Pyknosis, or karyopyknosis, is the irreversible condensation of chromatin in the nucleus of a cell undergoing programmed cell death or apoptosis . It is followed by karyorrhexis, or fragmentation of the nucleus. Pyknosis is also observed in the maturation of erythrocytes (a red blood cell) and the neutrophil (a type of white blood cell). The maturing metarubicyte (a stage in RBC maturation) will condense its nucleus before expelling it to become a reticulocyte. The maturing neutrophil will condense its nucleus into several connected lobes that stay in the cell until the end of its cell life.
It's also from the nuclear changes in some necrotic cells .
1. Pyknosis : Nucleus appear as a shrunk dark nucleus 2. Karyorrhexis : Fragmentation to the nucleus takes place 3. Karyolysis : Faint dissolved nucleus then cells lose their nucleus, as necrotics steatocytes (fat cells in pancreas ) 4. Karyopyknosis : The shrinkage of cell nuclei and condensation of chromatin