**2. Types of cell culture**

Cell culture broadly denotes maintaining cell population, enabling both growth and propagation of cells. There are different types of cell culture methods. The primary culture involves desegregation of cells from the mother tissue by application of enzymatic or shear processes. Cells are transferred to a sterile system with favorable media for growth. This is usually done in glass or plastic containments such as flasks, petri plates, dishes and so on. Primary cultures are often heterogeneous, that is they are a mixed collection of different cell types that are present in the source tissue. Depending on the application requirements, the mixed pool can directly be considered for a study or the different cell populations may be identified by examining biomarkers and further sorted to obtain a culture of a single type of cells. Primary cultures are often further classified into adherent and suspension cultures. Adherent cultures are anchorage dependent [4]. They require surface support for normal proliferation. Adherent cultures are grown in containers coated with a basal polymeric protein matrix such as lysine.

When normal cells are isolated for a period, detached from the surrounding extracellular matrix, it leads to growth arrest and even the induction of anoikis. The cell-cell contact [5] in the extracellular matrix provides a local niche that provides the necessary growth factors, cytokines and integrin binding molecules that favors cell survival and growth. On the other hand, cells growing as a monolayer in a culture dish, also experience a growth arrest when they have crowded and have no more space to spread out. Cells stop proliferating once they fill out the culture dish. As such routine sub culturing or passaging is necessary, to not only maintain the proliferating pool of cells but also to keep them healthy. Tissue from most organs are candidates for adherent cultures.

Other than primary, there is also the suspension type cultures. These cells do not require a support matrix to grow. Examples include cells of the hematopoietic system. From an industrial perspective, suspension cultures are easier to maintain and implement in a large scale set up. Optimization and quantification of various culture parameters are quicker, and this helps efficient development of protocol for production. Like passaging in adherent cultures, suspension cultures need dilution for further growth and propagation. Cell population in a culture is finite and a function of the concentration of cells in the medium. Constant agitation is required to avoid flocculation of cells. Although do not require enzymatic and mechanical detachment as in adherent cultures. Adherent cultures best suit cytological studies while suspension cultures enhance bulk protein production.

Although primary cells retain genetic integrity of the source tissue, there is a limit to its life span and proliferative ability. It varies between donor tissues, requires optimization of culture conditions and is time consuming to grow. The other alternative to primary are continuous cell lines. They are primary cells transformed through subsequent culturing. Transformation can occur naturally or be induced through chemical and viral means. They greatly benefit from their ease of culturing methods. They are characterized for markers and often are available with well-established protocols of handling and propagation. They enable quicker enable quicker biochemical and cellular analysis of mammalian cells. A large number of experiments can be conducted and repeated to add accuracy to experimental evaluation. This is highly desirable for research and industrial applications. Antibody production, screening of toxic compounds and gene expression studies can all be achieved in a time and cost-efficient manner. Drawbacks however include disparity of investigations with *in vivo* systems. Thus, any use of continuous cell line as a model of study, needs to be followed by *in vivo* analysis and medical trials before consideration for implementation.
