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

Question 1

Answer 1

• Trachea
• Longitudinal section of trachea
• Note the four layers
  
  • Mucosa (epithelium and lamina propria)
  • Submucosa (with glands and large blood vessels)
  • Cartilage rings
  • Skeletal muscle
• The structure of the trachea is maintained by C-shaped rings of hyaline cartilage seen as ovals in longitudinal sections
• The submucosal glands are particularly obvious

Question 2

Answer 2

• Trachea
• Longitudinal section
• The structure of the trachea is maintained by C-shaped rings of hyaline cartilage seen as ovals in longitudinal sections
• The submucosal glands are particularly obvious

Question 3

Answer 3

• Trachea
• Cross-section
• Identify mucosa, submucosa, cartilage & adventitia
• Note trachealis muscule at posterior aspect
• The structure of the trachea is maintained as a plate in true cross sections
• Illustrates the trachealis muscle joining the ends of the cartilage “C”

Question 4

Answer 4

• Trachea
• Cross-section
• Note cartilage ring and trachealis muscle
• The structure of the trachea is maintained as a plate in true cross sections
• Illustrates the trachealis muscle joining the ends of the cartilage “C”

Question 5

Answer 5

• Trachea
• The “respiratory epithelium” is a pseudostratified columnar epithelium with goblet cells and cilia
• In this slide, the trachea has
  
  • A fairly thick basement membrane, which thickens during chronic asthma
    
    • Normally, the basement membrane is only visible in electron microscopy
  • A thin, dense lamina propria
• The submucosa has glands and a lymphocyte cluster
• The cartilage rings are hyaline cartilage with a perichondrium
• Identify…
  
  • The pseudostratified columnar epithelium
    
    • The goblet cells and cilia are not clearly seen in this thick section
  • A narrow lamina propria
  • Submucosal glands, blood vessels and lymphocytes
  • The hyaline cartilage

Question 6

Answer 6

• Trachea
• In this tracheal section, the denser lamina propria is easily distinguished from the submucosa with seromucous glands
• Easier to distinguish the lamina propria from submucosal structures

Question 7

Answer 7

• Trachea
• Seromucous glands of the submucosa release their secretions via ducts (through the denser lamina propria), and eventually onto the respiratory epithelium
• Mucus and cilia combine to create the mucociliary escalator to trap and remove particulates from air in the conducting airways
• Follows the path of a mucus secreting submucosal gland, to its duct passing through the lamina propria, on its way to the epithelium and tracheal lumen

Question 8

Answer 8

• Trachea, respiratory epithelium
• The “respiratory epithelium” of the trachea is really a misnomer as no respiration or gas exchange occurs here
• Histologically, it is classified as a pseudostratified columnar epithelium with cilia and goblet cells
• While goblet cells are not obvious in this fairly thick section, the cilia are clearly identified because they are longer than the width of a nucleus, clumped and marked by a row of basal bodies in the apical cytoplasm
• How you can tell this epithelium has cilia & not microvilli
  
  • Darkly stained row of basal bodies

Question 9

Answer 9

• Trachea & esophagus
• The trachea lies anterior to the esophagus
• The trachea (right) requires cartilage rings to prevent collapse during inspiration and is well vascularized to warm the passing air
• The esophagus (left) is lined by a thicker stratified squamous epithelium designed to withstand friction of swallowed food, and a thick muscular tunic to propel the bolus

Question 10

Answer 10

• Bronchus
• Irregular cartilage patches characterize the bronchi
• Progressing to smaller bronchi, the amount of cartilage will diminish, the amount of smooth muscle will increase
• The epithelium is a low pseudostratified columnar, still with cilia and goblet cells
• Seromucous glands are present in the submucosa
• At the top are two blood vessels: a thicker walled bronchial artery and a thin walled pulmonary artery
• To the far left and far right are two bronchioles
• A bronchus is characterized by patches of cartilage, gradually replaced by smooth muscle

Question 11

Answer 11

• Bronchus
• Low pseudostratified “respiratory epithelium” with cilia & goblet cells
• Submucosa with seromucous glands and a few small clusters of smooth muscle
• Irregular cartilage patches
• Adventitia with blood vessels: a thin walled pulmonary artery & thicker walled bronchial artery
  
  • Both these vessels travel with the arborizing conducting airways
• Submucosal seromucous glands are seen that secrete onto the luminal surface
• The respiratory epithelium is a lower pseudostratified columnar than the trachea
• Accompanying the bronchus are a branch of both the pulmonary artery (thinner wall) and the bronchial artery (thicker wall of a systemic muscular artery)

Question 12

Answer 12

• Bronchus
• Good close up of a bronchus
• What would distinguish this slide from a close up of the trachea?
  
