3 Lab: Normal Histology of the Lung Parenchyma, Airways, & Blood Vessels

Question 1

Conducting vs. respiratory airways

• Conducting airways
• Respiratory airways

Answer 1

• Conducting airways
  
  • Structures
    
    • Nasal cavities
    • Pharynx
    • Larynx
    • Trachea
    • Bronchi
    • Preterminal & terminal bronchioles
  • Functions
    
    • Clear the air of particulates (movement of air)
    • Deliver warm and humid air to the deeper sites of gas exchange
    • Muscle regulated control of air passage
    • Mucociliary escalator
• Respiratory airways
  
  • Structures
    
    • Respiratory bronchioles
    • Alveolar ducts
    • Alveolar sacs
    • Alveoli
  • Functions
    
    • Actual respiration
    • Exchange of carbon dioxide in the blood for oxygen in the air (gas exchange)
    • Innate immunity
    • Surfactant

Question 2

Conducting airway generation & respiratory tree

• Conducting airway generation
• Respiratory tree

Answer 2

• Conducting airway generation
  
  • 16 generations of conducting airways (trachea to terminal bronchiole) develop by 16 weeks gestation
  • 16-23 generations of airway involve alveoli, respiratoyr bronchioles, & alveolar ducts
    
    • Many of these develop after birth up to age 5-8
• Respiratory tree
  
  • Cross-sectional area increases
  • Air flow velocity decreases
  • Air turbulence increases
    
    • Enhances circulation of air contacting the mucociliary escalator
  • Humidity & warmth increase

Question 3

Branching & cross-sectional area

• In total, the airways branch…
• Each parent gives rise to…
• As air moves distally into the lung, the following trend is observed

Answer 3

• In total, the airways branch…
  
  • 25-30 times
• Each parent gives rise to…
  
  • 2 daughters, the cross-sectional area of which exceeds the area of the parent tube
• As air moves distally into the lung, the following trend is observed
  
  • Cross-sectional area increases
  • Air flow velocity decreases
  • Air turbulence increases (enhances circulation of air contacting the mucociliary escalator and later the alveolar walls)
  • Humidity & warmth increase

Question 4

Blood supply & respiratory tree

• Blood supply
• The hollow tube of the respiratory tree from the trachea through respiratory bronchioles is built of four layers

Answer 4

• Blood supply
  
  • The lungs receive a dual arterial blood supply
  • The low pressure pulmonary arteries run alongside bronchi and bronchioles.
  • The pulmonary circuit continues to divide into the capillary networks of the alveolar walls
  • The high pressure bronchial arteries arise from the aorta, and accompany the bronchi to the terminal bronchioles only
• The hollow tube of the respiratory tree from the trachea through respiratory bronchioles is built of four layers
  
  • Mucosa
  • Submucosa
  • Muscularis &/or cartilage
  • Adventitia

Question 5

Trachea

• The mucosa consists of
• The trachea displays a classic “respiratory epithelium”
• Mucus secreted by the goblet cells and submucosal glands
• The columnar cells of the epithelium

Answer 5

• The mucosa consists of
  
  • Respiratory epithelium
  • Underlying connective tissue, the lamina propria
• The trachea displays a classic “respiratory epithelium”
  
  • Pseudostratified columnar with cilia and goblet cells
  • Note that this epithelium is a misnomer, it is not the site of actual respiration or gas exchange
  • The respiratory epithelium produces the mucociliary escalator for the trachea and bronchi
• Mucus secreted by the goblet cells and submucosal glands
  
  • Carried by the action of cilia up to the junction with the esophagus, where it is swallowed
• The columnar cells of the epithelium
  
  • Contain receptors for IgA
  • Via transcytosis, secrete the immunoglobulin into the lumen of the respiratory tree

Question 6

Trachea: mucosa

• Migrating lymphocytes
• Specialized secretory cells called Kulchitsky cells
• Concentrated granules at the basal aspect of the Kulchitsky cells

Answer 6

• Migrating lymphocytes
  
  • Can occasionally be seen penetrating the basal layer of the epithelium
  • These lymphocytes are part of the mucosal immune system of the GI and respiratory tracts
• Specialized secretory cells called Kulchitsky cells
  
  • Located in the base of the epithelium
  • Single endocrine cells, as a class referred to as APUD cells (Amine Precursor Uptake & Decarboxylase)
• Concentrated granules at the basal aspect of the Kulchitsky cells
  
