Types of Cell Cultures

The maintenance of growth of cells dissociated from the parental tissue (such as kidney, liver) using mechanical or enzymatic methods, in culture medium using suitable glass or plastic containers is called Primary Cell Culture.

The primary cell culture could be of two types depending upon the kind of cells in culture.

a) Anchorage Dependent / Adherent cells - Cells shown to require attachment for growth are said to be anchorage dependent cells. The adherent cells are usually derived from tissues of organs such as kidney where they are immobile and embedded in connective tissue. They grow adhering to the cell culture.

b) Suspension Culture / Anchorage Independent cells - Cells which do not require attachment for growth or do not attach to the surface of the culture vessels are anchorage independent cells/suspension cells. All suspension cultures are derived from cells of the blood system because these cells are also suspended in plasma in vitro e.g. lymphocytes.

Secondary Cell Cultures

When a primary culture is sub-cultured, it becomes known as a secondary culture or cell line. Subculture (or passage) refers to the transfer of cells from one culture vessel to another culture vessel.

Subculturing - Subculturing or splitting cells is required to periodically provide fresh nutrients and growing space for continuously growing cell lines. The process involves removing the growth media, washing the plate, disassociating the adhered cells, usually enzymatically. Such cultures may be called secondary cultures.

Cell Line

A Cell Line or Cell Strain may be finite or continuous depending upon whether it has a limited culture lifespan or it is immortal in culture. On the basis of the lifespan of culture, the cell lines are categorized into two types:

a) Finite Cell Lines - The cell lines which have a limited lifespan and go through a limited number of cell generations (usually 20–80 population doublings) are known as finite cell lines. These cell lines exhibit the property of contact inhibition, density limitation and anchorage dependence. The growth rate is slow and doubling time is around 24–96 hours.

b) Continuous Cell Lines - Cell lines transformed under laboratory conditions or in vitro culture conditions give rise to continuous cell lines. These cell lines show the property of ploidy (aneuploidy or heteroploidy), absence of contact inhibition and anchorage dependence. They grow in monolayer or suspension form. The growth rate is rapid and doubling time is 12–24 hours.

c) Monolayer Cultures - When the bottom of the culture vessel is covered with a continuous layer of cells, usually one cell in thickness, they are referred to as monolayer cultures.

d) Suspension Cultures - Majority of continuous cell lines grow as monolayers. Some of the cells which are non-adhesive, e.g. cells of leukemia, or certain cells which can be mechanically kept in suspension, can be propagated in suspension. There are certain advantages in propagation of cells by the suspension culture method.

These advantages are:
(a) The process of propagation is much faster.
(b) The frequent replacement of the medium is not required.
(c) Suspension cultures have a short lag period.
(d) Treatment with trypsin is not required.
(e) A homogenous suspension of cells is obtained.
(f) The maintenance of suspension cultures is easy and bulk production of the cells is easily achieved.
(g) Scale-up is also very convenient.

The cell lines are known by:
a) A code e.g. NHB for Normal Human Brain.
b) A cell line number – This is applicable when several cell lines are derived from the same cell culture source e.g. NHB1, NHB2.
c) Number of population doublings the cell line has already undergone e.g. NHB2/2 means two doublings.

CHARACTERIZATION OF CELL LINES

The cell lines are characterized by their a) growth rate and b) karyotyping.

a) Growth Rate - A growth curve of a particular cell line is established taking into consideration the population doubling time, a lag time, and a saturation density of a particular cell line. A growth curve consists of:

1) Lag Phase: The time the cell population takes to recover from subculture, attach to the culture vessel, and spread.

2) Log Phase: In this phase, the cell number begins to increase exponentially.

3) Plateau Phase: During this phase, the growth rate slows or stops due to exhaustion of growth medium or confluency.

b) Karyotyping - Karyotyping is important as it determines the species of origin and the extent of gross chromosomal changes in the line. Cell lines with abnormal karyotypes are also used if they continue to perform normal functions. Karyotype is affected by the growth conditions used, the way in which the cells are subcultured, and whether or not the cells are frozen.

c) There are certain terms that are associated with the cell lines.

These are as follows:
(i) Split ratio - The divisor of the dilution ratio of a cell culture at subculture.
(ii) Passage number - It is the number of times that the culture has been cultured.
(iii) Generation number - It refers to the number of doublings that a cell population has undergone.