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histology of lungs

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lung histology microscopy alveoli bronchioles

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lung alveolar cells type I type II pneumocytes histology

A high-resolution light microscopy image of lung parenchyma stained with hematoxylin and eosin. The tissue section exposes multiple pulmonary alveoli arranged in thin-walled saccules separated by delicate interalveolar septa. Alveolar spaces appear largely air-filled with minimal intra-alveolar material; capillary networks are present within septa, giving a fine reticular background. Type I pneumocytes line most alveolar surfaces with flat morphology, while occasional cuboidal type II pneumocytes are visible along septal borders. The alveolar septa show sparse cellularity and no conspicuous inflammatory infiltrate. There is no evidence of edema, hemorrhage, fibrosis, or hyaline membrane formation, and the overall architecture remains preserved without architectural distortion typical of fibrotic diseases. The stain highlights subtle nuclear detail in resident macrophages and endothelial cells, enabling evaluation of cellular morphology at this magnification. This image represents normal or non-pathologic pulmonary parenchyma, suitable as a reference for educational comparisons or as a control in studies of diffuse lung disease. Clinically, it serves as baseline histology against which interstitial lung diseases, edema, pneumonia, edema and alveolar proteinosis can be contrasted. Potential research use cases include training in normal alveolar histology, differential diagnosis workshops, and digital pathology algorithm development for tissue classification.

A high-resolution light microscopy image of lung parenchyma stained with hematoxylin and eosin. The tissue section exposes multiple pulmonary alveoli arranged in thin-walled saccules separated by delicate interalveolar septa. Alveolar spaces appear largely air-filled with minimal intra-alveolar material; capillary networks are present within septa, giving a fine reticular background. Type I pneumocytes line most alveolar surfaces with flat morphology, while occasional cuboidal type II pneumocytes are visible along septal borders. The alveolar septa show sparse cellularity and no conspicuous inflammatory infiltrate. There is no evidence of edema, hemorrhage, fibrosis, or hyaline membrane formation, and the overall architecture remains preserved without architectural distortion typical of fibrotic diseases. The stain highlights subtle nuclear detail in resident macrophages and endothelial cells, enabling evaluation of cellular morphology at this magnification. This image represents normal or non-pathologic pulmonary parenchyma, suitable as a reference for educational comparisons or as a control in studies of diffuse lung disease. Clinically, it serves as baseline histology against which interstitial lung diseases, edema, pneumonia, edema and alveolar proteinosis can be contrasted. Potential research use cases include training in normal alveolar histology, differential diagnosis workshops, and digital pathology algorithm development for tissue classification.

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bronchus bronchiole wall layers smooth muscle cartilage histology cross section

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I now have comprehensive material from Junqueira's Basic Histology and Schwartz's Surgery. Let me compile the full, well-illustrated answer.

Histology of the Lungs

The lung has two linked functional components: the conducting airway system (tracheobronchial tree) and the gas exchange system (alveolar spaces). Below is a systematic account from the largest airways down to the alveoli.

1. Tracheobronchial Tree (Conducting Airways)

Bronchi

The bronchial wall is organized into four layers:
LayerContents
MucosaPseudostratified ciliated columnar epithelium + goblet cells; rests on a basement membrane
Lamina propriaLoose connective tissue; contains a distinct smooth muscle layer encircling the entire bronchus
SubmucosaHyaline cartilage (as irregular plates); mixed submucosal glands (mucous + serous + neuroendocrine Kulchitsky cells)
AdventitiaFibrous connective tissue with blood vessels and nerves; merges with surrounding lung tissue
The submucosal glands are mixed salivary-type structures that can give rise to mucoepidermoid and adenoid cystic carcinomas. Goblet cells (mucous cells) significantly increase after injury such as cigarette smoke exposure.
Figure 19-10 (Schwartz's Surgery): Normal bronchial mucosa showing pseudostratified ciliated columnar cells (panel A) and a Kulchitsky neuroendocrine cell indicated by arrow (panel B).
Normal bronchial pseudostratified epithelium and Kulchitsky cell

Bronchioles

Bronchioles are intralobular airways with diameters of 1 mm or less (formed after ~10 generations of branching). Key distinguishing features:
  • No cartilage, no submucosal glands
  • Epithelium transitions progressively:
    • Large bronchioles: still ciliated pseudostratified columnar
    • Smaller bronchioles: simple ciliated columnar
    • Terminal bronchioles (last conducting segment): simple cuboidal with cilia
  • Smooth muscle becomes proportionally more prominent, with prominent elastic fibers
  • Diffuse mucosal lymphoid tissue (MALT) increases as cartilage decreases
  • Club cells (formerly Clara cells) are non-ciliated secretory cells that increase toward terminal bronchioles - they produce surfactant components, enzymes, and bacteriostatic agents
Figure 17-9 (Junqueira's Histology): Bronchioles at three levels - note the folded epithelium (E), prominent smooth muscle (arrows), and surrounding fibrous connective tissue (CT) in the absence of cartilage.
Bronchiole histology showing epithelium, smooth muscle, and connective tissue

2. Respiratory Zone

Respiratory Bronchioles

Terminal bronchioles subdivide into 2-3 respiratory bronchioles, which are the transition zone. They are lined by simple cuboidal epithelium interrupted by scattered alveolar outpouchings - the first site of gas exchange. They still contain a smooth muscle layer.

