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Home » Biology Homework Help » Biotechnology » Cell Cultures
Cell Cultures
Cell cultures may contain the following three types of cells: (1) stem cells, (2) precursor cells and (3) differentiated cells. Stem cells are undifferentiated cells which can differentiate under correct including conditions into one of several kinds of cells; different kinds of stem cells differ markedly in terms of the kinds of cells they will differentiate into. Precursor cells are derived from stem cells, are committed to proliferation. In contrast, differentiated cells usually do not have the capacity divide. Some cell cultures, e.g. epidermal keratinocytes cultures, contain all the three types of cells. In such cell cultures, stem cells constantly provide new cells which develop into precursors; the precursor cells proliferate and mature into the differentiated cell types. Thus stem cells are necessary for the maintenance of such cultures, which by their nature are heterogeneous.

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 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 and cell proliferation are affected by, in addition to cell density, factors like serum, Ca2+ ions, hormones, cell to cell and cell to matrix interactions etc. Generally, cell proliferation is promoted by low cell density, low Ca2+ (100-600 M), and high growth factor levels, while differentiation is promoted by the exact opposite conditions and by the presence of differentiation including factors, e.g. cortisone, nerve growth factor etc. The proportions of stem, precursor and differentiated cells are markedly affected by the source tissue used for obtaining the cultures. For example, cultures derived from embryos and those derived from even adult tissues where continuous cell renewal occurs naturally, e.g. intestinal epithelium, hemopoietic cells (cells producing blood cells) etc. stem cells are likely to be more frequent than in other cell cultures. In contrast, cell cultures from tissues where renewal occurs only under stress, e.g. fibroblasts, muscles etc. may contain only precursor cells.

Cell cultures can be grown as (1) monolayers or as (2) suspension cultures. Propagation in suspension cultures is limited to hemopoietic cell lines, ascites tumours and transformed cells (those cells that have become phenotypically modified during in vitro culture to become anchor-independent and are able to grow in layers of several cells thick, as against monolayer growth of non-transformed cells). Therefore, cells in culture need a surface to adhere to so that they are able to proliferate. Cells that are unable to adhere to a substrate are unable to divide, i.e. their growth is anchorage dependent.

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