Karyolysis

Morphological characteristics of karyolysis and other forms of nuclear destruction.

Karyolysis (from Greek κάρυον karyon—kernel, seed, or nucleus), and λύσις lysis from λύειν lyein, "to separate") is the complete dissolution of the chromatin of a dying cell due to the enzymatic degradation by endonucleases. The whole cell will eventually stain uniformly with eosin after karyolysis. It is usually associated with karyorrhexis and occurs mainly as a result of necrosis, while in apoptosis after karyorrhexis the nucleus usually dissolves into apoptotic bodies.

Disintegration of the cytoplasm, pyknosis of the nuclei, and karyolysis of the nuclei of scattered transitional cells may be seen in urine from healthy individuals as well as in urine containing malignant cells. Cells with an attached tag of partially preserved cytoplasm were initially described by Papanicolaou and are sometimes called comet or decoy cells. They may have some of the characteristics of malignancy, and it is therefore important that they be recognized for what they are.

Overview

Karyolysis is the culminating step in the process of necrosis. Necrosis is a form of cellular injury in which living tissue experiences irreversible damage through premature cell death. While both are forms of cell death, necrosis differs from apoptosis as an external factor triggers necrosis rather than it being a controlled and planned process.

Triggers

First, it is essential to understand the factors that can trigger a necrotic reaction. A typical example is ischemia, also known as reduced blood flow, in which the interrupted or decreased blood supply cannot sufficiently supply oxygen and nutrients, leading to necrosis. In this situation, blood flow restriction will deprive cells of oxygen (hypoxia), impairing cellular respiration and energy production. Ischemia triggers ATP depletion and accumulation of metabolic waste, leading to cell death. This process can occur in myocardial infarctions (heart attacks), where a blood clot blocks coronary arteries, and the lack of blood flow through said arteries causes heart muscle cell necrosis.

Some more commonly seen triggers of necrosis include physical trauma (such as crush injuries, burns, and frostbite), viral/bacterial infections, chemical/toxicant exposure, immune reactions (autoimmune and inflammatory), radiation exposure, and oxidative stress. While there are many causes of necrosis, the basic principle remains: an external factor affects the cells or tissue unexpectedly, eliciting a reaction of steps that terminates the cells prematurely.

Necrosis involves the nucleus undergoing an integral series of morphological changes that are critical indicators of the cell's deterioration. The three steps are pyknosis, karyorrhexis, and terminating in karyolysis.

Necrotic pathway leading to karyolysis

Pyknosis

Pyknosis (stemming from the Greek pyknos (πυκνός), meaning "dense" or "thick") is the first step in which the nucleus condenses, reflecting the root meaning. During pyknosis, the chromatin within the nucleus clumps together, resulting in a shrunken, hyperchromatic nucleus, seen as a compact and dark form in microscopic views; this dense appearance is characteristic of the cell preparing for the following stages of necrosis. Pyknosis is unique to the other steps of necrosis in that it is the only step that commonly occurs the same way in processes other than necrosis. While karyorrhexis and karyolysis are typically associated with necrosis (or in the case of karyorrhexis, has different mechanisms in apoptosis and necrosis), pyknosis is a characteristic step observed in both necrosis and apoptosis, as well as some normal cell differentiation–an example of this being normal erythrocyte (red blood cell) maturation.

Karyorrhexis

Following pyknosis, karyorrhexis (stemming from the Greek karyo- (κάρυον), meaning "nut" or "nucleus," with rhexis (ῥῆξις), meaning "bursting" or "breaking") ensues in which the nucleus fragments or bursts. During this phase, the nuclear envelope breaks down, causing the condensed chromatin to break apart and then distribute nuclear fragments throughout the cytoplasm. Karyorrhexis occurs in apoptosis– with a different cause and purpose than necrosis–and, on rare occasions, occurs in normal cell differentiation processes. The difference between apoptotic and necrotic karyorrhexis is essential to the karyolytic process and incidence. In an apoptotic cell, following chromatin condensation (pyknosis), the nucleus fragments in an organized way, breaking down into small, membrane-bound apoptotic bodies. Each body contains a portion of the nucleus and cytoplasmic material, neatly packaged. These fragmented and packaged membrane-bound bodies then signal nearby phagocytic cells (such as macrophages) to engulf them. This entire process serves as a clean removal process. It prevents the release of cellular contents into surrounding tissue and extracellular space, thus minimizing damage to neighboring cells and avoiding an inflammatory response. This measure is essential for maintaining normal tissue health when cells die, which is imperative to a cell's typical life cycle.

