Pathophysiology of tissue atrophy md pathology exam question robbins based answer
Reading File
Finding Sources
Finding Sources
Searching PubMed
tissue atrophy AND mechanisms
Searching the Web
Robbins Cotran Pathologic Basis of Disease atrophy mechanisms ubiquitin proteasome autophagy
Reading File
Reading File
Finding Sources
Tissue Atrophy: Pathophysiology
Robbins-based pathology exam answer
Definition
Atrophy is a reduction in the size of an organ or tissue due to a decrease in the size of cells and, in more prolonged cases, their number. It is an adaptive response that allows cells to survive under reduced functional, nutritional, vascular, or hormonal support.
Basic cellular basis
An atrophic cell reaches a new equilibrium in which its metabolic demands are lowered enough to match the reduced availability of nutrients, blood supply, workload, or trophic stimulation.
- Cells become smaller.
- Cytoplasmic organelles decrease in number.
- In skeletal muscle, there are fewer mitochondria, myofibrils, and rough endoplasmic reticulum.
- Function of the organ is diminished.
- Initially, the change is reversible if the cause is removed.
- Severe or persistent atrophy may progress to apoptotic cell death, causing reduction in cell number as well.
Molecular mechanisms of atrophy
Atrophy results from an imbalance between protein synthesis and protein degradation:
[
\textbf{Decreased protein synthesis} + \textbf{increased protein degradation} \rightarrow \textbf{cell shrinkage}
]
1. Decreased protein synthesis
Normally, growth factors and trophic signals stimulate nutrient uptake and protein synthesis through increased mRNA translation.
In atrophy, there is reduced trophic stimulation, for example:
- Decreased workload
- Reduced nerve supply
- Diminished blood flow
- Nutritional deficiency
- Withdrawal of hormonal stimulation
This leads to reduced synthesis of structural proteins and organelles, producing a smaller cell.
2. Increased protein degradation: ubiquitin-proteasome pathway
This is the principal mechanism responsible for degradation of cellular proteins during atrophy.
- Nutrient deficiency and disuse activate ubiquitin ligases.
- Ubiquitin ligases attach ubiquitin to intracellular proteins.
- Ubiquitinated proteins are targeted to and degraded in proteasomes.
- Loss of contractile and other cellular proteins leads to shrinkage of the cell.
This pathway is important in:
- Disuse muscle atrophy
- Starvation and protein-calorie malnutrition
- Cancer cachexia
- Chronic inflammatory states
3. Increased autophagy
Atrophic cells often show increased autophagy.
- Cellular organelles and cytoplasmic components are enclosed in autophagic vacuoles.
- These are delivered to lysosomes for digestion.
- Autophagy provides nutrients and energy during deprivation, thereby helping the cell survive.
- Some undigested material remains as membrane-bound residual bodies, especially lipofuscin granules.
Accumulation of lipofuscin can cause a brown discoloration of tissues, termed brown atrophy, classically seen in the heart and liver of elderly or cachectic individuals.
4. Apoptosis in advanced atrophy
If the adverse stimulus is prolonged or severe:
- Atrophic cells may cross a threshold of irreversible injury.
- Cells undergo apoptosis.
- This accounts for decrease in cell number in severe disuse atrophy and following hormone withdrawal in endocrine-dependent tissues.
Causes with examples
| Cause | Example |
|---|---|
| Decreased workload | Immobilization of a limb in plaster cast causing skeletal muscle atrophy |
| Loss of innervation | Denervation atrophy of muscle after nerve injury |
| Diminished blood supply | Cerebral atrophy in chronic atherosclerotic ischemia |
| Inadequate nutrition | Muscle wasting in starvation, marasmus, cancer cachexia |
| Loss of endocrine stimulation | Endometrial, vaginal, and breast atrophy after menopause; prostate atrophy after castration |
| Pressure | Atrophy of tissues adjacent to an expanding benign tumor |
| Aging | Senile atrophy, especially brain atrophy |
Flowchart for writing in exams
Reduced workload / nutrition / blood supply / nerve supply / hormones
↓
Reduced trophic and growth-factor signaling
↓
↓ Protein synthesis + ↑ protein degradation
↓
Ubiquitin-proteasome pathway and autophagy-lysosomal pathway
↓
Loss of cellular proteins and organelles
↓
Cell shrinkage and decreased organ function
↓
Persistent injury → apoptosis → reduced cell number
Key points
- Atrophy is generally a reversible adaptive response in the early stage.
- The major mechanisms are decreased protein synthesis, ubiquitin-proteasome-mediated proteolysis, and autophagy.
- Persistent atrophy can culminate in apoptosis.
- Lipofuscin accumulation in atrophic cells produces brown atrophy.
Source: Robbins, Cotran & Kumar Pathologic Basis of Disease, section “Atrophy,” pp. 70-71 (library pagination). A recent systematic-review search did not identify a result that changes this core general-pathology mechanism; the returned reviews were mainly about skeletal-muscle aging rather than tissue atrophy broadly.
This is a shared conversation. Sign in to Orris to start your own chat.