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apoptosis

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apoptosis NCBI Bookshelf mechanisms intrinsic extrinsic pathway

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https://www.ncbi.nlm.nih.gov/sites/books/NBK499821

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Apoptosis

Assuming you meant apoptosis.

Apoptosis is a regulated, energy-dependent form of programmed cell death. It removes unwanted, aged, infected, genetically damaged, or potentially malignant cells while usually avoiding inflammation. It helps maintain normal tissue size by counterbalancing cell proliferation. Sleisenger and Fordtran's Gastrointestinal and Liver Disease, p. 23.

Apoptosis pathways showing mitochondrial and death-receptor activation of caspases

Morphological features

Cell shrinkage and cytoplasmic condensation
Chromatin condensation (pyknosis)
Nuclear fragmentation (karyorrhexis)
Membrane blebbing
Formation of membrane-bound apoptotic bodies
Rapid phagocytosis by macrophages or adjacent cells
Minimal leakage of cell contents, hence little or no inflammatory reaction

Biochemical basis

Apoptosis is executed by caspases, cysteine proteases synthesized as inactive precursors. Their sequential activation produces an irreversible proteolytic cascade.

Initiator caspases: caspase-8, caspase-9
Executioner caspases: caspase-3, caspase-6, caspase-7

Executioner caspases break down structural proteins, activate endonucleases that fragment DNA, and lead to dismantling of the cell.

Pathways of apoptosis

1. Intrinsic or mitochondrial pathway

This pathway is initiated by intracellular stress, such as:

DNA damage
Hypoxia
Nutrient deprivation
Radiation, toxins, or chemotherapy
Misfolded proteins and severe cellular stress

Mechanism:

Stress activates pro-apoptotic Bcl-2 family members, particularly Bax and Bak.
They increase mitochondrial outer-membrane permeability.
Cytochrome c is released into the cytoplasm.
Cytochrome c combines with Apaf-1 and procaspase-9 to form the apoptosome.
Caspase-9 activates executioner caspases, especially caspase-3 and caspase-7.

Anti-apoptotic proteins such as Bcl-2 and Bcl-xL prevent mitochondrial permeabilization and oppose apoptosis.

2. Extrinsic or death-receptor pathway

This pathway is initiated by extracellular death signals.

Important death receptors include:

Fas (CD95)
TNF receptor 1 (TNFR1)

Binding of Fas ligand or TNF to these receptors triggers intracellular signaling that activates caspase-8, followed by executioner caspases.

3. Cytotoxic T-cell pathway

Cytotoxic T lymphocytes can induce apoptosis in virus-infected and tumor cells through:

Fas-Fas ligand interaction
Perforin, which creates membrane pores
Granzyme B, which enters the target cell and activates caspases

Physiological roles

Apoptosis occurs normally in:

Embryogenesis and tissue sculpting, for example separation of fingers and toes
Hormone-dependent involution, such as endometrial shedding and post-lactational breast involution
Elimination of self-reactive lymphocytes
Termination of immune responses
Removal of senescent, damaged, infected, or mutated cells

Clinical significance

Reduced apoptosis allows abnormal cells to survive and contributes to cancer and autoimmune disease. For example, excess Bcl-2 activity may promote tumour-cell survival.
Excess apoptosis contributes to neurodegenerative disease, ischemic injury, and some viral infections.
Defects in apoptosis can permit survival of cells with DNA damage, facilitating malignant transformation.

Apoptosis versus necrosis

Feature	Apoptosis	Necrosis
Nature	Programmed, regulated	Uncontrolled cell injury/death
Cell size	Shrinkage	Swelling
Plasma membrane	Intact until apoptotic bodies form	Disrupted
Cell contents	Contained and phagocytosed	Leak into surrounding tissue
Inflammation	Usually absent	Marked
DNA breakdown	Internucleosomal, orderly fragmentation	Random degradation

In summary, apoptosis is an orderly caspase-mediated cell-death process involving intrinsic mitochondrial and extrinsic death-receptor pathways. Its major purpose is to eliminate harmful or unnecessary cells without provoking inflammation. The NCBI overview of apoptosis similarly describes activation of caspase-9 through the mitochondrial pathway and caspase-8 through death receptors, with caspase-3, -6, and -7 executing cellular breakdown.

