A cell is the fundamental unit of an organism, and a collection of cells forms tissues that make various important parts of the organism, which together give a living being its characteristics. An injured cell is a case study for various malfunctioning states of a living being. Cell injury results from endogenous and exogenous stimuli, leading to significant changes in the cell and, often, cell death. Therefore, the basis of different pathological findings starts with the study of an abnormal or injured cell.

This guide will help you understand the various aspects of cell injury, its causes, mechanisms, types, and more!

What is the Definition of Cell Injury?

A cell can be damaged by internal and external influences that cause reversible or irreversible injuries and often lead to cell death. Reversible injuries allow the cell to regain its original shape and properties after temporary damage, whereas irreversible injuries do not, leading to permanent damage and sometimes cell death.

Cell injury is defined as the loss of homeostasis in any cellular constituent caused by various exogenous and endogenous factors, including microorganisms, genetic changes, invasion by toxic substances, and metabolic changes. Cells have an inbuilt mechanism for recovering from injury and maintaining homeostasis. Still, persistent injuries, if irrecoverable, lead to adaptation, degeneration, and cell death. 

What are the Types of Cell Injury?

Depending on the extent of cell recovery after injury, cell injury is categorised into two types: reversible and irreversible. In the first type, the cell recovers due to its own homeostatic mechanisms, whereas in the latter type, the cell’s original state and properties are not restored.

Cell injury is divided into two types as follows:

1. Reversible Cell Injury 

Reversible cell injury is a type of injury in which the cell can regain homeostasis and a functionally normal state after temporary internal or external influences are resolved. Acute swelling is the basic morphological change in reversible cell injury. 

In this case, regardless of the nature of the initial injury, hypoxia is the ultimate cause of cell swelling because it leads to depletion of adenosine triphosphate (ATP). In many situations, this is the early change of irreversible cell injury.

2. Irreversible Cell Injury

In the case of irreversible cell injury, if the internal or external factor causing the damage persists for a long time, the cell fails to maintain homeostasis and a functionally steady state, and its response to the injury may be adaptive or degenerative, sometimes causing cell death. 

What is the Difference between Reversible and Irreversible Cell Injury?

The reversible cell injury is mild and temporary. Upon removal of the adverse situation, the cell recovers and attains the functionally steady state. But irreversible cell injury does not recover; it leads to adaptation or degeneration and often to permanent cell rupture, resulting in cell death.

What are Myelin Figures?

It is the cytoplasmic figures that are visible in the cell by electron microscopy after injury. They are composed of whorls of membrane formed by damaged cytoplasmic constituents, such as mitochondria or RER. 

In Tay-Sachs disease of the neuron, the myelin figures are prominent, as are other inborn errors of metabolism that rupture the cytoplasmic membrane. Myelin figures are taken up by autophagosomes, similar to other residues of harmed organelles. 

What is the Mechanism of Cell Injury?

The fundamental pathogenesis of cell injury is disturbed homeostasis. Initiation of cell injury begins at the molecular level, and the specific causes are numerous. The mechanism is categorised into four types: mitochondrial dysfunction, disruption of cellular membranes, impaired protein synthesis, and disturbed calcium homeostasis.   

The different mechanisms that result in cell injury are as follows: 

1. Mitochondrial Dysfunction

The well-being of a cell is dependent on the healthy function of mitochondria, which is also called the powerhouse of a healthy cell and is primarily responsible for adenosine triphosphate (ATP) production. 

Mitochondrial dysfunction leads to ATP depletion, causing cell damage and swelling. These damaged cells due to an abnormality in mitochondrial function cannot be reversed.

2. Disruption of Cellular Membranes

When the cell membrane is exposed to adverse conditions, such as free radical formation or oxygen deprivation, its permeability increases. This change results in an increase in the sodium or water levels in the cell, leading to swelling, and is recognised as reversible cell injury.