  • The smooth muscle lying at the interface of the lamina propria & the submucosa
• Submucosal seromucous glands are seen that secrete onto the luminal surface
• The respiratory epithelium is a lower pseudostratified columnar than the trachea
• Goblet cells and cilia are present indicating an active mucociliary escalator in the bronchi
• Scattered patches of smooth muscle are seen beneath a thin lamina propria
• The smooth muscle will become a complete circle in the smaller branching of bronchi
• Stimulation of the parasympathetic nervous system will contract the smooth muscle

Question 13

Answer 13

• (Preterminal ) Bronchiole
• Bronchiole - Verhoeff’s stain
• Classic bronchiole with a continuous tunic of smooth muscle (and no cartilage)
• Lingering submucosal glands indicate the probably persistence of a few ciliated cells for mucus transport
• These will rapidly disappear from bronchioles
• Note the presence of black staining elastin, which is an increasing component of the lamina propria from the bronchioles through the alveoli
• Note, that often these largest bronchioles are simply called bronchioles (without the “preterminal” designation)
• The epithelium is simple columnar with a few clear goblet cells and probable cilia
• A complete ring of smooth muscle is the clear diagnostic feature here
• There is no cartilage, but seromucous glands persist in the submucosal layer
• Note that this slide is stained with Verhoeff’s to emphasize the elastin component

Question 14

Answer 14

• Terminal Bronchioles
• Bronchioles at bifurcation point
• Usually bronchioles are distinguished by their LACK of cartilage
  
  • Do be aware though, that small pieces may persist at bifurcation points
• The most important positive diagnostic of a bronchiole is the folded appearance of the simple cuboidal epithelium due to a continuous tunic of smooth muscle
  
  • The complete ring of smooth muscle contracts as it is fixed resulting in a highly folded lumen, which makes a bronchiole easy to identify
• The blood vessel in the upper right-hand corner is a thinner walled pulmonary vein (travels independently of the respiratory tree), while at “6 o’clock” is a thicker walled bronchial artery
• At “10 o’clock” is a thinner walled pulmonary artery
• The bronchial arteries only accompany conducting airways, until alveoli begin to bud from the respiratory bronchioles
• Lingering support of cartilage and submucosal glands at the split, but none around the bronchioles themselves
• Epithelium is simple cuboidal with no clearly discernable goblet cells

Question 15

Answer 15

• Terminal Bronchioles
• Simple cuboidal epithelium, with few/no goblet cells or cilia
• The tunic of smooth muscle contracts giving a folded appearance to the epithelium
• Cells in the lumen are alveolar macrophages moving by amoeboid action to a bronchus where they can be carried up to the esophagus by the mucociliary escalator
• The cartilage and cluster of submucosal glands are remnants of an earlier bronchial segment that linger into this branch point
• Lingering support of cartilage and submucosal glands at the split, but none around the bronchioles themselves
• Epithelium is simple cuboidal with no clearly discernable goblet cells

Question 16

Answer 16

• Terminal Bronchiole, Respiratory Bronchiole, Alveolar Duct & Sac, Alveoli
• (Diagonal from lower right)
  
  • Terminal bronchiole
  • Respiratory bronchiole
  • Alveolar duct
  • Alveolar sacs
  • Branching into alveoli
• Focus ont he numerous alveoli in this low power view of the lung parenchyma

Question 17

Answer 17

• Terminal Bronchiole (lower right) to Respiratory Bronchiole (upper left)
• Terminal bronchiole
  
  • Last portion of the conducting airways
  • Simple cuboidal epithelium, with no goblet cells or ciliated cells (80% replaced by Clara cells)
    
    • Clara cells are distinguished only in EM
  • Patches of smooth muscle
    
    • A complete ring of smooth muscle produces a folded epithelium
    • No cartilage or submucosal glands
• As soon as alveolar outpocketings interrupt, the airway is called a respiratory bronchiole
  
  • The first of the respiratory airways for actual gas exchange
  • There are three levels of bifurcations in the respiratory bronchioles, each with progressively less and less smooth muscle
  • The blood vessel is a pulmonary artery
  • The deepest level that the bronchial arteries penetrate is to the terminal bronchioles
  • Single alveolar outpockets interrupt a wall whose structure is otherwise similar to the terminal bronchiole
  • Note the minimal blue-staining collagenous connective tissue (Mallory stain) and the alveolar macrophages in the bronchiole lumen

