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This type of culture contains mostly heterogeneous cells and these cells are similar to their parents and is called primary cell lines. For the preparation of primary cultures and cell lines, a tissue is dissected in sterile conditions followed by mechanical or enzymatic disaggregation. The tissue can simply be chopped and the pieces attached to a dish. Now cells will grow out from the fragment and may be used directly for culture. The fragment of tissue is called explant that may be transferred to a fresh dish. Primary cultures can also be made by disaggregating tissue with the help of enzymes, such as trypsin or collagenase, etc., and the cell suspension are allowed to adhere, and spread out on the substrate. Cells may grow as adherent cultures or may remain in suspension. (ii) Secondary cell culture When a primary culture is sub-cultured or passaged, the resulting culture is called secondary cell culture. During sub-culturing, a portion of cells are transferred to a new vessel having fresh growth medium for continual growth of cells. Sub-culturing of primary cells leads to the generation of secondary cell lines. During the passage, the cells with highest growth capacity predominate, leading to genotypic and phenotypic uniformity in the population. As the cells are sub-cultured at regular intervals, they may become different from the original cell. On the basis of the culture life span, the cell lines are categorised into two types: (a) Finite cell lines  The cell lines with a limited number of cell division and having a limited life span are called finite cell lines. As the cells pass several times, they lose their ability to proliferate (i.e., senescence). Cell lines derived from primary cultures of normal cells are finite cell lines. (b) Continuous cell lines Some cells of the finite cell line undergo transformation and resume the ability to divide indefinitely, and become a continuous cell line. This type of transformation or mutation can occur spontaneously or can be chemically or virally induced. Cell cultures prepared in this way can be sub-cultured and grown indefinitely as permanent cell lines Animal Cell Culture 197 Reprint 2025-26