Karyorrhexis also follows pyknosis along the necrotic pathway; however, in the case of necrosis, the inducing stimulus causes the nuclear envelope and chromatin to break down chaotically. This disorganization of broken nuclear content and chromatin is the immediate difference between apoptosis and necrosis pathways, apart from the signal causing them, in which the steps will differ hereafter. The unregulated fragmentation causes the dispersal of nuclear fragments throughout the cytoplasm without an intentional or organized method to dispose of them from there. Biological reasons for the nuclear envelope and chromatin break down differently could be any of the following factors:

• Loss of regulatory mechanisms
• The sudden disruption of the cellular energy supply
• Uncontrolled enzymatic activity
• Damage to the nuclear envelope
• An inflammatory response and further breakdown

The last of these factors is significant in that it furthers the effects of a necrotic cell by eliciting necrosis in neighboring cells (localized response of cell death) as they recognize leaked cellular contents in the extracellular space as signs of damage.

Ending of Necrosis in Respect to Karyolysis

The third and final step in the necrotic pathway is karyolysis (stemming from roots karyo- (κάρυον), meaning "nut" or "nucleus," and lysis (λύσις), meaning "dissolution" or "loosening"). The remaining nuclear fragments from karyorrhexis degrade completely during this step. Necrosis, including karyolysis, concludes with complete disintegration of the cell. After karyolysis, the cell undergoes total degradation, often called "cytoplasmic dissolution" or "ghost cell formation," leaving behind cytoplasmic debris and inflammatory mediators in the extracellular space. Here is how the process ties together:

1. Structural Collapse:
   • The degradation of nuclear proteins, chromatin, and cytoskeletal elements culminates in the collapse of the cell structure.
   • The cell membrane often becomes porous or ruptures entirely, releasing intracellular components into the extracellular space.
2. Immune Clearance:
   • Signals recruit immune cells such as macrophages and neutrophils to the site of necrosis. They attempt to engulf and digest the cellular remnants, but in necrosis, the lack of apoptotic signaling often results in an incomplete or inefficient clearance.
   • If there is a delay or issue with the recruitment of immune cells, the cellular contents may leak out to the extracellular matrix, likely exposing surrounding cells. Exposure may lead to prolonged inflammation, scarring, or fibrosis in the affected tissue.
   • Necrosis triggers a robust inflammatory response due to the release of damage-associated molecular patterns (DAMPs). These include nuclear proteins, mitochondrial DNA, and other intracellular contents that act as "danger signals."
   • Key immune players include neutrophils and macrophages, recruited to clear necrotic debris. However, the inflammatory mediators they release (e.g., cytokines, ROS) can exacerbate damage to surrounding tissues.
3. Tissue Remodeling and Scarring
   1. Chronic inflammation from persistent necrotic debris can cause fibrosis, replacing viable tissue with collagen and scar tissue.
   2. The aftermath of necrosis varies depending on the tissue. For instance:
      1. In the brain, necrosis often leads to liquefactive necrosis, where tissue becomes soft and liquid-like.
      2. Coagulative necrosis occurs in the heart or kidneys, leaving behind a "ghost" framework of the affected cells.
   3. In specific tissues, like the liver, necrotic areas may regenerate if the surrounding cells are viable. However, non-functional scar tissue and collagen replace necrotic tissue in organs like the heart or brain.

Additionally, karyolysis occurs in necrosis and necroptosis but with key differences. In necrosis, karyolysis results from chaotic enzymatic degradation of nuclear material following lysosomal membrane permeabilization (LMP), driven by external stressors such as trauma or ischemia. In contrast, necroptosis, a regulated form of cell death different from both necrosis and apoptosis and serving almost as a blend, involves the same terminal event of karyolysis but within a programmed framework. The RIPK1-RIPK3-MLKL signaling axis directs the process, ensuring controlled steps before membrane rupture. Although both necrosis and necroptosis release intracellular contents that trigger inflammation, the regulated nature of necroptosis offers potential for targeted therapeutic intervention, especially in diseases where excessive or uncontrolled karyolysis contributes to pathology.

