Connective Tissue

Question 1

What are the functions of connective tissue?

Answer 1

1. Provide structural support (most important)
2. Used for storage of metabolites
3. Play important role in defense and protection by mediating immune, inflammatory, and allergic responses
4. Used to repair other tissues and organs through fibrosis

Question 2

Describe the fibrillar components of collagen and how they are organized into fibrils, fibers, and bundles.

Answer 2

• Most abundant protein in human body
• Flexible and have high tensile strength
• Produced principally by fibroblasts
• Structure:
  
  • Fibers composed of fibrils
  • Molecule measues ~ 300 nm
    
    • Consists of 3 polypeptide chains that form a triple helix
  • Molecules polymerize laterally head → tail to form Collagen Fibrils​
    
    • Fibrils CANNOT be visualized in light microscope
    • Have distinctive banding in electron microscope
      
      • Due to small gaps b/t collagen moleclues that absorb stain
  • Collagen Fibrils assemple into Collagen or Reticular Fibers
  • Type I Collagen Fibers can assemble into Collagen Bundles

Question 3

Describe the major types of collagen and where they are found.

Answer 3

• Over 20 different types
• Four Major Types:
  
  • Type I
    
    • Principal type
    • Found in dermis of skin, tendons, ligaments, fascia, bone, and most connective tissues proper (Loose CT, Dense Regular/Irregular CT)
    • Fibrils → Fibers that can be assembled → Bundles
  • Type II
    
    • Found in cartilage
    • Only forms fibrils
      
      • Gives cartilage “glassy” appearance
  • Type III (Reticular Fibers)
    
    • Found in cells of various tissues and organs
      
      • Loose CT
      • Walls of blood vessels
      • Lymphoid Tissues
      • Bone Marrow
      • Smooth Muscle
      • Nerves
      • Lung
    • Fibrils → Reticular Fibers
    • Form supporting framework for cells
  • Type IV
    
    • Does NOT form banded fibrils
    • Forms sheet-like meshwork of beaded filaments
    • Found in the basal lamina

Question 4

Describe the fibrillar components of elastin and fibrillin and how they are organized into fibrils, fibers, and bundles.

Answer 4

• Elastic Fibers
  
  • Elastin
    
    • Contains hydrophobic domain that causes the molecules to coil in aqeous environments
    • Elastin molecules are cross-linked by covalent bonds
    • Form either fibers of variable length or form lamellar layers (blood vessels)
    • Responsible for elasticity because they passively return to their normal configuration after stretch or deformation
  • Fibrillin
    
    • A glycoprotein that forms thin microfibrils
    • Surround developing elastic fibers
    • Provide substrate for their assembly
      
      • Organize elastin into elastic fibers

Question 5

List the major components of the ground substance.

Answer 5

1. Glycosaminoglycans (GAGs)
2. Proteoglycans
3. Multiadhesive Glycoproteins

Question 6

Describe glycosaminoglycans and their role in the extracellular matrix.

Answer 6

• Most abundant component of ground substance
• Long-chain, unbranched, highly neagatively charged polysaccharides
  
  • Makes them hydrophilic
• Can bind water which forms a highly hydrated gel
  
  • Helps resist compression/deformation
  • Allows quick diffusion of water-soluble particles
• Several types of GAGs found in CT:
  
  • Hyaluronic Acid (Hyaluronan)
    
    • Always present in ECM
    • Different form other GAGs in having a much longer molecule that doesn’t contain sulfer
    • Does NOT form proteoglycans
  • Chondroitin Sulfate, Dermatan Sulfate, Keratan Sulfate, Heparan Sulfate
    
    • Some of the more common types of GAGs
    • Attach to proteins to form proteoglycans

Question 7

Describe proteoglycans and their role in the extracellular matrix.

Answer 7

• Composed of GAGs covalently attache to a core protein
  
  • GAGs extend perpendicularly from the core protein
    
    • GAGs repel each other form a brush-like structure
• Form giant aggregates by non-covalently attaching to hyaluronic acid via a link protein
  
  • Aggregates are hydrophilic
    
    • Responsible for gel state of ECM
  • Account for ability of CT (esp. cartilage) to resist compression w/o losing flexibility

Question 8

Describe multiadhesive glycoproteins and their role in the extracellular matrix.

