Pneumonia and Pulmonary Edema
Gross Pathology and Histopathology of Pneumonia
Noah Stewart
Learning Objectives
By the end of this section, you will be able to:
- Distinguish between symptoms of lobar pneumonia, bronchopneumonia and interstitial pneumonia.
- Describe the unique features of each type of pneumonia.
- Explain how different types of pneumonia uniquely affect the gross features of the lung.
Histopathology of Pneumonia
Lobar Pneumonia
Lobar pneumonia occurs when entire lobe(s) become acutely inflamed, leading to the alveoli filling with fluid, cell debris, inflammatory cells, and fibrin (consolidation). Both grossly and under the microscope, the affected lobe(s) are clearly distinct compared to surrounding normal lobes and will be visibly discoloured, lose their spongy appearance, and will appear consolidated.
Presented by Dr. Jennifer Kong using DHPLC specimen A0201
Histologically, the alveolar walls will remain intact and thin, with stained material filling the lumen. The inflammation will affect the entire lobe(s) uniformly, with complete or nearly complete consolidation of the alveoli. As with other inflammatory processes, the blood vessels become congested. There are four main stages of lobar pneumonia that can be identified depending on what types of materials or cells can be found within the alveolar fluids:
| Stage | Alveolar fluid findings |
| Congestion | Alveolar fluid with microorganisms, few RBCs/neutrophils |
| Red hepatization | Many RBCs, neutrophils and fibrin |
| Grey hepatization | RBC breakdown, fibrinopurulent exudate (pus) |
| Resolution | Clearance of exudate by macrophages, possible scar tissue formation |
Presented by Noah Stewart using a histology slide of H&E stained lung with lobar pneumonia (DHPLC e-slide: PATH 425-001) under a CC-BY-NC license
Bronchopneumonia
Bronchopneumonia occurs when the infection begins within the bronchi/bronchioles, and then descends into the nearby alveoli. Because of this, consolidation of the alveoli occurs in patches around the affected bronchi/bronchioles. This pattern may or may not be localized to a single lobe of the lung.
Presented by Noah Stewart using a histology slide of H&E stained lung with bronchopneumonia (University of Michigan virtual Slide Box 151127)
Interstitial Pneumonia
Unlike lobar or bronchopneumonia, interstitial pneumonia is characterized by the inflammation and congestion of the alveolar walls/interstitial tissue. Interstitial pneumonia can be identified by diffuse, patchy areas of inflamed interstitial tissue. The alveolar lumens will be mostly normal. Within the inflamed areas, the alveolar walls will be congested and may appear thicker than the alveolar lumens as they become filled with fluid, inflammatory cells, debris, and possibly scar tissue.
Presented by Dr. Jennifer Kong using DHPLC specimen A0204. All rights reserved
Presented by Noah Stewart using a histology slide of H&E stained lung with interstitial pneumonia (DHPLC e-slide: PATH 425-014) under a CC-BY-NC license
Outcomes of Pneumonia
A patient may present with different signs and symptoms, based on the type of pneumonia affecting them. Lobar pneumonia and bronchopneumonia normally present as typical pneumonia. This includes malaise, fever/chills, productive cough with purulent sputum (pus), difficulty breathing (dyspnea) or an increased rate of breathing (tachypnea). Interstitial pneumonia often presents as atypical pneumonia, which has a slow onset, malaise, fever without chills, muscle soreness, headaches, nonproductive (dry) cough.
Pneumonia has a wide range of outcomes depending on patient factors, the specific pathogen, the severity of infection, and the length of infection, among others. Severe complications, such as the formation of lung abscesses can occur in rare cases. An abscess is a cavity that contains liquefied necrotic (dead) tissue, pus, and microorganisms.
Presented by Noah Stewart using a histology slide of H&E stained lung with abscess (DHPLC e-slide: PATH 425-003) under a CC-BY-NC license
Review Questions
Normal lung histology
1. Which of the following features can be found in the terminal bronchioles? Select all that apply.
- Smooth muscle
- Glands present in the submucosa
- Cartilaginous plates
- Goblet cells
- Ciliated epithelial cells
2. Which cells can be found in the alveoli of normal lungs? Select all that apply.
- Goblet cells
- Alveolar macrophages
- Type I pneumocytes
- Type III pneumocytes
- Ciliated epithelium
- Type II pneumocytes
- Chondrocytes
3. Describe the microscopic appearance of the normal lung under a) low magnification and b) high magnification, at different levels within the airway.
4. Fill in the blanks.
The thin walls that separate the vascular _____ and the air-filled _____ is made up of Type _____ pneumocytes and Type II pneumocytes which are responsible for making _____. This allows for the exchange of gases and known as the blood-air barrier.
Pneumonia histopathology
- The clinical symptoms seen in lobar pneumonia are primarily due to thickening of the alveolar walls, which impairs the exchange of gas across the blood-air barrier.
- Bronchopneumonia and interstitial pneumonia appear very similar under the microscope, except bronchopneumonia involves consolidation of alveoli around the bronchi whereas the entire lobe(s) of lung is uniformly affected in interstitial pneumonia.
- In bronchopneumonia, the infection occurs first within the larger airways and then spreads to adjacent alveoli.
- Interstitial pneumonia occurs uniformly throughout an affected lobe of lung.