Question 18

Answer 18

• Respiratory Bronchiole
• Verhoeff stain
• This elastin stain emphasizes the importance of the elastic component in the lamin propria/ interstitial area of the lung acinus
  
  • Elastin increases in the lower conducting airways and in the true respiratory portions of the lung
• The acinar unit passively recoils after inspiration, decreasing the need for a forced muscular expiration
  
  • Diseases that compromise elastin, such as emphysema, have substantial effects on the ability of the lung to recoil after inhalation

Question 19

Answer 19

• Lung Alveoli
• The attenuated lung epithelial cells, the type I pneumocytes, line 97% of the alveolar surfaces that face the air
• Capillary endothelial cells are found in the center of the alveolar septal walls
• Interstital cells occupy the thicker angular wedges of the alveoli
• Alveoli branch out from the alveolar duct
• Alveolar walls are built of ccapillaries coated by type I pneumocytes

Question 20

Answer 20

• Lung Alveoli
• Alveoli (L19 & 20) branch out from the alveolar duct
• Alveolar walls are built of ccapillaries coated by type I pneumocytes

Question 21

Answer 21

• Alveolar Macrophages
• Macrophages migrate freely in the airspace of the lungs
• These are from a smoker and have been given an undigestible “meal” of tar
  
  • These macrophages have ingested cigarette tar
• Note the tar deposits in the wedges of interstitial tissue as well
  
  • The black patches are tar; this slide was not stained with Verhoeff’s
• Since there are no lysosomal enzymes for digestion of the tar, residual bodies accumulate
  
  • The amoeboid macrophages migrate up to the preterminal bronchioles and bronchi where they are cleared by the mucociliary escalator
  • Macrophages also exit the lung via the minimal lymphatic drainage
  • The life span of an alveolar macrophage is about 24 hours

Question 22

Answer 22

• Type I Pneumocyte
• A transmission electron micrograph is necessary to visualize the gas diffusion barrier of the alveolus
• The capillary lumen is identified by the presence of two RBCs, another cell (WBC) and precipitated plasma proteins
• The air space is above & below
• The walls of the lung alveoli are composed of two cell types, type I and
  type II pneumocytes
• Type I pneumocytes are a thin attenuated cells that face the air space and are limited at their base by a basement membrane shared with the endothelial cells of alveolar capillaries
• Type I pneumocytes form a portion of the gas diffusion barrier (from blood to air):
  
  • Red blood cell plasma membrane
  • Blood plasma
  • Endothelial cell (continuous)
  • Basement membrane
  • Type I pneumocyte
  • Surfactant
  • Air

Question 23

Answer 23

• Respiratory Barrier
• The barrier for the diffusion of gases (from RBC to air sac)
  
  • RBC PM (red)
  • Plasma
  • Capillary endothelial cell (pink)
  • Basement membrane (green)
  • Type I pneumocyte (blue)
  • Surfactant (not visualized in this photo)
• The walls of the lung alveoli are composed of two cell types, type I and
  type II pneumocytes
• Type I pneumocytes are a thin attenuated cells that face the air space and are limited at their base by a basement membrane shared with the endothelial cells of alveolar capillaries
• Type I pneumocytes form a portion of the gas diffusion barrier (from blood to air):
  
  • Red blood cell plasma membrane
  • Blood plasma
  • Endothelial cell (continuous)
  • Basement membrane
  • Type I pneumocyte
  • Surfactant
  • Air

Question 24

Answer 24

• Type II Pneumocyte
• The epithelium of the alveolus is not only composed of type I pneumocytes, but type II pneumocytes as well
• The type II pneumocyte seen here is easily identified by the lamellar granules of surfactant
• As a TEM artifact, the lipid components of the surfactant aggregate into sheets upon processing
• Lung surfactant lowers the surface tension to allow for easier expansion of the air sacs
• Type II pneumocytes are rounded cells in the same sheet of epithelium as the type I cells
• Lamellar granules in the cytoplasm contain surfactant proteins and lipids that are secreted into the alveolar space and form a coat on the outer aspects of both the type I and type II pneumocytes
• Lung surfactant lowers the surface tension to allow for easier expansion of the air sacs
• Surfactant is composed of 50% cholesterol, 40% phospholipid dipalmitoyl- phosphotidylcholine (DPPC) and 10% proteins
• There are four surfactant proteins: SP- A, B, C and D
  
  • SP-A and SP-B have carbohydrate recognition receptors that bind bacteria and aid in the activation of alveolar macrophages
  • SP-B and SPC are hydrophobic proteins that stabilize the surfactant/air interface
  • Type II pneumocytes can also undergo mitosis and differentiate into both new type I or type II cells