PI session 1 Flashcards
What is cell death?
Irreversible severe cell injury that exceeds attempts at repair or adaptation induces cell death
What are the two distictive forms of cell death?
- Apoptosis
- Necrosis
Why does apoptosis occur?
- cell no longer needed by the body
- OR it is damaged beyond repair
Why is there no inflammatory response associated with apoptosis?
- Dissolution of nucleus without perforation of cell membrane
- Prevents cell contents from leaking into extracellular matrix
Why is there no inflammatory response associated with apoptosis?
- Dissolution of nucleus without perforation of cell membrane
- Prevents cell contents from leaking into extracellular matrix
Cells destined to die activate intrinsic enzymes that degrade genomic DNA and nuclear and cytoplasmic proteins. These enzymes are called what?
Caspases
What 2 pathways converge on caspase activation?
- Mitochondrial pathway (intrinsic) - mitochondrial membrane becomes more permeable and releases pro-apoptotic molecules
- Death receptor pathway (extrinsic) - activates inflammatory cascade
What are the anti-apoptotic proteins in the intrinsic pathway?
BCL2, BCL-xL, and MCL1
What are the pro-apoptotic proteins in the intrinsic pathway?
BAX and BAK
What are the regulated apoptosis initiator proteins?
BAD, BIM, BID, Puma, and Noxa
sensors of cellular stress/damage & initiate apoptosis when activated
What occurs in the intrinsic (mitochondrial) pathway of apoptosis?
- Growth factors and other survival signals stimulate BCL2, protecting cells from apoptosis
- When no signals are sent, or in the case of DNA damage or ER stress, BH3-only proteins (apoptosis initiators) are upregulated
- BH3-only proteins activate BAX and BAK which insert into mitochondrial membrane forming the permeability transition pore
- Results in cytochrome C leaking into the cytoplasm and binding to APAF-1 (apoptosis activating factor) and forms an apoptosome which binds CASPASE-9
Explain the extrinsic (death receptor-activated) pathway with regards to Fas.
- When FasL binds to Fas, 3 or more molecules of Fas are brought together along the inner cell membrane, and their collective cytoplasmic death domains form a binding site for an adapter protein called FADD (Fas-associated death domain)
- FADD then binds to Caspase-8 (or caspase-10), activating the EXECUTIONER caspase sequence of the extrinsic path
What is karyolysis?
- Basophilia of the nucleus fades
- Loss of DNA due to enzymatic destruction
What is pyknosis?
- Nuclear shrinkage and increased basophilia
- Chromatin condenses into a ‘dense, shrunken basophilic mass
What is karyorrhexis?
- Pyknotic nucleus undergoes fragmentation
- Within 1-2 days, the nucleus in the necrotic cell totally disappears
What are the 6 patterns of tissue necrosis?
- Coagulative
- Liquefactive
- Gangrenous
- Caseous
- Fat
- Fibrinoid
What occurs in coagulative necrosis?
- Denaturation of structural proteins and enzymes
- Shadow of dead cells/tissue persists for days
- Leukocytes eventually remove dead cells
- Classic example: infarcts (ischemic necrosis) in solid organs
Brain infarcts are an exception–> Liquefactive instead
On histology slides, necrotic cells will lack what organelle?
Nucleus
What are the gross appearance characteristics of infarcts?
- Firm
- Located near periphery of organ
- “Wedge shaped” (like triangle pointing toward center of organ)
- Pale/white, except for in lungs where it is red
What is liquefactive necrosis?
- Dead cells completely digested into viscous liquid
- Examples: Abscess d/t bacterial infection; Infarct in brain/CNS
- If abundant inflammation (neutrophils): pus
What is gangrenous necrosis?
- Not a specific pattern of cell death, but commonly used in clinical practice
- Coagulative necrosis involving a limb (lower leg)
- Superimposed bacterial infection attracting leukocytes and degradative enzymes causing liquefactive necrosis
- Combination causes so-called wet gangrene
What is caseous necrosis?
- Grossly has a soft, pale, crumbly/friable “cheesy” look
- Buzzword for Tuberculosis
- Characteristic of a focus of inflammation called a granuloma
What is fat necrosis?
- Focal areas of fat destruction
- Enzymes leak out of damage cells and liquefy membranes of fat cells in the peritoneum
- Typical of NECROTIZING PANCREATITIS - (abdominal emergency caused by leaking pancreatic lipases)
- Fat saponification: chalky white material - broken down lipid combines with calcium to make soap-like substance
What is fibrinoid necrosis?
- Special form of vascular damage - Amorphous pink material (fibrin) associated with blood vessel walls
- Generally immune-mediated - Immune vasculitis (leukocytoclastic vasculitis, polyarteritis nodosa, etc)
- Only detected microscopically
What is the ECM?
