thumb|275px|[[Apoptosis]]
Pyknosis, or karyopyknosis, is the irreversible condensation of chromatin in the nucleus of a cell undergoing necrosis or apoptosis. It is typically followed by karyorrhexis, the fragmentation of the nucleus.
The term derives from Ancient Greek meaning "thick, closed or condensed". Pyknosis also occurs as part of normal cell maturation in certain blood cells.
Occurrence
thumb|275px|[[Micrograph of an infarct in the biliary tract, with pyknotic nuclei (arrows) (400x).]]
In erythropoiesis, the maturing metarubricyte condenses its nucleus before expelling it to form a reticulocyte. In granulopoiesis, the developing neutrophil condenses its nucleus into several connected lobes, which remain in the cell until its death.
Pyknotic nuclei are also commonly seen in the zona reticularis of the adrenal gland and in the keratinocytes of the outermost layer of parakeratinised epithelium.
Mechanism
Pyknosis represents irreversible nuclear condensation in a cell undergoing apoptosis or necrosis. Two main types are described: nucleolytic pyknosis, which occurs during apoptosis (programmed cell death), and anucleolytic pyknosis, which occurs during necrosis.
Necrosis is a regulated form of cell death triggered by toxins, infections, or other acute stressors. Karyorrhexis refers to the subsequent fragmentation of this condensed nucleus and cleavage of chromatin. Both necrosis and apoptosis are regulated by a few of the same proteins: caspase-activated DNase (CAD), endonuclease G and DNase I. Pyknosis occurs in both an apoptotic and a necrotic cell. Pyknosis in an apoptotic cell is identified by nuclear condensation, chromatin fragmentation, and the formation of a few large clumps which are enveloped by apoptotic extracellular vesicles, which are to be released when the cell dies.
ISEL (in-situ labeling technique) is a labeling/tagging technique of apoptotic or necrotic cells.]]
Pyknosis is a stage in the apoptotic or necrotic cell death pathways. It is an important stage that involves fragmentation and condensation of damaged DNA/chromatin. Without it, the apoptotic or necrotic cell death pathways would be interrupted. This disruption, in turn, may prompt the improper destruction or removal of a cell with damaged elements as well as other related issues. These issues include cell accumulation and uncontrolled cell growth, which results in the formation of cancerous and abnormal tissue masses known as tumors. Therefore, being able to observe or identify when a cell is pyknotic (which may indicate that the cell is undergoing apoptosis or necrosis) and if it then successfully undergoes apoptosis or necrosis, may be crucial in determining if the cell will undergo uncontrolled cell growth and contribute to the formation of a tumor.
See also
- Apoptosis
- Necrosis
- Karyolysis
- Karyorrhexis