固缩（又叫细胞致密变化，英语：Pyknosis）是细胞经历坏死或凋亡后核内染色质不可逆的压缩。接着会发生核破裂。红血球或中性白血球成熟时也会观察到固缩。

Pyknosis, or karyopyknosis, is the irreversible condensation of chromatin in the nucleus of a cell undergoing necrosis or apoptosis. It is followed by karyorrhexis, or fragmentation of the nucleus. Pyknosis (from Greek pyknono meaning "to thicken up, to close or to condense") is also observed in the maturation of erythrocytes (a red blood cell) and the neutrophil (a type of white blood cell). The maturing metarubricyte (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.