Answer 8

• Assemble into thin fibrils
• Serve as cross-links b/t collagen, ECM, and cells
• Play important role in stabilizing the ECM
  
  • Bind ECM to transmembran proteins from integrin family
    
    • Hemidesmosomes
    • Focal adhesions
• Two Types:
  
  • Fibronectin
    
    • Most abundant glycoprotein in CT
    • Molecule composed of a dimer
      
      • Contains binding domains that interact with diff. ECM molecules
        
        • Type I, II, III Collagen, GAGs, Fibrin, Integrin, and others
  • Laminin
    
    • Present in the basal lamina
    • Binding sites for Type IV Collagen, Integrin, and others

Question 9

List the resident cells found in connective tissue.

Answer 9

1. Fibroblasts
2. Adipocytes
3. Macrophages
4. Mast Cells

Question 10

Describe fibroblasts.

Answer 10

• Principal resident cell of CT
• Responsible for producing components of ECM:
  
  • Collagen
  • Elastic Fibers
    
    • Elastin
    • Fibrillin
  • GAGs
  • Proteoglycans
  • Multiadhesive Glycoproteins
• Have different appearances in different types of CT
• Usually reside in close proximity to collagen fibers
• Four Main Types:
  
  • Active Fibroblasts
    
    • Present in actively growing CT or during wound repair
    • Abundant cytoplasm w/ many thin processes
      
      • Contiains abundant RER and Golgi Apparatus
      • Oval-shaped Nucleus
        
        • Pale, Euchromatic
        • One or more well-expressed nucleoli
    • Commonly found in Loose CT
  • Inactive Fibroblasts
    
    • Present in Dense CT
    • Have more limited cytoplasm
      
      • Contains less develope RER and Golgi
      • More elongated nucleus
        
        • Dark, Heterochromatic
  • Myofibroblasts
    
    • Display properties of both fibroblasts and smooth muscle
      
      • Contain bundles of actin filaments and dense bodies similar to smooth muscle cells (ability to contract)
      • NOT a type of smooth muscle
        
        • Don’t have an external lamina
    • Often numerous at wound sites
      
      • Thought to be important for closure of wound
  • Mesenchymal Cells
    
    • Present in Embryonic CT
    • Superficially resemble Active Fibroblasts
      
      • Pale, Euchromatic Nuclei
      • Well develope RER and Golgi
    • Pluripotent
      
      • Give rise to fibroblasts, chondroblasts, osteoblasts, and adipocytes

Question 11

Describe adipocytes.

Answer 11

• CT specialized for storing fat and producing a variety of hormones
• Two Types:
  
  • Uniloculuar
    
    • Most common
    • Aggregate to form white adipose tissue
    • Large spherical cells
      
      • Single large lipid droplet
      • Cytoplasm and flattened nucleus form thin rim around lipid droplet
  • Multilocular
    
    • Rare in adult humans
      
      • Found within brown adipose tissue of newborns around the root of the aorta and renal arteries
        
        • Help sustain body temp.
    • Characterized by:
      
      • Multiple lipid droplets
      • Round nuclei
      • Large number of mitochondria (give tissue brown color)
    • Metabolize lipid to produce heat

Question 12

Describe macrophages.

Answer 12

• Phagocytic cells derived from monocytes
  
  • Originate from precursor in bone marrow → circulate through blood → migrate into connective tissue → become macrophages
• One in CT can undergo mitosis and increase their number
• Ways to identify: (Both features can be difficult to see)
  
  • Presence of ingested material in the cytoplasm
  • Kidney-shaped nucleus
• Functions:
  
  • Phagocytosis of bacteria, senescent cells, and tissue clean up
  • Have MHC II on cell surface, which allows them to present antigens to other cells of the immune system (including T-cells)
  • Produce cytokines that activate cells of the immune system
• Mononuclear Phagocytic System
  
  • Histiocytes (CT)
  • Alveolar (Lungs)
  • Kupffer (Liver)
  • Osteoclasts (Bone)
  • Langerhans (Skin)
  • Dendritic (Lymphoid- lymph nodes, tonsils, and spleen)
  • Microglial (CNS) - Associated w/ Alzheimers)

Question 13

Describe mast cells.

Answer 13

• Originate in bone marrow → migrate into CT → proliferate and differentiate
• Structure:
  
  • Largest cells in CT (20-30 µm)
  • Oval
    
    • Centrally placed spherical nucleus
    • Lage number of large basophilic granules
      
      • Contain histamine, heparin sulfate or chondroitin sulfat, and chemotactic mediators
    • Surface contains receptors for IgE
  • Similar to basophils but differ in nuclear morphology
• Function:
  
  • Activation occurs when first exposure of antigen results in and increase in IgE that binds to the surface of the cell
  • Subsequent exposure → antigen binds to surface bound IgE
  • Causes cross-linking of IgE → Degranulation
  • Degranulation → release of histamine which results in:
    
    • Vasodilation and Increased Vascular Permeability that promotes emigration of WBCs into tissue)
    • Bronchochonstriction and Increased Mucus Production (caused by several other agents too)
    • Chemotactic Mediators attract WBCs, Eosinophils, and Neutrophils
  • These reactions are usually mild and local, but can be serious and life threatening

Question 14

List the transient cells found in connective tissue.

