Pulmonary Histology

Question 1

Conducting Portions of the Respiratory System

Answer 1

• Cleans and humidifies air and brings it down to the lungs (serves as a conduit)
• Consists of:
  
  • The nasal cavities
  • Nasopharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles
  • Terminal Bronchioles
• To ensure an uninterrupted supply of air, it has the following general histological structure:
  
  • Combination of cartilage, elastic and collagen fibers, and smooth muscle provide both rigid structural support and some flexibility/extensibility

Question 2

Respiratory Portions of Respiratory System

Answer 2

• Where gas exchange actually occurs
• Consists of:
  
  • Respiratory bronchioles
  • Alveolar ducts
  • Alveoli = sac-like structures that make up most of the lungs

Question 3

Nasal Cavities

Answer 3

• Keratinized stratified squamous epithelium transitions to pseudostratified columnar epithelium at the vestibule
• Middle and inferior conchae are covered in respiratory epithelium
• The roof of the nasal cavities and the superior conchae are covered in olfactory epithelium

Question 4

Respiratory Epithelium

Answer 4

= Ciliated pseudostratified columnar epithelium that rests on a thick basement membrane

• Found in most of the nasal cavities and the conducting portion
• Can have several differents cells that all contact the BM, among them are:
  
  • Ciliated columnar cells - cilia beat mucus particulate matter to pharynx for elimination
  • Goblet cells - filled w/ mucin glycoproteins - mucus traps airborne dust particles & microorgs
  • Brush cells - columnar cell w/ microvilli, function as chemoreceptors like taste cells
  • Small granule cells - neuroendocrine cells like those in the gut
  • Basal cells - stem and progenitor cells

Question 5

Olfactory epithelium

Answer 5

• A thick, pseudostratified columnar epithelium in the cribiform plate of the ethmoid bone with three major cell types:
  
  • bipolar Olfactory neurons (ONs)
    
    • dendrites at luminal ends
    • Nuclei lie in the middle of the epithelium (closer to basal cells)
    • have cilia w/ mb-Rs for odor molecules
    • Signal travels from mb-Rs to axons at basal ends, into small nn. in LP eventually thru foramina in cribiform plate to form olfactory n. (CNI)
  • Supporting cells
    
    • Columnar cells w/ microvilli
    • Lie above ON nuclei near apical end
    • Express many ion channels
  • Basal cells
    
    • Small, spherical or cone-shaped
    • Stem cells and progenitor cells
      
      • replace ONs every 2-3 months
      • replace supporting cells less freq.
    • Only a thin basement mb separates the basal cells from the underlying LP

Question 6

Nasopharynx

Answer 6

Lined with respiratory epithelium

Mucosa contains medial pharyngeal tonsil and bilateral openings to the auditory tubes

Question 7

Larynx

Answer 7

• Short 4 cm passage for air b/w pharynx & trachea
• Wall: cartilages connected by ligaments
  
  • Hyaline cartilage (thyroid,cricoid, & inferior arytenoid)
  • Smaller elastic cartilage (epiglottis, cuneiform, corniculate, inferior arytenoid)
• Epiglottis: transitions from stratified squamous epithelium → respiratory epithelium
• Laryngeal vestibule (LV) surrounded by seromucus glands and bulges out as vestibular folds (VFs) w/ MALT. VFs mostly respiratory epithelium, but sometimes stratified squamous epithelium
• Vocal folds or cords (VC) covered by stratified squamous epithelium contain vocalis m.
  *

Question 8

Trachea

Answer 8

• Epithelium: respiratory
• LP: CT with abundant seromucus glands produce watery mucus
• Submucosa:
  
  • C-shaped hyaline cartilage rings covered by perichondrium: reinforce wall and keep tracheal lumen open
    
    • Open end of rings on posterior surface against esophagus and bridged by trachealis muscle (smooth)
  • Also contains seromucus glands
• Adventitia surrounds the entire organ

Question 9

Bronchi

Answer 9

• Mucosa:
  
  • respiratory epithelium (1°) to ciliated columnar cells w/ fewer goblet cells (smaller branches)
  • LP w/
    
    • Also w/ crisscrossing bundles of spirally arranged smooth m. and elastic fibers become more prominent in smaller branches → contractions give mucosa folded appearance on slides
    • MALT becomes more abundant in smaller branches as well
  • Submucosa
    
    • Seromucus glands
    • Cartilage rings transition to plates of hyaline cartilage in smaller branches
  • Adventitia: contains BVs and nn.
    
    • All surrounded by distinctive lung tissue with empty spaces (of the alveoli)
• Fxn: repeated branching → conduct air deeper into lungs

Question 10

Bronchioles

Answer 10

• Formed after about the 10th gen. of branching
• Mucosa:
  
  • Ciliated simple columnar epithelium w/ Clara cells in larger bronchioles
  • Ciliated simple cuboidal epithelium w/ Clara cells in smallest terminal bronchioles
  • Clara cells = exocrine bronchiolar cells
    
    • not ciliated
    • secrete surfactant
    • detoxify inhaled xenobiotics in sER
    • secrete antimicrobial peptides/cytokines
    • stem cell fxn → injury-induced mitosis
  • LP with prominent smooth m. → folded app.
• Submucosa: no supporting cartilage or glands

Question 11

Respiratory Bronchioles

Answer 11

• Mucosa:
  
  • Ciliated simple cuboidal cells w/ Clara cells
  • Alveolar openings w/ simple squamous cells
  • LP w/ smooth m. and elastic CT

Question 12

Alveolar ducts and alveolar sacs

Answer 12

• Alveolar ducts end in two or more clusters or alveoli called alveolar sacs
• Each alveolus = small rounded pouch open on one side to an alveolar duct or sac
• Lined by thin squamous cells
• Thin LP
• Strand of smooth m. cells surrounds each alveolar opening
• Matrix of elastic and collagen fibers supports both the duct and alveoli

Question 13

Alveoli

Answer 13

• Interalveolar septa b/w adjacent alveoli
  
  • Fibroblasts
  • Sparse ECM of CT with elastic and reticular fibers
    
    • elastic fibers allows alveoli to expand during inspiration and passively contract during expiration
    • reticular fibers prevent both collapse and excessive distension of the alveoli
  • Structures within the septa
    
    • Capillary endothelial cells
      
      • continuous, but not fenestrated
      • very thin b/c most organelles surround the nucleus
    • Type I pneumocytes - gas exchange
      
      • no mitotic capacity
      • Have desmosomes
      • Have occluding jxns that prevent leakage of tissue fluid into the alveolar space
    • Type II pneumocytes - produce surfactant (stored in lamellar bodies)
      
      • mitotic capacity → fxn as stem cells to regenerate the epi.
    • Alveolar macrophages
      
      • Active ones are darker because of carbon from the air and hemosiderin from engulfment of old RBCs
    • Alveolar pores (pore of Kohn)
      
      • connect neighboring alveoli that open to different bronchioles
      • equalize pressure w/in alveoli
      • permit collateral circulation of air when a bronchiole is obstructed
• Reticular and elastic fibers of anatomosing capillaries also provide structural support of alveoli
• Dust cells or macrophages (Kuppfer cells) can be in alveoli or in the interalveolar septa

Question 14

Blood-Air Barrier

Answer 14

• aka alveolar-capillary interface or respiratory mb
• Alveolar air is separated from capillary blood by 3 components that form the blood-air barrier:
  
  • 2-3 very thin cells lining the alveolus
    
    • = Type I pneumocyte
  • fused basal laminae of the cells and of the capillary endothelial cells
  • thin endothelial cells of the capillary
• Surfactant is another important component of the blood-air barrier