Create a one-page PDF revision note on apoptosis

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apoptosis

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mkdir -p "/tmp/workspace/apoptosis-revision-note/src" "/tmp/workspace/apoptosis-revision-note/output" && python - <<'PY' from reportlab.lib.pagesizes import A4 from reportlab.pdfgen import canvas from reportlab.lib import colors from reportlab.lib.units import mm from reportlab.pdfbase.pdfmetrics import stringWidth from pypdf import PdfReader out='/tmp/workspace/apoptosis-revision-note/output/Apoptosis_One_Page_Revision_Note.pdf' W,H=A4 c=canvas.Canvas(out,pagesize=A4) c.setTitle('Apoptosis - One-Page Revision Note') c.setAuthor('Orris') # Palette NAVY=colors.HexColor('#10324A'); TEAL=colors.HexColor('#167A7A'); BLUE=colors.HexColor('#EAF5F6') INK=colors.HexColor('#1E2933'); MUTED=colors.HexColor('#52616B'); PALE=colors.HexColor('#F5F8FA') RED=colors.HexColor('#9D3145'); GREEN=colors.HexColor('#286B4D'); GOLD=colors.HexColor('#C58728') # Helpers def text(x,y,s,font='Helvetica',size=8,color=INK): c.setFont(font,size); c.setFillColor(color); c.drawString(x,y,s) def wrap(s,font,size,width): words=s.split(); lines=[]; line='' for w in words: trial=(line+' '+w).strip() if stringWidth(trial,font,size)<=width: line=trial else: lines.append(line); line=w if line: lines.append(line) return lines def para(x,y,s,width,font='Helvetica',size=7.3,leading=9,color=INK,bullet=None): c.setFillColor(color); c.setFont(font,size) indent=0 if bullet: c.drawString(x,y,bullet); indent=8 for i,line in enumerate(wrap(s,font,size,width-indent)): c.drawString(x+indent,y-i*leading,line) return y-len(wrap(s,font,size,width-indent))*leading def box(x,y,w,h,title,accent=TEAL): c.setFillColor(colors.white); c.setStrokeColor(colors.HexColor('#D8E1E6')); c.setLineWidth(.7) c.roundRect(x,y-h,w,h,4,fill=1,stroke=1) c.setFillColor(accent); c.roundRect(x,y-20,w,20,4,fill=1,stroke=0) c.rect(x,y-20,w,5,fill=1,stroke=0) text(x+8,y-13,title,'Helvetica-Bold',9,colors.white) # header c.setFillColor(NAVY); c.rect(0,H-43*mm,W,43*mm,fill=1,stroke=0) c.setFillColor(colors.white); c.setFont('Helvetica-Bold',24); c.drawString(16*mm,H-21*mm,'APOPTOSIS') c.setFont('Helvetica',9); c.drawString(16*mm,H-29*mm,'Programmed cell death | High-yield pathology revision') c.setFillColor(colors.HexColor('#64D3CC')); c.roundRect(W-58*mm,H-28*mm,40*mm,12*mm,6,fill=1,stroke=0) c.setFillColor(NAVY); c.setFont('Helvetica-Bold',8.5); c.drawCentredString(W-38*mm,H-23.7*mm,'CASPASE-MEDIATED') m=12*mm; gap=5*mm; col=(W-2*m-gap)/2; top=H-48*mm # left: definition + morphology box(m,top,col,56*mm,'CORE CONCEPT',TEAL) y=top-29 para(m+8,y,'A regulated, energy-dependent form of cell death that removes unwanted, damaged or potentially harmful cells without provoking significant inflammation.',col-16,'Helvetica',8,10) y-=31 text(m+8,y,'KEY IDEA', 'Helvetica-Bold',7.5,TEAL); y-=11 para(m+8,y,'Balances cell proliferation and preserves tissue homeostasis.',col-16,'Helvetica-Oblique',7.6,9,MUTED,bullet='•') box(m,top-61*mm,col,69*mm,'MORPHOLOGY & BIOCHEMISTRY',RED) y=top-61*mm-29 items=[ ('Cell shrinkage','dense cytoplasm; membrane blebs'), ('Nucleus','chromatin condensation → fragmentation'), ('DNA','orderly internucleosomal fragmentation'), ('Membrane','apoptotic bodies form; membrane remains intact'), ('Clearance','phagocytosis by macrophages/adjacent cells; little inflammation')] for a,b in items: text(m+8,y,'• '+a+': ','Helvetica-Bold',7.2,INK) x=m+8+stringWidth('• '+a+': ','Helvetica-Bold',7.2) text(x,y,b,'Helvetica',7.2,MUTED) y-=9.2 text(m+8,y-1,'Initiators: caspase-8, -9 | Effectors: caspase-3, -6, -7','Helvetica-Bold',7.1,RED) # right pathways x=m+col+gap box(x,top,col,73*mm,'TWO MAJOR PATHWAYS',GOLD) y=top-29 text(x+8,y,'1. INTRINSIC (MITOCHONDRIAL)', 'Helvetica-Bold',8,RED); y-=10 para(x+8,y,'Triggers: DNA damage, hypoxia, nutrient deprivation, toxins, misfolded proteins.',col-16,'Helvetica',7.2,8.5,MUTED) y-=28 text(x+8,y,'Bax / Bak ↑ → mitochondrial permeabilization → cytochrome c release → Apaf-1 apoptosome → caspase-9 → caspase-3/7','Helvetica-Bold',7.15,INK) y-=21 text(x+8,y,'Anti-apoptotic brakes: Bcl-2 and Bcl-xL', 'Helvetica-Oblique',7.3,GREEN); y-=15 text(x+8,y,'2. EXTRINSIC (DEATH RECEPTOR)', 'Helvetica-Bold',8,RED); y-=10 para(x+8,y,'Fas (CD95) or TNFR1 stimulation by their ligands → adaptor signalling → caspase-8 → caspase-3/7.',col-16,'Helvetica',7.2,8.5,MUTED) box(x,top-78*mm,col,48*mm,'PHYSIOLOGY & CLINICAL LINKS',GREEN) y=top-78*mm-29 for line in ['Embryogenesis: tissue sculpting, e.g. separation of digits.','Hormone-dependent involution and immune-cell selection.','Too little apoptosis: cancer, autoimmunity.','Too much apoptosis: neurodegeneration, ischemic injury.']: y=para(x+8,y,line,col-16,'Helvetica',7.25,8.7,INK,bullet='•')-1.3 # lower full panel comparison lower=top-135*mm box(m,lower,W-2*m,45*mm,'APOPTOSIS VS NECROSIS',NAVY) # table headings left=m+8; mid=m+(W-2*m)/2 text(left,lower-31,'APOPTOSIS','Helvetica-Bold',8,TEAL) text(mid,lower-31,'NECROSIS','Helvetica-Bold',8,RED) rows=[('Cell shrinks','Cell swells'),('Membrane largely intact','Membrane ruptures'),('Cell contents contained','Cell contents leak'),('No / minimal inflammation','Prominent inflammation'),('Orderly, caspase-dependent','Unregulated injury-related death')] y=lower-42 for a,b in rows: text(left,y,'• '+a,'Helvetica',7.45,INK); text(mid,y,'• '+b,'Helvetica',7.45,INK); y-=7.8 # footer c.setStrokeColor(colors.HexColor('#D8E1E6')); c.line(m,13*mm,W-m,13*mm) text(m,8.5*mm,'Memory hook: “8 = outside (death receptor); 9 = inside (mitochondria); 3 = execution.”','Helvetica-Oblique',7.4,MUTED) text(W-m-53*mm,8.5*mm,'Source: Sleisenger & Fordtran, p. 23','Helvetica',6.7,MUTED) c.showPage(); c.save() reader=PdfReader(out) assert len(reader.pages)==1, len(reader.pages) print(out, len(reader.pages), 'pages', 'size', __import__('os').path.getsize(out)) PY

Created a one-page PDF revision note with definition, morphology, intrinsic and extrinsic pathways, clinical relevance, and a comparison with necrosis.

Apoptosis One Page Revision Note

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