Lung and lymphoid histology
Lung, blood vessel and lymphoid organ histology: spleen, thymus, lymph node
Histology of the airways and alveoli, arteries, veins, capillaries and lymphatics, and the lymph node, spleen and thymus for FCPS Part 1. Explains how to tell an artery from a vein, which cells line and protect the alveolus, and where B and T cells live.
Airways
As air moves deeper the wall thins: cartilage and glands disappear first, then goblet cells, then cilia, until only a thin layer is left for diffusion.
| Level | Epithelium | Cartilage | Glands | Other |
|---|---|---|---|---|
| Trachea and bronchi | Pseudostratified ciliated columnar with goblet cells | C-shaped rings (trachea), irregular plates (bronchi) | Seromucous submucosal glands | Trachealis; spiral smooth muscle in bronchi |
| Bronchioles (under 1 mm) | Ciliated columnar to cuboidal | None | None | Relatively thick smooth muscle; club (Clara) cells (secretory proteins, detoxification, stem cells) |
| Terminal bronchiole | Ciliated cuboidal, no goblet cells | None | None | Last purely conducting part |
| Respiratory bronchiole | Cuboidal, interrupted by alveoli | None | None | First gas exchange |
| Alveoli | Type I and type II pneumocytes | None | None | Alveolar macrophages, pores of Kohn |
- In chronic bronchitis, mucous gland hypertrophy raises the Reid index above 0.4; smoking also causes squamous metaplasia and loss of cilia.
- A tube about 2 mm wide with cartilage plates and glands among alveoli is a small bronchus; without cartilage it is a bronchiole.
The alveolus
| Cell | Shape | Share | Function |
|---|---|---|---|
| Type I pneumocyte | Very thin, squamous | About 40% of cells but about 95% of surface area | Gas exchange; cannot divide |
| Type II pneumocyte | Cuboidal, with lamellar bodies | About 60% of cells, 5% of area | Surfactant; stem cells that replace type I cells after injury |
| Alveolar macrophage (dust cell) | Free in alveoli and septa | From blood monocytes | Main host defence in the alveoli; ingest dust and microbes |
- Blood–air barrier: surfactant film, type I cell cytoplasm, fused basement membranes of epithelium and endothelium, capillary endothelium (about 0.5 micrometres). The fused basement membrane is what separates epithelium from endothelium in the thin part of the septum.
- In chronic congestion (mitral stenosis), alveoli contain haemosiderin-laden heart failure cells. Pink fluid without cells in the alveoli is pulmonary oedema (transudate).
Blood vessels
| Vessel | Wall |
|---|---|
| Elastic artery (aorta, pulmonary trunk, common carotid) | Thick media of many concentric fenestrated elastic lamellae; vasa vasorum |
| Muscular artery (radial, coronary, femoral) | Thick smooth muscle media; most prominent wavy internal elastic lamina (and external elastic lamina) |
| Arteriole | One or two smooth muscle layers; main site of peripheral resistance |
| Capillary | Endothelium, basement membrane and pericytes |
| Vein | Thin media, adventitia the thickest layer, wide irregular lumen, valves in limb veins |
- Artery versus companion vein on a slide: the artery has a thick muscular media, a clear wavy internal elastic lamina and a round lumen; the vein has a thin media, a thick adventitia and a large collapsed lumen.
| Capillary type | Structure | Sites |
|---|---|---|
| Continuous | Unbroken endothelium with tight junctions | Muscle, skin, lung, brain (blood–brain barrier) |
| Fenestrated | Pores in endothelium, continuous basement membrane | Glomerulus, gut villi, endocrine glands, choroid plexus |
| Sinusoidal (discontinuous) | Wide gaps, incomplete basement membrane | Liver, spleen, bone marrow |
- Weibel–Palade bodies: rod-shaped endothelial granules storing von Willebrand factor and P-selectin; released on activation to recruit platelets and leucocytes. Platelet alpha granules also hold vWF.
- Leucocyte exit: selectins mediate rolling; integrins binding ICAM-1 and VCAM-1 (up-regulated by TNF and IL-1) give firm adhesion; transmigration (diapedesis) between endothelial cells uses PECAM-1 (CD31).
- Marfan syndrome: defective fibrillin weakens the tunica media: fragmented elastic lamellae with pools of mucoid material (cystic medial degeneration), leading to aortic dilatation and dissection. Hypertension is the commonest predisposing cause overall.
- Berry aneurysms form where the media is deficient at arterial branch points in the circle of Willis. A capillary haemangioma is a mass of anastomosing capillaries lined by a single layer of endothelium.