Answer 14

1. Plasma cells
2. Leukocytes

Question 15

Describe plasma cells.

Answer 15

• Anti-body producing cells derived from B-lymphocytes
• Most numerous in areas of inflammations and where foreign particls and microorganisms have entered tissue
• Characteristics
  
  • Prominent RER and Golgi
  • Dark Heterochromatin alternates with pale euchromatin
    
    • Creates “clock-face” pattern

Question 16

Describe leukocytes.

Answer 16

• Otherwise known as white blood cells (WBCs)
• Found in CT, especially in areas of inflammation

Question 17

List and describe the types of embryonic connective tissue.

Answer 17

• Composed of:
  
  • Mesenchyme
  • Mucous connective tissue
• Mesenchyme
  
  • Found in the embryo
  • Composed of mesenchymal cells
    
    • Pluripotent
  • Form 3-D network
  • Very few fibers
  • Abundant ground substance
• Mucous connective tissue
  
  • Found only in the umbilical cord
  • Contains large amounts of hyaluronic acid
  • Very few fibers
  • Gelatin-like appearance (Wharton’s Jelly)

Question 18

List and describe the connective tissues proper.

Answer 18

• Subdivisions:
  
  • Loose (areolar) Connective
  • Dense Connective
    
    • Dense Irregular
    • Dense Regular
• Loose (areolar) connective tissue
  
  • Highly cellular w/ thin and relatively sparse Type I Fibers
    
    • Usually don’t contain bundles
  • Abundant ground substance
  • Most fibroblasts are active
  • Contain large number of transient cells (plasma cells and leukocytes)
  • Located beneath epithelia that cover internal surfaces of the body (e.g. lamina propria of intestine)
    
    • Acts as filter tissue
  • Also surrounds blood and lymphatic vessels and nerves, excretory ducts
• Dense Connective Tissue
  
  • ​Contain larger amounts of Collagen Type I fibers and bundles
  • Little ground substance and fewer cells than in Loose CT
  • Dense Irregular
    
    • _​_Fibers oriented in various directions
    • Main type of cell is fibroblast
      
      • Most are inactive
    • Transient cells (plasma cells and leukocytes) are scarce or absent
    • Fine networks of elastice fibers scattered around collagen
    • Found:
      
      • Dermis of skin
      • Capsule of internal organs
        
        • Spleen
        • Testis
  • Dense Regular
    
    • Densely packed collagen bundles
      
      • Oriented in same direction
    • Inactive fibroblasts
      
      • Thin, sheet-like
      • Located in b/t bundles
      • Compressed, heterochromatic nuclei
    • Found:
      
      • Tendons
      • Ligaments
      • Aponeuroses

Question 19

List and describe specialized connective tissues.

Answer 19

• Adipose Tissue
  
  • ​Largest respository of energy via triglycerides
  • Connective tissue septa separate groups of adipocytes into lobules
  • Receive rich blood supply
  • Innervated by autonomic system
  • Contains several receptors for several hormones
  • ​​White adipose tissue
    
    • Composed of unilocular adipocytes
    • Accumulates energy
    • Contains receptors for GH, Insulin, and noradrenalin
    • Poor heat conductor
    • Attributes to insulation of body
    • Visceral adipose helps to keep organs in place and plays important role in shock-absorption
  • Brown adipose tissue
    
    • Not present in human, but found in newborns
    • Composed of multilocular adipocytes
      
      • Contain large amounts of mitochondria
      • Oxidize fatty acids up to 20 times the rate of white adipose tissue
        
        • Increasing body heat production threefold

Question 20

List the main components of blood.

Answer 20

1. Plasma
2. Formed Elements
   
   1. Erythrocytes
   2. Neutrophils
   3. Eosinophils
   4. Basophils
   5. Lymphocytes
   6. Monocytes

Question 21

Describe the structure and function of plasma.

Answer 21

• Structure:
  
  • Solution of nutrients and gases
  • More than 90% water
  • Various solutes
    
    • Albumin
    • Globulins
    • Clotting Factors

Question 22

Describe the abundance, structure, and function of erythrocytes.

