Days 4+5: CT, Collagen, ECM

Question 1

functions of connective tissue

Answer 1

• Binding other tissue types together
• Structural cohesion
• Resist forces
• Protect
• Energy reservoirs
• Transport nutrients
• Controlling the passage of fluids, the migration of cells, and the spread of infections

Question 2

classifications of connective tissue

Answer 2

Connective tissue proper: structural framework of organs, form the stroma

• loose (areolar) connective tissue (most widespread CT)
  
  • thin fibers, many cells (includes plasma, mast, fibroblast, lymphocytes, etc)
  • tissue is pliable, well-vascularized, and only slightly resistant to tensile forces
• dense irregular connective tissue
  
  • _​_thick fibers, few cells (mostly fibroblasts)

Special: have a unique function based on a modification in either the cells, fibers, or ground substance

• dense regular connective tissue: only found in ligaments and tendons, in tendons, the fibroblast may be called tendinocytes

Question 3

embryonic precursor of connective tissue cells

Answer 3

• mesenchymal stem cells (MSCs) are precusors of the “resident” cells of adult connective tissues:
  
  • fibroblast
    
    • myofibroblasts
  • chondroblast
  • osteoblast
  • adipocyte
    
    • unilocular
    • multilocular
• In the fetus, CT is known as “fetal mesenchymal tissue”
  
  • originates from embryonic mesoderm layer

Question 4

fibroblast

Answer 4

• native cell to the CT
• produces/degrades fibers + ground substance of matrix
• can undergo mitosis
• has overall fusiform shape and thin protoplasmic processes, which may contact the processes of other fibroblasts
• nuceli are very thin, basophilic, and elongated
• synthesize, secrete, and organize all the fibers of connective tissue, as well as most of the interfibrillar ground substance
• fibroblasts that are actively repairing or forming new connective tissue have more cytoplasm than ones involved in ongoing low-level matrix turnover of established connective tissue
• can trans-differentiation into other mesenchyme-derived cells (adipocyte, chondroblast, osteoblast)

Myofibroblasts

• modified, similar to fibroblasts and smooth muscle cells
• basophilic cytoplasms (lots of prot generation)
• abundant in areas of wound healing

Question 5

adipocytes

Answer 5

• lipid storage & mobilization
• mesenchymal origin

unilocular:

• triglycerides fuse, a blood supply heavily supplies, the dominant extracellular structural elements are reticular fibers
• secrete chem factors that relay the state of energy storage to brain, liver, muscle, etc

multilocular:

• thermogenic and direct innervation by sympathetic nervous system controls metabolism of its stored fatty acids
• distrubuted near the scapula, clavicle, and along vertebral column

Question 6

macrophage

Answer 6

• frothy appearance due to vacuole contents
• cell born in BM, resides in CT
• circulate blood as monocytes, after 40 hrs, enter the CT and increase in size, lysosomal content, and endocytic capacity
• some serve as antigen-presenting cells: process antigens and present to lymphocytes to stimulate the immune response

Question 7

plasma cell

Answer 7

• when antigenically stimulated, B-lymphocytes undergo mitotic division, plasma cell clones that synthesize and secrete abs against the antigen
• they are the effector cells of B-lymphocytes, producing antibodies against one specific antigenic epitope
• end stage cells, short lifespan
• usually not present in blood stream
• no secretion granules because the release of product (antibodies) is constitutive
• morphology:
  
  • oval to round shape (12 um in diameter)
  • cytoplasm is basophilic, with no intracellular granules
  • nucleus is eccentric (away from cell center)
  • pale cytocentrum (cell center) that corresponds to location of Golgi
  • nucleus is

Question 8

mast cell

Answer 8

• large cell, central oval nucleus, metachromatic (may be red) granules (take on color separate from stain)
  
  • granules contain pharmacologic agents that create inflammation
  • can appear red in H&E, and dark blue in other preparations
  • can be confused with eosinophils, but differ in nuclear morphology
• long-lived, enhances WBC response through inflammation
• antibodies of IgE class are associated with mast cell plasma membrane
  
  • enables rapid response to antigens (allergens specifically)
• functionally related to basopils

Question 9

extracellular matrix components

Answer 9

• Role of the ECM is to distribute stresses of movement + gravity while maintaining the body shape

Comprised of:

• insoluble protein fibers
  
  • collagen
  • reticular fibers
  • elastic fibers
• interfibrillar ground substance
  
  • made up of soluble complexes, composed of long carbohyrate polymers linked to protein molecules that attract + bind to water
• the resident cells (fibroblasts, chondrocytes, osteoblasts) produce both the fibers and ground substance

Question 10

collagens

Answer 10

• long, stiff, triple-stranded helical structures
  
  • three polypeptide alpha chains twist around to form triple helices
• type I:
  
  • most common and abundant fiber of CT
  • composed of fibrils (made of staggered tropocollagen molecules)
  • aggregate as their density increases
• type II:
  
  • supports cartilage matrix
• type III (reticular fibers):
  
  • smaller, more flexible than type I
  • eosinophilic, agyrophilic (stained with silver treatments)
• type IV
  
