ABSTRACT
Hyperplasia: an increase in the number of cells within an organ in response to a physiological stimulus (e.g. breast tissue during pregnancy and lactation) or injury. An example is liver hyperplasia and enlargement to compensate for tissue loss, even if massive in magnitude. Hyperplasia can be induced by hormonal stimulation, e.g. endometrial hyperplasia in response to oestrogen. Hypertrophy: an increase in the size of cells, e.g. skeletal muscle hypertrophy in response to exercise. Apoptosis: programmed cell death (PCD). The cells are damaged either by intrinsic changes, which inevitably lead to cell death (e.g. endometrial sloughing with menstruation), or by extrinsic factors such as viruses. Apoptosis can be advantageous, e.g. removal of tissues between the fingers and toes during embryonic development prevents syndactyly. Necrosis: cell death due to acute cellular injury, such as from trauma or virus infection.
