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Home » Biology Homework Help » Biotechnology » Callus, Suspension Cultures
Callus, Suspension Cultures
When explants are cultured on a suitable GR combination, many of its cells undergo division. Even mature and certain differentiated, e.g. parenchyma and often colenchyma, cells undergo changes to become meristematic; this is called dedifferentiation. Dedifferentiation involves, among other things, renewed and enhanced RNA and protein syntheses leading to the formation of new cellular components needed for meristematic activity. Initially, cell divisions are confined to the cut ends, but subsequently it covers the entire explant. The resulting cell mass is ordinarily unorganized, but it often consists of several cell types including fibres, and vascular elements.

Callus cultures

Tissues and cells cultured on an agar gelled medium form an unorganised mass of cells called callus. Callus cultures need to be subcultured every 3-5 weeks in view of cell growth, nutrient depletion and medium drying. Therefore, calluses are easy to maintain and are most widely used.

Suspension cultures

Tissues and cells cultured in a liquid medium produce a suspension of single cells and cells clumps of few to many cells; these are called suspension cultures. Liquid cultures must be constantly agitated, generally by a gyratory shaker at 100-250 rpm, to facilitate aeration and dissociation of cell clumps into smaller pieces. Suspension cultures grow much faster than callus cultures, need to be subcultured about every week, allow a more accurate determination of the nutritional requirements of cells and are the only system amenable to scaling up for a large scale production of cells and even somatic embryos (SEs). The suspension cultures are broadly grouped as: (1) batch cultures, (2) continuous cultures, and (3) immobilized cell cultures.

Batch cultures

In a batch culture the same medium and all the cells produced are retained in the culture vessel, e.g. culture flasks (100-250 ml), fermenters (variable size) etc. the cell number or biomass of a batch culture exhibits a typical sigmoidal curve, having a lag phase during which the cell number of biomass remains unchanged, followed by a logarithmic (log) phase when there is a rapid increase in cell number and finally ending in a stationary phase during which cell number does not change. The lag phase duration depends mainly on inoculums size and growth phase of the culture from which inoculums is taken. The log phase lasts about 3-4 cell generations (time taken for doubling of cell number), and the duration of a cell generation may vary from 22-48 hr mainly depending on the plant species. The stationary phase is forced on the culture by a depletion of the nutrients and possibly due to an accumulation of cellular wastes. If the culture is kept in stationary phase for a prolonged period the cells may die.

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