2. Fill in the provided table to compare and contrast lobar, broncho-, and interstitial pneumonias according to their clinical symptoms, unique histological features, and unique gross features.
| Pneumonia | Clinical Symptoms | Unique Histological Features | Unique Gross Features |
| Lobar pneumonia |
|
||
| Bronchopneumonia |
|
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| Interstitial Pneumonia |
|
Answer Key
Normal lung histology
- Ciliated epithelial cells
- Alveolar macrophages, Type I pneumocytes, Type II pneumocytes
- a) At low magnification, the lung architecture appears spongy, with plenty of white space as the majority of the lung will be made up of the alveoli. You can also identify larger airways (bronchi) with their outer cartilaginous plates, smaller airways (bronchioles) and larger blood vessels. Veins and arteries can be distinguished primarily by the thickness of the layer of muscle surrounding the endothelium, however because the arteries of the lung are under lower pressure than other areas in the body, this difference can be less obvious.
b) At high magnification, bronchi are distinguishable by the ciliated columnar epithelium with interspersed goblet cells, larger size, with glands present in the submucosa and cartilage present around the outer edge of the structure. Smooth muscle is also present below the epithelium, which allows for contraction of the bronchi. The submucosal glands will appear cuboidal (box-like) and in circular arrangements surrounding a common duct into which they will empty their secretory products (which can be seen in their cytoplasm). Under H&E staining, cartilage will appear bluish compared to the surrounding tissue, with chondrocytes present in “holes” (lacunae) within the cartilage. Moving down the airway, bronchioles can be identified by having a similar organization to the bronchi, but lacking cartilage and submucosal glands. They will generally be thinner, and will still be surrounded by a layer of smooth muscle. Depending on the depth within the bronchiole, the epithelium may appear less columnar (more cuboidal) and will have a higher proportion of goblet cells present. Continuing down the airway, the alveoli lose the ciliated epithelium, and instead contain thin septae with an epithelium consisting predominantly of type I pneumocytes (thin, spindly cells with elongated nuclei). Nuclei stain black/dark blue using H&E staining. Within the septae in high proportions are capillaries, made up typically of a single endothelial cell (blood vessel cell) wrapped around itself to create a thin wall within which blood can flow. The thin capillary wall and alveolar epithelium allow for easy gas exchange within the lungs. Type II pneumocytes appear as cuboidal cells also within the alveolar epithelium, but take up only ~5% of the space. They are responsible for secreting surfactant, and thus appear foamy due to the products present in their cytoplasm. Lastly, alveolar macrophages can be identified by their large, round or ovoid appearance with a single acentric (off-centred) nucleus, and commonly appear with a dark/brown pigment due to update of various debris in the lungs, giving them the name “dust cells”. - Capillaries/pulmonary capillaries, alveoli, I/1, surfactant
Pneumonia histopathology
- In bronchopneumonia, the infection occurs first within the larger airways and then spreads to adjacent alveoli.
- See table below.
| Pneumonia | Clinical Symptoms | Unique Histological Features | Unique Gross Features |
| Lobar pneumonia | Typical pneumonia: Malaise, fever/chills, productive cough with purulent sputum (pus), difficulty breathing/increased rate of breathing (dyspnea, tachypnea). |
Involves one or more lobes uniformly. Appearance may change depending on stage. Alveoli become consolidated (filled with fluid, cell debris, inflammatory cells, fibrin). Blood vessels become more prominent due to inflammation (dilate and increase in volume). Inflammation takes place within the lumen of the alveoli. Minimal damage to interstitium; alveoli are mostly intact and remain thin. | One (or more) lobes will appear affected while other lobes may appear normal. The affected lung will appear consolidated, larger and more dense due to exudate filling the alveoli. The lobe may appear similar to liver tissue (intermediate lobar pneumonia). Depending on the stage, the colour of the affected lobe(s) may change. For example, at one stage (gray hepatization), much of the blood in the tissue will have been broken down, giving the affected lobes a gray appearance. |
| Bronchopneumonia | Typical pneumonia: Malaise, fever/chills, productive cough with purulent sputum (pus), difficulty breathing/increased rate of breathing (dyspnea, tachypnea)
|
Accumulation of fluid, inflammatory cells, cell debris, fibrin in the bronchi and/or bronchioles in inflamed patches around the lung. May appear very similar to lobar pneumonia, and commonly progresses to lobar pneumonia | The lung will exhibit diffuse consolidation. It may spread to multiple lobes (unevenly), and appear in patchy areas where the lung tissue is obviously affected (difference in colour, white/black versus the normal healthy red/brown lung colour). NB black spots around the lung are found in normal lung (anthrocotic pigments) due to everyday pollution, and not the result of pneumonia.
|
| Interstitial Pneumonia | Atypical pneumonia: Slow onset, malaise, fever without chills, muscle soreness, headaches, nonproductive (dry) cough | Unlike other inflammatory patterns, there is no consolidation (exudate buildup) of alveoli. Instead, interstitial pneumonia is marked by the diffuse thickening of interstitial tissue (alveolar septae) by inflammation (inflammatory cells, fluid, debris) and scar tissue. Alveolar walls will appear very thick in some parts of the lung, but others will appear relatively normal within the same lobe. | Unique appearance on chest X-ray. In interstitial pneumonia, affected areas may appear as fine white nodules (e.g. on the millimeter scale) where there are areas of acute inflammation. Visible airways (bronchi) in affected areas will not appear empty and open as they normally would due to thickening of the interstitial tissue. |
Section Summary
- Lobar pneumonia presents as a typical pneumonia and affects one or more lobes of the lung uniformly. Inflammation makes blood vessels more prominent while consolidation contributes to hepatization of the affected lobe(s).
- Bronchopneumonia presents as a typical pneumonia, originating in the larger airways and radiating to nearby alveoli. Consolidation is patchy and may be diffuse throughout the lung, but may progress to the more uniformly distributed lobar pneumonia.
- Interstitial pneumonia presents as an atypical pneumonia. Consolidation is absent but interstitial tissue may become inflammed and thicken and/or scar.
