Cell Culture

Cell cultures may contain following three types of cells:

   1. Stem cells
   2. Precursor cells
   3. Differentiated cells

stem cells are undifferentiated cells, which can differentiate under correct inducing conditions into several kinds of cells; different kinds of stem cells differ markedly in terms of the kinds of the cells in which they will differentiate into. Precursor cells are derived from stem cells, are committed to differentiation, but are not yet differentiated; these cells retain the capacity for proliferation. In contrast Differentiated cells usually do not have the capacity to divide. some cell cultures, e.g. epidermal keratinocyte cultures, contain all the 3 types of cells. In such cultures stem cells constantly provide, new cells which develop into precursor cells, which further proliferate and mature into differentiated cell types. Thus stem cells are necessary for maintenance of such cultures, which by their nature are heterogenous.
  
On the other hand, other cell cultures, e.g., fibroblast cultures, contain a more or less uniform population of dividing cells at low cell densities(<10⁴ cells/cm² ), but at high cell densities( 10⁵ cells/ cm²) are uniformly composed of non- proliferating differentiated cells. The cells begin to proliferate once the cell density is approximately reduced.
      Differentiation & cell  proliferation are affected by, in addition to cell density, factors like serum, Ca²⁺ ion, hormones, cell to cell & cell to matrix interactions etc. Generally, cell proliferations promoted by low cell density, low Ca²⁺ ions (100-600 μM), & high growth factor levels, While differentiation is promoted by the exact opposite conditions & by the presence of differentiation inducing factors, e.g. cortisone, nerve growth factor, etc. The proportion of stem, Precursor & differentiated cells are markedly affected by the source tissue used for obtaining the cultures. For example cultures derived from embryos & those derived from even adult tissues where continuous cell renewal occurs naturally, e.g., intestinal epithelium, haemopoietic cells etc, stem cells are likely to be more frequent than in other cultures.

 In contrast, cell cultures from tissues where renewal occurs only under stress, e.g., fibroblast, muscle, etc, may contain only precursor cells.

Cell cultures can be grown as:

   1. Monolayer or as
   2. suspension cultures.

Propagation in suspension cultures is limited to haemopoietic cell lines, ascites tumors & transformed cells. Transformed cells are those cells that have been phenotypically modified  during in vitro culture to become anchor independent and are able to grow in layers of several cell thick, as against monolayer growth of non transformed cells. Therefore, cells in culture need to surface or substrate to adhere to so that they are able to proliferate. In contrast, cells that are unable to adhere to a substrate are unable to divide, i.e., their growth is anchorage dependent.