  • Store a variety of pharmacologically active peptides (bombesin, calcitonin & leu-enkephalin) and amines (serotonin, dopamine & norepinephrin)
  • When stimulated, the secretions are released into the basal lamina and carried away by the blood stream
  • These local agents act as paracrine and endocrine factors that respond to hypoxia and help to regulate the respiratory tract

Question 7

Trachea

• Lamina propria of the mucosa
• Submucosa

Answer 7

• Lamina propria of the mucosa
  
  • Thin wedge of dense connective tissue containing lymphocytes, plasma cells, mast cells and occasional polymorphonuclear leukocytes
  • Displays a rich capillary network with accompanying non-myelinated nerves
• Submucosa
  
  • Looser area of connective tissue outside of the lamina propria
  • Characterized by blood vessels and glands that secrete both mucous and serous components

Question 8

Trachea

• Muscularis/cartilage layer
• Trachealis muscle
• Outer adventitia

Answer 8

• Muscularis/cartilage layer
  
  • Contains C-shaped rings of hyaline cartilage
  • Posterior gap in the cartilage is spanned by smooth muscle,the trachealis muscle
• Trachealis muscle
  
  • Keeps the trachea open during the pressure swings of inspiration and expiration
  • Lies in front of the esophagus, as the anterior-most structure of the neck
• Outer adventitia
  
  • Contains large nerves, lymphatic vessels, fat, and occasional lymph nodules (like that of blood vessels)
  • Blends into the general connective tissue of the central mediastium of the chest cavity

Question 9

Bronchi

• The trachea bifurcates into two mainstem bronchi
• In the bronchi, the respiratory epithelium
• Lamina propria
• Plasma cells
• Submucosal glands

Answer 9

• The trachea bifurcates into two mainstem bronchi
  
  • Right mainstem bronchus lies in direct line with the trachea and therefore increases the possibility of aspiration down this tube
• In the bronchi, the respiratory epithelium
  
  • Is still pseudostratified columnar, but of a lower height than the
    tracheal epithelium
• Lamina propria
  
  • Often contains lymphocytes
• Plasma cells
  
  • Secrete IgA to be carried across the epithelium
• Submucosal glands
  
  • Produce both mucous and serous secretions
  • Contain myoepithelial cells

Question 10

Bronchi

• Myoepithelial cells
• Hyaline cartilage
• In fixed sections of bronchi, the smooth muscle

Answer 10

• Myoepithelial cells
  
  • Innervated by the autonomic nerves
  • Hhelp to squeeze secretions onto the luminal surface
• Hyaline cartilage
  
  • Discontinuous, in irregular plates
  • The amount of cartilage decreases as the bronchi move distally into the lungs
  • As the cartilage decreases, smooth muscle slowly increases to form a complete ring in the smaller bronchi
  • The smooth muscle is sandwiched between the epithelium & cartilage
• In fixed sections of bronchi, the smooth muscle
  
  • Contracts producing a convoluted lumen that distinguishes the bronchi
    from the trachea
  • The same contraction can constrict the airways in vivo
    (ex: asthma)

Question 11

As the bronchi progress into the lung, they

Answer 11

• Branch two-three more times
• Decrease in diameter
• Decrease the height of the respiratory epithelium
• Decrease the number of goblet cells
• Decrease the presence of submucosal glands
• Decrease the amount of hyaline cartilage
• Increase the amount of smooth muscle

Question 12

Bronchioles: general features

Answer 12

• Epithelium
  
  • Largest with low pseudostrat to simple columnar
  • Smaller with simple cuboidal
• Smooth muscle complete ring
• Loss of submucosal glands
• No cartilage
• No goblet cells (a few in largest)
• Cilia decrease and disappear
• Clara cells appear

Question 13

Bronchioles

• Bronchi divide to become
• Structure of (preterminal) bronchioles

Answer 13

• Bronchi divide to become
  
  • (Preterminal) bronchioles
  • Further divide to become terminal bronchioles (
  • Develop single alveolar pockets along their walls and are then named respiratory bronchioles
• Structure of (preterminal) bronchioles
  
  • Within lung segments
  • Simple columnar epithelium
  • Decreasing cilia and goblet cells
  • Full circle of smooth muscle
  • Decreasing submucosal seromucous glands
  • Increasing clara cells
  • Travels w/ large pulmonary artery
  • No cartilage