- Lymphatic capillaries: thin overlapping endothelial cells acting as one-way flaps, no continuous basement membrane, and anchoring filaments tying them to surrounding tissue, so when tissue swells the vessels are pulled open and take up fluid, protein and cells. Protein-rich lymphoedema fluid drives fibrosis.
Mononuclear phagocyte system
All derive from blood monocytes (bone marrow): Kupffer cells (liver), alveolar macrophages, microglia (from early yolk sac precursors), osteoclasts, splenic and lymph node macrophages, Langerhans cells of skin (dendritic).
Lymph node
- Cortex: B-cell follicles; secondary follicles have germinal centres after antigen exposure.
- Paracortex: T cells and dendritic cells; high endothelial venules where blood lymphocytes enter the node.
- Medulla: medullary cords with plasma cells, and sinuses lined with macrophages.
- Lymph enters through afferent vessels into the subcapsular sinus and leaves through the efferent vessel at the hilum.
- DiGeorge syndrome empties the paracortex; X-linked agammaglobulinaemia gives no follicles or germinal centres.
Spleen
- White pulp: periarteriolar lymphoid sheath (PALS, T cells) around central arterioles, with B-cell follicles.
- Marginal zone: B cells and macrophages that trap blood-borne antigens, especially encapsulated bacteria.
- Red pulp: cords of Billroth and sinusoids with slit-like gaps; rigid, old or antibody-coated red cells cannot squeeze through and are eaten by macrophages.
- The spleen has afferent blood but no afferent lymphatics.
Thymus
- Lobules with a dark cortex (dense immature T cells, positive selection for MHC recognition) and pale medulla (negative selection against self; Hassall corpuscles of concentric epithelial cells).
- No lymphoid follicles and no afferent lymphatics normally; blood–thymus barrier in the cortex.
- Largest relative to body size at birth and largest in absolute weight around puberty, then involutes into fat. In a child it is large and lies in the anterior (superior) mediastinum.
- Thymoma and thymic follicular hyperplasia are associated with myasthenia gravis.
Tonsils and MALT
Palatine tonsils: stratified squamous epithelium dipping into crypts, lymphoid follicles, a capsule on the deep side. Peyer patches have M cells that pass gut antigens to underlying lymphocytes.
Remember Artery: thick media and wavy internal elastic lamina; vein: thick adventitia and valves.
Exam trap Type I cells cover most of the alveolar surface; type II cells make surfactant and regenerate type I.
Exam trap T cells live in the paracortex and PALS; B cells in follicles.
How FCPS asks about this
- Cells lining the alveolar area and making surfactant: type I line about 95%; type II secrete surfactant.
- Main alveolar host defence: alveolar macrophages (dust cells).
- Vessel with the most prominent wavy internal elastic lamina: muscular artery; veins have the thickest adventitia.
- Contents of Weibel–Palade bodies: vWF and P-selectin.
- Layer damaged in Marfan dissection: tunica media (cystic medial degeneration).
- Why lymphatic capillaries stay open in oedema: overlapping cells held by anchoring filaments.
- Cell derived from blood monocytes: Kupffer cell; Hassall corpuscles identify the thymus.
Test yourself on this topic
5 of the 40 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Which type of capillary, with large gaps between endothelial cells and a discontinuous basement membrane, is found in the liver, spleen and bone marrow?
Answer: Sinusoidal capillary
Continuous capillaries have tight junctions and an intact basement membrane: skeletal muscle, skin, lungs, and the brain (blood-brain barrier). Fenestrated capillaries have pores (often with diaphragms): renal glomeruli (no diaphragm), endocrine glands, intestinal villi, choroid plexus. Sinusoidal capillaries are wide with gaps and an incomplete basement membrane, allowing passage of proteins and cells: liver, spleen, bone marrow, some endocrine glands.
Capillary types
- Continuous: muscle, skin, brain
- Fenestrated: glomerulus, gut, endocrine
- Sinusoidal: liver, spleen, marrow
Remember Sinusoids let cells slip through.
Why the other options are wrong
- Continuous capillary Tight; muscle, brain.
- Fenestrated capillary Pores; kidney, gut, endocrine.
- Arteriovenous anastomosis Bypass channels.
- Venule Post-capillary vessel.
Which blood vessel has the most prominent wavy internal elastic lamina between intima and media?
Answer: Muscular artery
Muscular arteries such as the radial or coronary arteries have a distinct internal elastic lamina, a thick smooth-muscle media and an external elastic lamina. Elastic arteries such as the aorta have many concentric elastic lamellae instead. Veins have thin media, large lumens, an adventitia that is the thickest layer, and valves.