- A secreted network of proteins that surrounds cells and constitutes a significant proportion of any tissue in the body
- Cell interactions within the ECM are critical for tissue development, healing, and maintenance of normal tissue architecture
What are the two basic forms of ECM?
- Interstitial Matrix
- Basement Membrane
What is the interstitial matrix?
- 3-dimensional, amorphous, semi-fluid gel
- Synthesized by mesenchymal cells (e.g., fibroblasts)
- Occupies the spaces between stromal cells (in connective tissue)
- Assists with support/scaffolding and in some tissues acts as a fluid that cushions tissue compression
What is the basement membrane?
- Highly organized meshwork
- Lies between epithelial cells and underlying connective tissue
- Synthesized by overlying epithelial cells and underlying mesenchymal cells
- Forms a flat, porous mesh that represents a specialized surface for cell growth
What are the functions of the ECM?
- Mechanical support
- Regulation of cell proliferation
- Scaffolding for tissue renewal - Disruption of the ECM prevents effective tissue regeneration and repair!
- Foundation for establishment of tissue microenvironments
What are the components of the basement membrane?
- Type IV collagen
- Laminin
- Proteoglycan
What are the components of the interstitial matrix?
- Fibrillar collagens
- Elastin
- Proteoglycan & Hyaluronan
Fibrillar collagen does what?
types I, II, III, & V
Form linear fibrils that comprise a major proportion of connective tissue as well as in healing wounds and scars
Nonfibrillar collagen does what?
type IV
Contributes to basement membrane structure, provides an anchor that maintains structure of some tissues, such as stratified squamous epithelium (e.g., skin)
What is the function of elastic fibers?
Provide tissue recoil and return to baseline structure after physical stress
What are water-hydrated gels?
comprised of?
- Provide compressibility to tissues, resilience, and lubrication
- Reservoir for growth factors secreted into the ECM
- Proteoglycans (glycosaminoglycans: ie- Keratan sulfate and chondroitin sulfate) are linked to hyaluronan. some are integral cell membrane proteins w/ roles in cell proliferation, migration, and adhesion
- These proteoglycan – hyaluronan molecules attract sodium and water, producing a viscous, gel-like matrix
- Present in skin, cartilage, vitreous humor, synovial fluid
What is fibronectin?
- component of interstitial matrix
- Provides the scaffolding for ECM deposition, angiogenesis, and re-epithelialization in healing wounds
What is laminin?
- most abundant glycoprotein in the basement membrane
- Connects cells to underlying basement membrane; modulates cell proliferation and differentiation
What are integrins?
- a type of adhesion receptor, also known as cell adhesion molecules [CAMs]
- Receptors that allow cells to attach to the adhesive glycoproteins (fibronectin, laminin)
- Facilitate cell-cell adhesive interactions
- Involved in signaling cascades that regulate cell locomotion, proliferation, shape, and differentiation
What is tissue regeneration and when does it occur?
- Restoration of normal cells
- Ability to regenerate is determined by
(1) the ability of the cells to proliferate, and (2)
the presence of tissue [adult] stem cells
What is tissue scarring and when does it occur?
- Deposition of connective tissue
- Occurs if (1) the injured tissues cannot regenerate, or
(2) if the supporting structures (ECM) are too severely damaged to support regeneration
With mild injury (damage to epithelium but not the underlying tissue), what type of tissue repair occurs?
Regeneration
With severe injury (damage to connective tissue), what type of tissue repair occurs?
Scarring
What is Labile tissue?
- continuously dividing; never enter G0 of cell cycle and divide rapidly with short G1)
- Examples: Hematopoietic cells, skin, and surface epithelia of the GI tract and genitourinary tissues
- Cells of these tissues are constantly being lost and replaced by (1) maturation from tissue stem cells and (2) proliferation of mature cells
- Must preserve pool of stem cells for regeneration!
- Most affected by chemotherapy!
What is stable tissue?
- quiescent – G0 of cell cycle – but capable of dividing – enter G1 when stimulated
- Examples: Endothelial cells (blood vessels), smooth muscle cells, parenchymal cells of most solid tissues (liver, kidney, pancreas), lymphocytes
- With the exception of the liver, these cells have limited capacity to regenerate after injury
- Liver has remarkable ability to regenerate after partial hepatectomy – (1) hepatocyte proliferation and (2) regeneration from “progenitor” cells in “canals of Hering” (adult stem cell niche!)
- Healing ability is dependent on ability of these cells to proliferate
What is permenant tissue?
