The removal of cells from animals and their subsequent growth in a favorable artificial environment is called animal cell culture. Animal cells are
more difficult to culture than microorganisms because animal cells require many
more nutrients and typically grow only when attached to specially coated
surfaces. The environment consists of a suitable glass or plastic culture
vessel containing a liquid or semisolid medium that supplies the nutrients
essential for survival and extra proliferation. The cells are removed from the
tissue directly and disaggregated by enzymatic or mechanical means, and then
cultured in artificial media under an aseptic environment similar to the culture
of microorganisms. This in vitro cell culture system explores cell growth and
differentiation and performs genetic manipulations required to
understand gene structure and function.
Animal cell cultures are initiated by the dispersion of a
piece of tissue into a suspension of its component cells, which is then added
to a culture dish containing nutrient media. In animal cells, fibroblasts and
epithelial cells are selected because these cells grow very fast. The plastic
surface of dishes is used for cell culture. In 1907, Harrison cultivated frog
nerve cells in a lymph clot and observed the growth of nerve fibres in vitro
for several weeks; hence, he is recognized as the father of cell culture.
Animal cell culture is helpful to study the effect of new
drugs, to investigate cancerous cells, to study the interactions between
disease-causing agents and cells, and is useful for gene therapy, genetic
engineering, etc.
Types of Animal Cell Culture:
Based on the number of cell divisions, cell culture is
classified as primary cell culture and cell lines. Cell lines can undergo
finite or infinite cell divisions (Flowchart).
Flowchart: Types of Animal Cell Culture
Primary Cell Culture:
This cell culture is obtained from the cells of a host
tissue. The cells dissociated from the parental tissue are grown on a suitable
container, and the culture thus obtained is called a primary cell culture. The
culture is comprised of heterogeneous cells, and most of the cells divide only
for a limited time. Based on their origin, primary cells grow either as an
adherent monolayer or in a suspension.
Adherent Cells:
These cells are propagated as a monolayer and are anchorage-dependent.
Monolayer cultures are defined as when the bottom of the culture vessel is
covered with a continuous layer of cells that need to be attached to a solid or
semi-solid substrate for proliferation. They adhere to the culture vessel with
the use of an extracellular matrix, which is derived from tissues of organs that
are immobile and embedded as connective tissue. For example, Fibroblasts and
epithelial cells.
The majority of continuous cell lines grow as monolayers, and
such types of cells are transferred directly to a cover slip for examination
under a microscope.
Suspension Cells:
These types of cells do not attach to the surface of the
culture vessels. Hence, they are also known as anchorage-independent or
non-adherent cells, which are grown in liquid culture medium. Hematopoietic stem
cells (derived from blood, spleen, and bone marrow) and tumor cells are grown
in suspension much faster, which does not require frequent replacement of the
medium. These cultures have short lag periods.
Secondary Cell Culture and Cell Line (Figure)
Secondary culture is the culture when a primary culture is
sub-cultured. It is also known as a cell line or sub-clone. The process
involves removing the growth media and disassociating the adhered cells by
enzymatic treatment. Sub-culturing of primary cells to different divisions
leads to the generation of cell lines. During the growth period, cells with the
highest growth capacity predominate and result in genotypic and phenotypic
uniformity in the population. Based on the life span of the culture, cell lines
are two types viz. Finite cell lines and Continuous cell lines.
Finite cell lines:
It is defined as the cell line that undergoes a limited
number of cell divisions with a limited life span. The cells pass several times
and then lose their ability to proliferate, which is a genetically determined
event known as senescence. Cell lines derived from primary cultures of normal
cells are finite cell lines.
Continuous cell lines:
It is defined as a finite cell line that undergoes
transformation and acquires the ability to divide indefinitely. This
transformation occurs spontaneously, chemically, or virally, or from the
establishment of cell cultures from malignant tissue. Prepared cell cultures
are then sub-cultured and grown indefinitely as permanent cell lines. These
cells are less adherent and fast-growing; as a result, the cell density becomes
higher and different in phenotypes from the original tissue. They grow more in
suspension medium.
Fig.: Isolation of cell lines for in vitro culture