3. Impaired Protein Synthesis

Protein synthesis is an essential process of a cell to maintain its function. Adverse influences that harm the cell disrupt protein synthesis, including RNA and ribosome production, leading to irreversible cell injury and, in some cases, cell death. 

4. Disturbed Calcium Homeostasis

Disturbance in calcium homeostasis reduces ATP generation and leads to cell swelling due to an imbalance in ion equilibrium. This imbalance causes irreversible cell injury by disrupting energy-dependent ion pumps that slow sodium ion influx.

What are the Causes of Cell Injury?

Exogenous and endogenous factors can harm a cell. Factors responsible for cell injury from outside the organism are referred to as exogenous, and disturbances in certain constituents and internal processes of the cell that cause cell injury are referred to as endogenous factors. 

The causes of cell injury can be classified as 

1. Exogenous factors responsible for cell injury include physical, chemical, and biological agents such as heat and cold, drugs and toxins, and bacteria and viruses.
2. Endogenous factors that are responsible for cell injury include genetic defects, metabolites, hormones, cytokines, and other ‘bioactive’ substances.

What is Cell Adaptation?

When a cell undergoes irreversible injury, it sometimes undergoes adaptive changes. Changes due to injury may involve structural, morphological, or functional aspects. There are several types of adaptation, including atrophy, hypertrophy, hyperplasia, metaplasia, and dysplasia.

Cellular adaptation is the physiological changes that occur in response to a new environment, including physical/chemical factors and exposure to other stimuli. This is reflected in changes in cell size and number, as well as in morphological and functional changes. 

It is divided into two categories: reversible damage resulting from temporary changes induced by exposure to influencing factors, and irreversible damage resulting from continuous exposure to adverse stimuli. 

The adaptations are of different types as listed below:

1. Atrophy.
2. Hypertrophy.
3. Hyperplasia.
4. Metaplasia.
5. Dysplasia.

FAQs about Cell Injury

1. What is the key reason for cell injury?

Hypoxia is the primary cause of cell injury. It is a deficiency of oxygen, which is caused by disturbed aerobic oxidative respiration.

2. How does cell injury affect organs?

In living organisms, cell injury triggers various vascular, inflammatory, and immune-mediated responses, as well as disrupting growth.

3. What are the types of cell injury?

There are two types of cell injury: reversible and irreversible. 

4. Which ions are important in cell injury?

Calcium ions are responsible for toxic cell injury and are implicated as mediators of cell injury mechanisms under a variety of pathological conditions.

5. What is released when a cell suffers injury?

Different parts of an organism behave differently when they are injured. For example, damaged tissues release inflammatory chemicals that trigger dilation of blood vessels.

6. How does cell injury affect cellular function?

While a cell sustains injury, it elicits a variety of vascular, inflammatory, and immune-mediated responses, as well as disturbances in growth. 

7. Why does cell swelling occur in cell injury?

Cell swelling is a degenerative change characterised by an influx of sodium and water into the cell, mainly due to the malfunction of sodium and potassium ion pumps. If not reversed, it results in cell lysis and death. 

8. How does the body repair cell injury?

Oxygen-rich red blood cells arrive at injured tissues to build new cells and maintain homeostasis. 

9. What are the irreversible forms of cell injury?

All injuries that lead to cell death are irreversible forms of cell injury. There are six types of irreversible injury, such as Apoptosis, Necrosis, Autophagy, Oxidative Stress, Ischaemia, and Hypoxia. 

10. Which cell never regenerates after injury?

The most common cell that does not regenerate after injury is Neurons (nerve cells). Most neurons in the central nervous system never regenerate after injury.

Conclusion

Monitoring cell injury, such as DNA damage, cell membrane damage, or tissue damage, can provide insights into the disease and aid its diagnosis and progression. Molecular imaging methods combined with biochemical analysis are essential tools to identify individual cell injury with great precision. Thus, gaining knowledge and an in-depth understanding of cell injury can deepen your understanding of the illness and help you improve your clinical skills throughout your medical career.  

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