It has been seen that the exact replacement of a part or organ or tissue is not the just only way to regenerate in animals so many of the body's internal organs compensate for their loss by enlarging what remains, in place of re-growing the missing part Figure. This process, called compensatory hypertrophy, is feasible as the remaining mass is usually as good as that which was lost. Liver regeneration in mammals is a well documented instance of this process, where the size of the residual lobes expand, so restoring the original mass of hepatic tissues as well as its function.
Figure: Comparison between qualitative and quantitative modes of regeneration. Some structures are replaced in situ by morphological regeneration. Others are not re-grown but their residual portion enlarges by compensatory hypertrophy.
Compensatory hypertrophy in liver is accompanied through hyperplasia of its cells and of the histological functional units into which they are organized. Identical mechanisms have been noted in many endocrine and exocrine glands following surgery or physiological insufficiency. Pancreas, thyroid, adrenals and ovaries are another organs that regenerate by compensatory hypertrophy. The way they compensate for such loss is similar way they grow during ontogeny. However, not all organs are capable to multiply their functional units in such a way. Nerves, muscles, lungs and kidney are unable to do so.