animal organisms for biophysics techniques are those that optimized for in
vivo light microscopy investigations, including the zebrafish Danio rerio and the nematode
flatworm Caenorhabditis elegans. The C. elegans flatworm is ~1 mm in length and ~80 μm
in diameter, which lives naturally in soil. It is the simplest eukaryotic multicellular organism
known to possess only ~1000 cells in its adult form. It also breeds relatively easily and fast
taking three days to reach maturation, which allows experiments to be performed reasonably
quickly, is genetically very well characterized and has many tissue systems that have gen
eric similarities to those of other more complex organisms, including a complex network of
nerves, blood vessels and heart, and a gut. D. rerio is more complex in having ~106 cells in
total in the adult form, and a length of a few centimeters and several hundred microns thick
and takes more like ~3 months to reach maturation.
These characteristics set more technical challenges on the use of D. rerio compared
to C. elegans; however, it has a significant advantage in possessing a spinal cord in which
C. elegans does not, making it the model organism of choice for investigating specifically
vertebrate features, though C. elegans has been used in particular for studies of the nervous
system. These investigations were first pioneered by the Nobel Laureate Sydney Brenner in
the 1960s, but later involved the use of advanced biophysics optical imaging and stimulation