Physiological Regeneration
This type of regeneration is a regular physiological function including the continuous replacements of cells and tissues, and thus is indispensible for the maintenance of life in all animals. It is a primary attribute of all living systems. With no such type of regeneration there would be no life, as the extremely maintenance of an organism depends upon the incessant turnover by which all tissues and organs renew themselves. For instance, regular replacement of epidermal skin layers and RBC etc., in our body is a must. In a few instances quite substantial quantities of tissues are replaced periodically, as in the successive production of follicles in the ovary or the moulting and the replacement of feathers and hairs. In our body one per cent (%) of the total 25*1012 RBCs present in active circulation, die every day and thus are replaced daily. The time span of regeneration varies; as for example, mammalian skin epidermal cells, generated at the basal level may take several weeks to reach the outer surface and be sloughed off, whereas the life span of an individual epithelial cell in the intestine may be limited to a few days. The motile hairlike flagella and cilia of single-celled organisms have the capability to regenerate within an hour or two after amputation.