apoptotic bodies

**Definition:** Apoptotic bodies are the membrane-enclosed fragments produced in the final stage of apoptosis, when the shrunken cell undergoes "blebbing" and breaks apart. They contain intact organelles and/or nuclear fragments (condensed chromatin).

**Morphology (histology):** - Cell shrinkage with hypereosinophilic (pink) cytoplasm - Nuclear changes: pyknosis (condensation) → karyorrhexis (fragmentation) → karyolysis (dissolution; this is more typical of necrosis) - Plasma membrane blebbing with intact membrane (no leakage of contents) - Formation of membrane-bound apoptotic bodies - Classic example: Councilman bodies (apoptotic hepatocytes in viral hepatitis, e.g., yellow fever)

**Pathophysiology/mechanism:** - Initiation via two main pathways: - *Intrinsic (mitochondrial):* cell stress/DNA damage/growth factor withdrawal → ↑BAX/BAK, ↓BCL-2/BCL-XL → cytochrome c release → Apaf-1 → apoptosome → caspase-9 → caspase-3/6/7 (executioners). p53 promotes this pathway. - *Extrinsic (death receptor):* FasL–Fas or TNF–TNFR binding → FADD → caspase-8 (or 10) → executioner caspases. Also CTL/NK cell-mediated killing via perforin/granzyme B. - Executioner caspases activate endonucleases (DNA cleavage into ~180-bp multiples → "DNA laddering" on gel electrophoresis) and cleave cytoskeletal proteins. - Phosphatidylserine flips from inner to outer membrane leaflet (detected by Annexin V staining); also opsonization by C3b/thrombospondin. - Macrophages phagocytose apoptotic bodies → no inflammatory response, since cell contents are not released.

**Contrast with necrosis:** Necrosis involves cell swelling, membrane rupture, release of intracellular contents, and inflammation; it's usually pathologic and affects groups of cells. Apoptosis is ATP-dependent, affects single cells, and is caspase-mediated, with no inflammation.

**Physiologic examples:** embryogenesis (e.g., interdigital webbing regression, Müllerian/Wolffian duct regression), menstrual endometrial shedding, thymic negative selection, involution of hormone-dependent tissues, and elimination of self-reactive lymphocytes.

**Pathologic examples:** DNA damage (radiation, chemo), misfolded proteins (ER stress), viral infection (CTL-mediated), graft-versus-host disease, and ischemic injury (mild).

**Clinical correlations:** - ↓Apoptosis: cancer (e.g., follicular lymphoma t(14;18) → BCL-2 overexpression), autoimmune disease (e.g., Fas/FasL defects → autoimmune lymphoproliferative syndrome) - ↑Apoptosis: neurodegeneration, AIDS (CD4+ T cell loss), ischemic injury

**High-yield pearls:** Councilman bodies, caspases, BCL-2 family, Annexin V, DNA laddering, and no inflammation are the key associations tested.