Enzymes of Karyolysis

The enzymes involved in this process are critical in understanding karyolysis. Key enzymes involved in karyolysis include deoxyribonucleases (DNases), ribonucleases (RNases), proteases, and lysozymes.

• DNases
  • Are contained in lysosomes and released from dying lysosomes to contribute to this process and potentially released from recruited immune cells such as macrophages and neutrophils. Under normal cellular conditions, DNases maintain cellular homeostasis by breaking down old or damaged DNA; during necrosis, these enzymes release uncontrollably, cleaving DNA into small fragments, leading to chromatin dissolution. During karyolysis specifically, they degrade the highly compacted chromatin remaining following pyknosis and karyorrhexis, causing the nuclear material to lose its staining properties and appear "dissolved" under the microscope.
• RNases
  • These enzymes stem from the same sources as DNases and typically degrade RNA molecules no longer needed for cellular processes. During karyolysis, RNases help break down RNA, contributing to the overall dissolution of nuclear material.
• Proteases
  • Caspase-independent proteases include cathepsins and calpains. Necrotic lysosomes release cathepsins. Cytoplasmic calpains (and potential other cytoplasmic proteases) activate due to calcium influx during necrosis. Proteases ordinarily function to degrade proteins for recycling or removal in healthy cells. During karyolysis, proteases break down nuclear structural proteins such as histones, lamins, and nuclear scaffold proteins. This breakdown dismantles the nuclear architecture and facilitates the dissolution of chromatin.
• Lysozymes
  • Normally functioning by degrading bacterial walls and other cellular debris in controlled immune responses, lysosomes are another example of enzymes released from necrotic lysosomes or recruited immune cells during necrosis. Lysosomes contribute to karyolysis through the degradation of nuclear-associated proteins and the facilitation of DNase and RNase activity.

Mechanisms of Karyolysis

Mechanisms of these enzymatic reactions often link to lysosomal membrane permeabilization (LMP). LMP occurs under stressed conditions, releasing hydrolytic enzymes from the internal portion of the lysosome into the cytosol. Various factors, such as oxidative stress, exposure to lysosomotropic agents, or the action of specific lipids, can spur LMP. Once hydrolytic enzymes–DNases, RNases, and proteases–are freed from lysosomes, they translocate to the nucleus. Without lysosomal sequestration, the active enzymes can unintentionally and chaotically degrade nuclear components. In conjunction with other karyolytic mechanisms, the concerted action of these enzymes causes the nucleus to lose structural integrity and staining properties, a hallmark of karyolysis in microscopy.

Specifically, it is DNA cleavage, in which DNases cut chromatin into smaller fragments until eventually reducing it to mononucleotides or oligonucleotides, contributing to the "ghost" nucleus appearance since degraded DNA is no longer detectable with basic dyes. Another case is proteases, which target histones for degradation; histones function to bind and protect DNA, so degradation augments DNases enzymatic attack due to the lack of histone protection. Additionally, proteins like nuclear lamins–typically providing structural support to the nuclear envelope–are degrading, contributing to the disintegration of the nuclear structure. Finally, RNases target ribosomal (rRNA) and messenger RNA (mRNA) within the nucleus in RNA degradation, completing the dissolution of nuclear contents. While the mechanisms above reflect the general sequence of events making up karyolysis, these enzymatic reactions are dynamic and interdependent, with many processes occurring concurrently. The release of lysosomal enzymes occurs first and triggers multiple enzymatic reactions due to the chaotic release of typically contained enzymes. DNases and RNases act on DNA and RNA contemporaneously, while proteases also work to degrade histones and other structural proteins.

Additional images

• 
  Micrograph showing karyolysis and contraction band necrosis in an individual that had a myocardial infarction (heart attack).
• 
  Micrograph showing karyolysis and contraction band necrosis (left of image) and ischemic (nucleated) cardiac myocytes (right of image) in an individual that had a myocardial infarction.

See also

• Apoptosis
• Necrosis
• Pyknosis
• Karyorrhexis