  • major component of basal/external lamina

Question 11

elastic fibers

Answer 11

• occurs in tissues that need to be strong + elastic
• comprised of elastin (hydrophobic protein) and held in place by surrounding scaffolding of tiny fibrils
• stain lightly with eosin, special stains are required to render them visible for light microscopy
  
  • ie, resorcin fuchsin, Verhoeff’s stain
• fibers branch, but do not provide supporting network for cells like reticular fibers do

Question 12

fibrosis

Answer 12

• the pathological accumulation of dense type I collagen
  
  • early fibrosis (forming granulation tissue): primarily type III collagen that is replaced by type I
• occurs as a backup process to prevent fluid loss after an epithelial wound if re-epithelialization is not successful
• cyst: when occurs internal to a CT compartment
  
  • common when glands are damaged / lose exit to epithelium
• scarring: elevated type III collagen
  
  • keloid:
    
    • raised, collagen is laid down perpendicularly to epithelium, interfering with re-epithelialization
    • low myofibroblast count
  • hypertrophic:
    
    • flat, collagen parallel to surface
    • higher myofibroblast count

Question 13

edema

Answer 13

• ground substance pathology
• extracellular fluid normally arises from plasma filtered through vascular endothelium and leaves through lymphatics
• disturbances in the amount of fluid leaving the tissue causes edema d/t extracellular fluid accumulation

Question 14

primary + secondary structure of collagen

Answer 14

• alpha chains of collagen (2 identical alpha 1, one alpha 2 chain) comprised of:
  
  • 33% glycine
  • 17% proline (half are hydroxylated)
  • many lysines in collagen are also hydroxylated
  • mature collagen doesn’t usually contain cysteine
  • repetitive structure with tandemly repeating sequence: Gly-X-Y (usually X: Pro, Y: Hydroxyproline)
  • has alternating regions of polar/nonpolar
    
    • (Glyc-Pro-Y) (Gly-X-Y) (Gly-Pro-Y)
      
      • Gly-Pro-Y: usually non-polar
      • Gly-X-Y: polar d/t polar aa in X/Y position
  • assembly of collagen triple helix (procollagen) requires further processing + assembly to become mature collage

Question 15

primary + secondary structure of elastin

Answer 15

• in all tissues that require elasticity
• single, non-glycosylated molecule
• 33% Gly, 50% Pro
• synthesized as 70kDa monomer (tropoelastin)
  
  • ​secreted into extracellular fluid, where is cross-linked (by lysyl oxidase) to form highly insoluble elastin fibers
  • slow turnover, catalyzed by elastase
• does not form a regular secondary structure
  
  • stretched (longitudinally): becomes more ordered
  • released: relaxes back to non-stretched configuration

Question 16

alpha-1 antitrypsin (AAT) deficiency

Answer 16

• AAT: serine protease inhibitor produced by hepatocytes
  
  • inhibits human neutrophil elastase
• Smoke, dust, pollutants trigger oxidative stress + inflammation in lung
  
  • recruits macrophages, which recruit neutrophils
• In AAT defiency: neutrophil elastase is not inhibited and lung elastin is destroyed, resulting in emphysema
  
  • also susceptible to liver damage:
    
    • retention + polymerization of mutant AAT in ER of hepatocytes

Question 17

proteoglycans

Answer 17

• core protein covalently bound to long, linear polysaccharides of repeating disaccharide units called glycoaminoglycans (GAGs-negatively charged)
• large complexes can form by binding to collagen, hyaluronic acid, etc
• Functions:
  
  • ​reservoir for growth factors
  • facilitate contact beween proteins and their cell-surface receptors (neg or pos signaling)
  • protect proteins from proteolytic degradation

Question 18

multiadhesive matrix proteins

Answer 18

• long, flexible proteins that contain binding domains for other components of the ECM:
  
  • collagen and other matrix types
  • signaling molecules such as growth factors
  • cell surface receptors
  • polysaccharides
• Roles:
  
  • regulating cell attachment to ECM
  • organization of the ECM
  • mediate processes such as adhesion, migration, growth, differentiation

Question 19

fibronectins

Answer 19

• example of a multiadhesive matrix protein
• Structure:
  
  • high-molecular weight glycoprotein
  • dimer of 2 nearly identical monomers linked by 2 disulfide bonds
  • produced by a single gene, but alternative splicing of its pre-mRNA creates several isoforms
• Binds:
  
  • ECM components (collagen, fibrin, heparin sulfate proteoglycans)
  • signaling molecules (growth factors/cytokines)
  • polysaccharides (heparin)
  • cell surface receptors (integrins)

Question 20

reticular fibers

Answer 20

• thin, branching fibers composed of type III collagen. They typically form a delicate, flexible, and supportive network surrounding smooth muscle cells, adipocytes, and cells of lymphoid and hematopoietic origins
• fibers are heavily glycosylated
• eosinophilic, but thin compared to collagen so easily get lost in H+E stain
• glycosyl coating is agyrophilic, so fibers are stained with silver stains

Question 21

stroma vs. parenchyma

Answer 21

stroma: connective tissue supporting network
parenchyma: cells that carry out organ’s function