14. Which of the following is NOT a characteristic morphological feature of apoptotic cells?
(A) Disassembly of nuclear envelope
(B) DNA fragmentation
(C) Increased cell size
(D) Membrane blebbing
Apoptosis: Morphological Features on Programmed Cell Death
Introduction
Apoptosis is a highly regulated and energy-dependent form of programmed cell death that plays an essential role in embryonic development, tissue homeostasis, immune regulation, and elimination of damaged or infected cells. Unlike necrosis, which results from accidental injury and triggers inflammation, apoptosis is a controlled biological process that allows cells to die without damaging surrounding tissues. Throughout apoptosis, cells undergo a sequence of characteristic morphological and biochemical changes that facilitate their orderly removal by phagocytic cells.
Correct Answer
Correct Option: (C) Increased cell size
Detailed Explanation
Apoptosis is characterized by a series of precisely coordinated structural changes that occur due to activation of caspases, the major executioner enzymes of programmed cell death. These enzymes cleave numerous cellular proteins, including cytoskeletal proteins, nuclear lamins, and DNA repair enzymes. As apoptosis progresses, cells shrink rather than swell, chromatin condenses, DNA becomes fragmented, the nuclear envelope is dismantled, membrane blebs develop, and the cell eventually breaks into membrane-bound apoptotic bodies that are rapidly engulfed by macrophages or neighboring cells without provoking inflammation.
One of the defining features of apoptosis is cell shrinkage. In contrast, cell swelling and increased cell size are characteristic of necrosis, where loss of membrane integrity leads to osmotic imbalance, membrane rupture, and inflammation. Therefore, increased cell size is not considered a morphological feature of apoptosis.
Explanation of Each Option
Option (A): Disassembly of Nuclear Envelope
This statement is correct as a characteristic feature of apoptosis. During apoptosis, executioner caspases cleave nuclear lamins, which provide structural support to the nuclear envelope. This cleavage results in disassembly of the nuclear envelope, facilitating chromatin condensation, nuclear fragmentation, and the formation of apoptotic bodies. Thus, nuclear envelope breakdown is a well-recognized morphological event during programmed cell death.
Option (B): DNA Fragmentation
This statement is correct. DNA fragmentation is one of the hallmark biochemical and morphological features of apoptosis. Caspase activation stimulates Caspase-Activated DNase (CAD), which cleaves chromosomal DNA at internucleosomal regions, producing fragments of approximately 180–200 base pairs or multiples thereof. This characteristic DNA ladder pattern is widely used as an experimental marker of apoptosis.
Option (C): Increased Cell Size
This statement is incorrect and is therefore the correct answer to the question. Apoptotic cells do not increase in size; instead, they undergo progressive cell shrinkage due to cytoplasmic condensation and loss of cellular water. Increased cell volume, organelle swelling, plasma membrane rupture, and release of intracellular contents are characteristic features of necrosis rather than apoptosis. Therefore, increased cell size is not a morphological hallmark of programmed cell death.
Option (D): Membrane Blebbing
This statement is correct. Membrane blebbing is one of the earliest visible morphological changes observed during apoptosis. Caspase-mediated degradation of cytoskeletal proteins weakens the attachment between the plasma membrane and the underlying cytoskeleton, causing the membrane to bulge outward and form blebs. These membrane blebs eventually contribute to the formation of apoptotic bodies, which are safely removed by phagocytic cells without inducing inflammation.
Why Option (C) is Correct
The question asks for the feature that is not characteristic of apoptosis. Programmed cell death is associated with cell shrinkage, membrane blebbing, chromatin condensation, DNA fragmentation, nuclear envelope disassembly, and apoptotic body formation. Increased cell size occurs during necrosis because damaged cells lose membrane integrity, accumulate water, and swell before rupturing. Therefore, increased cell size is not a feature of apoptosis, making Option (C) the correct answer.
Why the Other Options are Correct Features of Apoptosis
Why Option (A) is Correct
Cleavage of nuclear lamins by executioner caspases leads to disassembly of the nuclear envelope, allowing the nucleus to fragment during apoptosis.
Why Option (B) is Correct
Activation of Caspase-Activated DNase produces characteristic internucleosomal DNA fragmentation, a hallmark of programmed cell death.
Why Option (D) is Correct
Membrane blebbing results from caspase-mediated cytoskeletal breakdown and contributes to the formation of apoptotic bodies that are later engulfed by phagocytes.
Comparison of All Options
| Option | Morphological Feature | Characteristic of Apoptosis? | Explanation |
|---|---|---|---|
| A | Disassembly of nuclear envelope | Yes | Occurs due to cleavage of nuclear lamins by caspases. |
| B | DNA fragmentation | Yes | Produced by Caspase-Activated DNase. |
| C | Increased cell size | No | Apoptotic cells shrink; swelling is typical of necrosis. |
| D | Membrane blebbing | Yes | Occurs due to cytoskeletal collapse during apoptosis. |
Sequence of Morphological Changes During Apoptosis
| Step | Morphological Change |
|---|---|
| 1 | Cell shrinkage |
| 2 | Chromatin condensation (Pyknosis) |
| 3 | Disassembly of nuclear envelope |
| 4 | DNA fragmentation |
| 5 | Membrane blebbing |
| 6 | Formation of apoptotic bodies |
| 7 | Phagocytosis without inflammation |
Comparison Between Apoptosis and Necrosis
| Feature | Apoptosis | Necrosis |
|---|---|---|
| Cell Size | Cell shrinks | Cell swells |
| Plasma Membrane | Intact with blebbing | Ruptures |
| DNA | Internucleosomal fragmentation | Random degradation |
| Nuclear Changes | Chromatin condensation and fragmentation | Nuclear swelling and disintegration |
| Inflammation | Absent | Present |
| Energy Requirement | ATP-dependent | Usually ATP-independent |
Molecular Basis of Apoptotic Morphology
The characteristic structural changes of apoptosis are driven by activation of initiator and executioner caspases. These proteases degrade cytoskeletal proteins, nuclear lamins, DNA repair enzymes, and structural proteins, producing membrane blebbing, nuclear envelope disassembly, chromatin condensation, and apoptotic body formation. Caspase-Activated DNase fragments DNA into nucleosomal units, while exposure of phosphatidylserine on the outer surface of the plasma membrane serves as an “eat-me” signal for macrophages, ensuring rapid clearance of apoptotic cells without inflammation.
Biological Significance of Apoptosis
Apoptosis is essential for maintaining tissue homeostasis, regulating embryonic development, eliminating damaged or mutated cells, controlling immune cell populations, and preventing tumor formation. Because apoptotic cells are removed without releasing inflammatory mediators, surrounding tissues remain undamaged. Dysregulation of apoptosis contributes to numerous diseases, including cancer, autoimmune disorders, neurodegenerative diseases, and developmental abnormalities.
Final Answer
Correct Option: (C) Increased cell size
Apoptotic cells characteristically undergo cell shrinkage, DNA fragmentation, disassembly of the nuclear envelope, chromatin condensation, and membrane blebbing. In contrast, increased cell size is a hallmark of necrosis, where cells swell before membrane rupture and inflammation occur. Therefore, increased cell size is not a characteristic morphological feature of apoptosis.
