The source of cells to be used within studies are divided into two main groups depending upon the origin. The first group, Primary Culture, refers to those cells which are directly obtained from tissue, whereas Non-primary (Secondary, Tertiary, etc.) sourced cells are those obtained from existing isolated populations.

Primary Culture Sources

The primary sourced cells must be isolated from the tissue in some way. Most tissues surround cells with a biologically fabricated matrix which makes it hard to remove the cells. To effectively remove cells a number of techniques exist.

Isolation from fluid tissue
From fluid tissue such as blood the isolation of cells is a generally simple process. The cell populations may be differentiated by various centrifugation processes and more specialised techniques exist for hard to distinguish cells.

Solid Tissue - Outgrowth

First tissue is dissected into a number of fine pieces. These form the primary explants, and cells are encouraged to migrate out of the tissue into a provided substrate, possibly tissue culture plastic. The cells can then be removed from the substrate and culture can commence.
Solid Tissue - Mechanical Disaggregation

This involves attempting to physically remove cells from the matrix. Techniques invlove sieving, syringing, and vigorous pipetting. Normally it starts with fine chopping, and is used for isolation of articular chondrocytes.

Enzymatic Disaggregation
The process involved here is the use of various enzymes to digest the matrix, leaving the cells hopefull free. Commonly used enzymes are trypsin and collagenase. The use of trypsin is highly temperature dependant and takes between hours up to a day. A cold trypsin will give a better result generally but takes longer, wherease warm trypsin will give fast disaggregation, possibly being less harmful to the cells, but needing repeated sampling and centrifugation. Collagenase requires the cells to be incubated in complete medium for some time following digestion. Collagenase typically contains a number of impurities, which in fact help matrix digestion, but hinder cell viability. In the pure form collagenase is somewhat expensive, leading to a trade off between viability and cost.

Other Source

Passage from other cultures
Following freeing cells using trypsin in a buffered saline from a culture, they may be transfered into new cultures. This process known as passage.

Cryopreserved Cells
Following isolation surplus cells may be preserved at low temperature in liquid nitrogen. These may be resuscitated when needed.

Cell lines from commerical sources
Immortalised cell lines are available for purchase. These are generally well characterised which has great benefits in terms of comparison of results and being able to repeat measurements without cell population variability.