Vessel walls
- Muscular artery: clear internal elastic lamina
- Aorta: many elastic lamellae
- Vein: thick adventitia, valves
- Capillary: endothelium only
Remember A crisp wavy line means muscular artery.
Exam trap In veins the adventitia, not the media, is the thickest layer.
Why the other options are wrong
- Large vein Thin media; lamina indistinct.
- Capillary Endothelium and basement membrane only.
- Postcapillary venule No elastic lamina.
- Lymphatic vessel Thin wall with valves.
Which feature of lymphatic capillaries allows them to take up interstitial protein, fluid and even cells, and to stay open when tissues swell?
Answer: Overlapping cells held by anchoring filaments
Lymphatic capillaries begin blindly in the tissues and have a very thin, incomplete basement membrane. Their endothelial cells overlap loosely, forming flap valves that open inwards, and are tethered to surrounding tissue by anchoring filaments. When interstitial fluid accumulates, the tissue pulls on these filaments and holds the vessels open, allowing protein, fluid and cells to enter. This is why lymphatics are the main route for returning protein to the blood.
Lymphatic capillaries
- Blind-ended, thin wall
- Overlapping flap-valve junctions
- Anchoring filaments hold them open
- Return protein to blood
Remember Anchored flaps let protein in.
Exam trap Lymphoedema fluid is protein-rich, which drives fibrosis and makes it pit less over time.
Why the other options are wrong
- A thick continuous basement membrane Lymphatic capillaries have little or no basement membrane.
- Fenestrated endothelium with diaphragms Fenestrations are typical of glomerular and gut blood capillaries.
- Pericytes forming a contractile coat Initial lymphatics lack pericytes.
- Tight junctions sealing every cell Their junctions are loose, not sealed.
Which of these tissue cells is derived from blood monocytes?
Answer: Kupffer cell
Monocytes circulate for about a day and then enter tissues to become macrophages with local names: Kupffer cells (liver sinusoids), alveolar macrophages (dust cells), microglia (CNS), osteoclasts (bone, by fusion), histiocytes (connective tissue), and splenic and lymph node macrophages. Pancreatic islets of Langerhans are endocrine cells; the epidermal Langerhans cell is a dendritic cell. Plasma cells come from B cells; mast cells have their own marrow precursor.
Mononuclear phagocyte system
- Liver: Kupffer cells
- Lung: alveolar macrophages
- Brain: microglia
- Bone: osteoclasts
Remember Monocyte goes to tissue and changes its name.
Why the other options are wrong
- Langerhans cell of the pancreatic islet Endocrine islet cells, not immune cells.
- Mast cell Separate marrow precursor.
- Plasma cell Derived from B lymphocytes.
- Schwann cell Neural crest derived.
A 60-year-old heavy smoker has had a productive cough for at least three months in each of the last three years. A lobectomy specimen taken for a nodule shows the bronchial wall. Which of the following histological changes best explains his sputum?
Answer: Submucosal mucous gland hypertrophy
Chronic bronchitis is defined clinically by a productive cough for at least three months in two consecutive years. Its main histological feature is hypertrophy and hyperplasia of the submucosal mucous glands in the bronchi, with goblet cell hyperplasia in the surface epithelium. The Reid index (gland thickness divided by wall thickness) rises above 0.4. Squamous metaplasia and loss of cilia further impair clearance.
Chronic bronchitis
- Mucous gland hypertrophy
- Raised Reid index (> 0.4)
- Goblet cell hyperplasia
- Loss of cilia, squamous metaplasia
Remember Big glands, big sputum.
Exam trap Chronic bronchitis is a clinical diagnosis; the Reid index is a histological correlate.
Why the other options are wrong
- Loss of alveolar walls This is emphysema; it causes breathlessness rather than sputum.
- Fibrosis of alveolar septa Seen in interstitial lung disease.
- Thickened basement membrane with eosinophils Typical of asthma.
- Granulomas around bronchioles Seen in sarcoidosis or tuberculosis.
Where this sits in the CPSP syllabus
- Paper 1: General and special histology, Respiratory, cardiovascular and lymphoid histology (lung, vessels, spleen, thymus, nodes)
- Surgery & Allied, Paper 2: Histology for surgery, Respiratory, cardiovascular and lymphoid histology (lung, vessels, spleen, thymus, nodes)
- Medicine & Allied, Paper 2: Histology: liver, pancreas, spleen, kidney, brain, gut, Respiratory, cardiovascular and lymphoid histology (lung, vessels, spleen, thymus, nodes)