- terminally differentiated; non-proliferative – G0 only)
- Examples: Neurons, cardiac muscle, skeletal muscle, red blood cells
- Injury to the brain, spinal cord, or heart is generally irreversible and results in a scar because this tissue cannot regenerate (very limited exceptions)!
- Repair of these tissues is dominated by scar formation
What are the steps in scar formation?
Within minutes, hemostasis occurs, followed by:
1. Inflammatory response (aka inflammatory phase of wound healing)
2. Cell proliferation and angiogenesis (aka proliferative phase)
3. Formation of granulation tissue (proliferative phase continued) - Collagen deposition and ECM protein synthesis
4. Remodeling of connective tissue and wound contraction (aka remodeling phase)
5. Acquisition of wound strength (remodeling phase continued)
What are the mediators in step 1 (inflammation) of scar formation?
- Break-down products of complement activation; cytokines (from activated platelets) – function as chemotactic agents and recruit (1) neutrophils followed by (2) macrophages
- Growth factors [GF] (from platelet plug, endothelial cells, other inflammatory cells)
- PDGF (platelets, macrophages) - induces vascular remodeling, smooth muscle cell migration; stimulates fibroblast growth for collagen synthesis
After clot formation (hemostasis), step 1 of scar formation (inflammation) is characterized by what?
- Increased vessel permeability and neutrophil migration into tissue
- Inflammatory cells eliminate the injurious agents (such as microbes)
- Macrophages clear debris, necrotic cells
What occurs in step 2 of scar formation (proliferative phase)?
Several cell types proliferate and migrate to close the now clean wound
* Epithelial cells – respond to locally released GF and migrate to the wound to cover it (skin: keratinocytes)
* Endothelial cells and pericytes – undergo angiogenesis (proliferation to form new blood vessels)
* Fibroblasts – proliferate and migrate to the wound; lay down collagen fibers that form the scar; deposit ECM proteins
What is angiogenesis?
the process of new blood vessel development from existing vessels
What growth factors are involved in angiogenesis?
VEGF (Vascular Endothelial GF)
* Stimulates migration and proliferation of endothelial cells
* Promotes vasodilation (stimulates production of nitric oxide [NO])
FGF-2 (Fibroblast GF-2)
* Stimulates endothelial cell proliferation
* Promote migration of macrophages and fibroblasts to wound
* Stimulate epithelial cell migration to cover wound
Angiopoietins 1 and 1 (Ang 1 and Ang 2)
* GFs that promote angiogenesis and structural maturation of new vessels
What occurs in step 3 of scar formation (formation of granulation tissue)?
Fibroblasts continue to proliferate, deposit loose connective tissue (mediator: TGF-beta), and synthesize ECM proteins forming granulation tissue
How is granulation tissue characterized?
- Proliferation of fibroblasts
- New, thin-walled, delicate vessels (angiogenesis)
- Loose ECM, often with admixed inflammatory cells (mostly macrophages)
- Progressively fills the wound; the amount formed depends on size of tissue defect, and intensity of inflammation
What occurs in step 3 of scar formation (deposition of connective tissue, ECM)?
- Granulation tissue is progressively replaced by the deposition of collagen
- Collagen is critical for the development of wound strength
- Mediated by locally produced cytokines and GF: PDGF, FGF-2, TGF-beta
Explain the role of TGF-beta in step 3 of scar formation.
- Potent fibrinogenic agent: most important cytokine for the synthesis and deposition of connective tissue proteins
- Produced by most of the cells in granulation tissue, including macrophages
- Also acts as an anti-inflammatory cytokine by inhibiting lymphocytes and the activity of other leukocytes
What is the end result of step 3 of scar formation?
Highly vascularized granulation tissue eventually transforms into a pale, largely avascular scar
* Amount of connective tissue increases; deposition of ECM increases
* In contrast, there is progressive vascular regression
* Some fibroblasts acquire features of smooth muscle cells called myofibroblasts - cause scar contraction
What occurs in step 4 (remodeling of connective tissue) of scar formation?
Matrix metalloproteinases (MMPs)
* Dependent on metal ions (e.g., zinc) for their activity
* Essential for remodeling
* Produced by various cell types (fibroblasts, macrophages, neutrophils, some epithelial cells)
* Mediate degradation of collagens and other ECM components
* Activity is tightly regulated
* Produced as inactive precursors that must be activated by proteases that are expected to be present only at site of injury
* Can be rapidly inhibited by specific tissue inhibitors of metalloproteinases (TIMPs), produced by most mesenchymal cells
What occurs in step 5 of scar formation?
- Excess of collagen synthesis over collagen degradation (due to cross-linking of collagen fibers and increased fiber size)
- Type III collagen is replaced by type I collagen (mediated by fibroblasts)
- Results in increased tensile strength of tissue
