Ultrastructural Changes of the Surface and Cytoplasm
A cell entering apoptosis undergoes a series of characteristic morphological changes. First, specialized surface structures are lost: microvilli disappear, and intercellular bridges and desmosomal contacts break down. The cell detaches from its neighbors and assumes a rounded shape.
Intracellularly, cytoplasmic condensation and a reduction in overall volume occur. This process is driven by two mechanisms:
- Dehydration mediated by the action of selective ion pumps regulating potassium and sodium exchange.
- Protein cross-linking facilitated by transglutaminase enzymes.
Unlike necrosis, hydrolytic enzymes are not massively activated, so intracellular structures remain intact for a prolonged period.
Transformation of Organelles and the Nucleus
Despite overall cellular shrinkage, mitochondria and other organelles maintain their integrity. The smooth endoplasmic reticulum undergoes dilation, expanding to form fluid-filled vesicles that are subsequently extruded. Ribosomes aggregate to form specific semi-crystalloid structures, and cytoskeleton elements form bundles of microfilaments running strictly parallel to the plasma membrane.
The nucleus undergoes dramatic changes. Under the action of $Ca^{2+}/Mg^{2+}$-dependent endonucleases, DNA is cleaved into uniform-sized fragments. Chromatin condenses into clumps and hemispheres, and osmiophilic bodies appear. Nuclear pores concentrate exclusively in regions devoid of marginated chromatin. Ultimately, the nucleus shrinks, its contours become jagged, and it fragments.
Formation and Fate of Apoptotic Bodies
The final stage of morphological remodeling is the partitioning of the cell into several fragments known as apoptotic bodies. These are small, rounded structures with a high nuclear-to-cytoplasmic ratio.
Each body contains a set of intact organelles, a nuclear fragment surrounded by a double-layered membrane, as well as water, sodium, and chloride ions. The tight membrane plays a critical role: it isolates the intracellular contents from the immune system and prevents the leakage of modified proteins into the extracellular space. This is precisely why inflammation does not develop during apoptosis. In vivo, apoptotic bodies exist for only a few minutes before being immediately phagocytosed by neighboring cells.
Genetic Regulation and Verification Methods
Apoptosis is triggered by genetic alterations. Normally, the activation of cellular proto-oncogenes (c-fos, c-myc, c-bcl-2) drives proliferation. However, upon their simultaneous expression with the tumor suppressor gene p53, the cell undergoes programmed death. This highlights the close genetic coordination between cell division and cell death machinery.
To reliably verify the process morphologically, methods detecting specific DNA and enzymatic alterations are used:
- TUNEL assay (Terminal deoxynucleotidyl transferase dUTP nick end labeling). Detects single-stranded DNA breaks at sites cleaved by endonucleases. It allows visualization of cells with shrunken and pyknotic nuclei, as well as apoptotic bodies themselves.
- Immunohistochemistry (IHC). Used to identify specific apoptosis-related enzymes, particularly caspases.
Comparison with Necrosis
Apoptosis and necrosis are two fundamentally different forms of cell death. While necrosis affects a large territory (ranging from a portion of a cell to an entire organ) and is accompanied by random DNA degradation and the activation of hydrolytic enzymes, apoptosis is a targeted process. It affects only individual cells, features ordered chromatin destruction, and lacks any surrounding inflammatory response.