Question 14

Structure of terminal bronchiole

Answer 14

• Root/stem of lung lobule
• Simple cuboidal epithelium
• No goblet cells or cilia
• Increasing Clara cells (80% of epithelial cells)
• Patches of smooth muscle (many)
• No cartilage or submucosal glands

Question 15

Structure of respiratory bronchioles

Answer 15

• Root/stem of lung acinus
• Low cuboidal epithelium
• Interrupted with outpockets of alveoli along wall
• First respiratory gas exchange
• Clara cells
• Patches of smooth muscle (fewer)
• Pulmonary artery
• No bronchial artery
• Lead into the alveolar duct

Question 16

Bronchioles

• As bronchioles become smaller, they
• Inflammation can cause

Answer 16

• As bronchioles become smaller, they
  
  • Progressively lose goblet cells, cilia, and seromucous glands
  • i.e. everything needed for the mucociliary escalator
• Inflammation can cause
  
  • Bronchiolar goblet cell metaplasia
  • Increased mucus production
  • In some cases leading to airflow obstruction

Question 17

Bronchioles

• Goblet cells
• Clara cells

Answer 17

• Goblet cells
  
  • Are replaced by Clara cells that have a domed-shaped appearance in TEM
• Clara cells
  
  • Secrete CC10, a Clara cell specific surfactant protein
    
    • The surfactant decreases surface tension, allowing the bronchiolar lumen to remain patent, even in the face of decreasing muscle and no cartilage
  • Secrete surfactant protein A (opsonize bacteria), lysozyme (digest bacterial cell walls), and transcytose immunoglobulins into the airspace

Question 18

Pulmonary acinus

• Components
• Functions

Answer 18

• Components
  
  • ​Respiratory bronchiole
  • Alveolar ducts
  • Alveolar scas
  • Alveoli
• Functions
  
  • Gas exchange
  • Immune functions
  • Angiotensin converting enzyme
• Image
  
  • Red = smooth muscle
  • Green = elastin

Question 19

Acinus

• Acinus
• Consists of
• The length of an alveolar duct contains
• The final alveolar sacs
• The walls of individual alveoli have three main layers through which gas diffusion occurs

Answer 19

• Acinus
  
  • Functional unit of the lung
  • Structure distal to a terminal bronchiole, with the main stem being a respiratory bronchiole
• Consists of
  
  • A respiratory bronchiole, alveolar ducts, alveolar sacs and alveoli.
• The length of an alveolar duct contains
  
  • ~30 alveoli budding off its wall in a spiral fashion
• The final alveolar sacs
  
  • Are blind ending cul-de-sacs lined by alveoli
• The walls of individual alveoli have three main layers through which gas diffusion occurs
  
  • Pulmonary capillary endothelium
  • Shared basement membrane
  • Alveolar epithelium

Question 20

Acinus

• There are also wedges of
• The alveolar epithelium consists mainly of
• Electron micrographs of the type II pneumocytes demonstrate
• Type I pneumocytes

Answer 20

• There are also wedges of
  
  • Intervening collagen, elastin, fibroblasts and a few inflammatory cells
• The alveolar epithelium consists mainly of
  
  • Type I pneumocytes (an attenuated cell that covers 95% of the alveolar surface)
  • Type II pneumocytes (secretes lung surfactant)
• Electron micrographs of the type II pneumocytes demonstrate
  
  • A rounded cell with limited surface area and unique granules filled with lamella of surfactant
• Type I pneumocytes
  
  • Terminally differentiated cell incapable of proliferation when
    damaged. Alveolar wall repair occurs from primitive type II pneumocytes
    that later differentiate into new type I cells

Question 21

Image

Answer 21

Question 22

Image

Answer 22

• Top: alveolar ducts w/ smooth muscle knobs
• Bottom: white space = alveoli

Question 23

Alveoli cell types

Answer 23

• Type I pneumocytes
• Type II pneumocytes
• Interstitial cells
• Alveolar macrophages

Question 24

Type II pneumocytes

• General
• Main function
• Other functions

Answer 24

• Lamellar granules
• Main function
  
  • Secrete pulmonary surfactant (tubular myelin)
• Other functions
  
  • Turnover/ recycling of surfactant
  • Superoxide dismutase antioxidant
  • SP-A = activates macrophage phagocytosis
  • Proliferate & differentiate into Type I

Question 25

Image

Answer 25

• Alveolar macrophages