Answer 22

• Abundance:
  
  • 600 erythrocytes for every 1 leukocyte
• Structure:
  
  • Anucleated biconcave disks
  • 8 x 2.5 µm
  • NO organelles
  • Sacks of hemoglobin
• Function:
  
  • Carry oxygen
  • Biconcave shape allows more surface area for gas exchange
    
    • Deacreases distance oxygen must diffuse through the cell

Question 23

Describe the abundance, structure, and function of neutrophils.

Answer 23

• Abundance:
  
  • Most common leukocyte (55-70% of leukocytes)
• Structure:
  
  • Fairly large cells (10-12 µm)
    
    • Multilobed nuclei
    • Cytoplasm contains granules
      
      • Bactericidal proteins
• Function:
  
  • Selectively phagocytize bacteria
  • Produce IL-1
    
    • Increases body temperature and causes fever
  • Dead neutrophils and cell debris form pus

Question 24

Describe the abundance, structure, and function of eosinophils.

Answer 24

• Abundance:
  
  • Rare (2-4% of Leukocytes)
• Structure:
  
  • Slightly larger thatn neutrophils (11-14 µm)
  • Bilobed nucleus
  • Cytoplasm contains eosinophilic granules
    
    • Cytotoxins designed to destroy protozoan and helminthic parasites
• Function:
  
  • Destroy protozoan and helminthic parasites
    
    • Most common in blood during parasitic infections and allergic reactions
  • Modulate allergic response
    
    • Break down histamine
    • Inhibit mast cell degranulation

Question 25

Describe the abundance, structure, and function of basophils.

Answer 25

• Abundance:
  
  • Rarest leukocytes (0.5-1.5% of all leukocytes)
• Structure:
  
  • Smallest of granulocytes (8-10 µm)
  • Lobed nucleus
  • Cytoplasm contains basophilic granules
    
    • Contain histamine and heparin
• Function:
  
  • Produce histamine
  • Very similar to mast cells

Question 26

Describe the abundance, structure, and function of lymphocytes.

Answer 26

• Abundance:
  
  • Second most common (20-30% of all leukocytes)
• Structure:
  
  • Large spherical
  • Slightly indented hetorchromatic nucleus
  • Tiny rim of cytoplasm
  • NO granules
• Function:
  
  • 3 Main Populations
    
    • T-Cells
      
      • Arise from bone marrow and hemopoietic organs → transmit through blood into thymus → mature and multiply → reenter blood stream
      • Characterized by T-cell receptor (TCR) on cell surface
        
        • Binds to antigen-MHC-II complexes
          
          • Activates CD4⁺ T cells
            
            • Produce cytokines
        • Binds to MHC-I complexes
          
          • Activates CD8⁺ T cells
            
            • Recognize defective MHC-I complexes and destroys cells and destroys them
    • B-lymphocytes
      
      • Arise and mature in the bone marrow
      • Characterized by surface immunoglobulins bound to plasma membrane
      • Reside in lympoid and connective tissues (spleen and lymph nodes)
        
        • Where they are programmed for viruses, bacteria, or other foreign particles
      • When they reach target tissue → proliferate into plasma cells → produce antibodies
    • Natural Killer Cells
      
      • Large lyphocytes
      • Programmed to kill certain virus infected cells and some types of tumors
      • Play important role in transplant rejection

Question 27

Describe the abundance, structure, and function of monocytes.

Answer 27

• Abundance:
  
  • 4-6% of all leukocytes
• Structure:
  
  • Large cells (12-20 µm)
  • Bean/Kidney shaped nucleus
  • Cytoplasm contains lysosomes and small mitochondria
• Function:
  
  • Circulate through peripheral blood → adhere to walls of blood vessels → use ameboid movement to reach target → become macrophages

Question 28

Describe the abundance, structure, and function of platelets.

Answer 28

• Abundance:
  
  • 200,000-300,000 per mm³
• Structure:
  
  • Biconvex discoid particles
    
    • Diameter of 2-4 µm
  • Contain:
    
    • Few mitochondria
    • Many granules
      
      • Adhesion proteins and clotting factors
  • Have lifespan of 8-10 days

Question 29

Describe the composition and function of blood.

Answer 29

• Composed of:
  
  • RBC’s
  • WBC’s
  • Platelets
  • Plasma
• Function:
  
  • Transport System for:
    
    • O₂
    • CO₂
    • Nutrients
    • Wastes
  • Maintains homeostasis by acting as a buffer
  • Regulates temp., hormone levels, pH, osmotic pressure
  • Clots if exposed to air/collagen