what is callus in tissue culture

In tissue culture, callus is defined as a group of cells derived from competent source tissue that is cultured under in vitro conditions to form an undifferentiated mass of cells. Callus formation is induced from plant tissue samples (explants) after surface sterilization and plating onto tissue culture medium in vitro (in a closed culture vessel such as a Petri dish). The culture medium is supplemented with plant growth regulators, such as auxin, cytokinin, and gibberellin, to initiate callus formation or somatic embryogenesis). Callus cultures are often broadly classified as being either compact or friable. Compact calluses are typically green and sturdy, while friable calluses are softer and more delicate). Callus culture is a mass of differentiated cells and formed morphological irregularities on the surface of injured tissue. Callus is a result of cellular/tissue reprogramming due to conditions overriding cell/tissue differentiation constrains (hormone gradients, chromatin regulation, cell division block, etc.). Callus can only be considered as a “dedifferentiated” tissue if the above plant-specific definition for dedifferentiation is considered.