Alveolar Ducts

Alveolar ducts are completely lined by the openings of alveoli - they have no wall of their own other than smooth muscle "knobs" (eosinophilic nodules visible on H&E) around each alveolar opening, and a sparse lamina propria of elastic and reticular fibers plus capillaries.

Alveolar Sacs

Clusters of alveoli opening into a common chamber at the terminal end of alveolar ducts. The lamina propria here is essentially a delicate web of elastic and reticular fibers.
Figure 17-12 (Junqueira's Histology): Low-power view of lung parenchyma showing respiratory bronchioles (RB), alveolar ducts (AD), alveolar sacs (AS), individual alveoli (A), pulmonary artery (PA), and veins (V).
Respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli - low power H&E

3. Alveoli

Each adult lung contains approximately 200 million alveoli, each ~200 µm in diameter, giving a total surface area of ~75 m².

Alveolar Wall (Interalveolar Septa)

The septa contain:
  • The richest capillary network in the body
  • Sparse connective tissue: elastic fibers (allow expansion/contraction) + reticular fibers (prevent collapse and overdistension)
  • Fibroblasts
  • Alveolar macrophages (dust cells)

Cell Types of the Alveolar Epithelium

CellProportionSurface coverageKey features
Type I pneumocyte40% of cells95% of surface areaExtremely flattened (attenuated); non-mitotic; forms the gas exchange surface; continuous (non-fenestrated)
Type II pneumocyte60% of cells~3-5% of surface areaCuboidal, bulge into lumen; contain lamellar bodies (surfactant precursors); produce surfactant (DPPC + SP-A, SP-B, SP-C, SP-D); regenerate after injury
Alveolar macrophagesVariableLuminal/septalDerived from monocytes; phagocytose surfactant, particulate matter, erythrocytes; contain carbon/hemosiderin; "dust cells"
Figure 17-13 (Junqueira's Histology): Diagram of alveolar structure showing Type I cells, Type II cells, alveolar macrophages, pulmonary capillaries, alveolar pores of Kohn, and the interalveolar septum:
Alveolar structure diagram with blood-air barrier detail
H&E photomicrograph of normal lung parenchyma showing multiple thin-walled alveoli with delicate interalveolar septa and capillary networks:
Normal lung parenchyma H&E - alveoli with type I and II pneumocytes

4. Blood-Air Barrier (Respiratory Membrane)

The barrier between alveolar air and capillary blood consists of three layers:
  1. Type I alveolar cell cytoplasm (extremely thin, 0.1-0.2 µm)
  2. Fused basement membranes of the alveolar epithelium and capillary endothelium
  3. Capillary endothelial cell (continuous, non-fenestrated; thin with perinuclear organelle clustering)
Total thickness: 0.1 to 1.5 µm - designed for maximum diffusion efficiency.
Alveolar pores of Kohn (10-15 µm diameter) penetrate interalveolar septa and equalize air pressure between adjacent alveoli and allow collateral air circulation if a bronchiole is blocked.

5. Surfactant

Produced by type II pneumocytes, surfactant contains:
  • DPPC (dipalmitoylphosphatidylcholine) - the main phospholipid; reduces surface tension
  • Cholesterol
  • SP-A and SP-D - hydrophilic glycoproteins; innate immune protection
  • SP-B and SP-C - hydrophobic proteins; orient DPPC in the surfactant film
Surfactant appears in the last weeks of fetal gestation. Inadequate surfactant causes neonatal respiratory distress syndrome in premature infants. Type II pneumocytes are the primary target of SARS-CoV-2 (COVID-19) via ACE2 receptor docking, leading to reduced surfactant production and alveolar collapse.

Summary Table

StructureEpitheliumKey Feature
Bronchi (large)Pseudostratified ciliated columnar + goblet cellsCartilage plates + submucosal glands
Bronchi (small)Columnar ciliatedLess cartilage, more smooth muscle
BronchiolesCiliated columnar → simple cuboidalNo cartilage, no glands; Club cells present
Terminal bronchiolesSimple cuboidal, ciliatedLast conducting segment
Respiratory bronchiolesSimple cuboidal + alveolar outpouchingsFirst gas exchange site
Alveolar ductsAlveolar lining onlySmooth muscle "knobs" at openings
AlveoliType I + Type II pneumocytesBlood-air barrier; surfactant
Sources: Junqueira's Basic Histology, 17e; Schwartz's Principles of Surgery, 11e
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