Histology · Year 1 · from Histology
Case 2: Cystic Fibrosis - Defective Chloride Transport in Exocrine Glands
Clinical Presentation
A 6-month-old male infant presents with failure to thrive, recurrent respiratory infections, and steatorrhea (fatty, foul-smelling stools). Birth history reveals meconium ileus requiring surgical intervention on day 2 of life.
Family History
- Parents are both of Northern European descent
- No known family history of cystic fibrosis
- One healthy older sibling
Physical Examination
- Weight: 5th percentile (was 25th at birth)
- Length: 10th percentile
- Respiratory: Scattered crackles and wheezes bilaterally
- Abdomen: Mild hepatomegaly
- Skin: Salty taste noted by parents when kissing the infant
Diagnostic Workup
Sweat Chloride Test:
- Result: 78 mEq/L (markedly elevated; normal <30, diagnostic >60)
Genetic Testing:
- Homozygous for F508del mutation in CFTR gene
Fecal Elastase:
- <100 mcg/g (severely reduced, indicating pancreatic insufficiency)
The Histology Question
While the diagnosis is established by sweat test and genetics, understanding the histopathology explains the clinical manifestations. Pancreatic changes on imaging (dense, fibrotic pancreas) and autopsy specimens in severe cases reveal the classic pathology.
Histopathological Features
Pancreas (autopsy/surgical specimens):
- Exocrine acini are markedly atrophic or absent
- Ducts are dilated and filled with inspissated (thick, dried) secretions
- Dense fibrosis replaces normal acinar tissue
- Islets of Langerhans (endocrine) initially preserved but may be lost over time
- This pattern explains the term "cystic fibrosis" - dilated ducts (cystic) with surrounding fibrosis
Lungs:
- Bronchi filled with thick, tenacious mucus
- Mucus gland hypertrophy in bronchial walls
- Chronic inflammation and bronchiectasis
- Goblet cell hyperplasia in bronchial epithelium
Intestine:
- Goblet cells are present but produce abnormally viscous mucus
- Meconium ileus results from inspissated intestinal contents
Understanding the Pathophysiology
Normal CFTR Function:
- CFTR = Cystic Fibrosis Transmembrane Conductance Regulator
- Functions as a chloride channel in apical membrane of epithelial cells
- Also regulates sodium absorption (inhibits ENaC)
- Normal: Chloride secretion draws water into luminal secretions, keeping them hydrated
CFTR Dysfunction:
| Organ | Normal Secretion | CF Secretion | Consequence |
|---|---|---|---|
| Pancreas | Thin, bicarbonate-rich | Thick, protein-rich plugs | Ductal obstruction, autodigestion |
| Lungs | Thin, easily cleared mucus | Thick, tenacious mucus | Impaired clearance, infection |
| Sweat glands | Chloride reabsorbed in duct | Chloride not reabsorbed | Salty sweat (diagnostic) |
| Vas deferens | Normal secretion | Obstruction/agenesis | Male infertility |
Glandular Histology Implications:
- Merocrine secretion (pancreas, salivary): Proteins are secreted but can't be properly hydrated without CFTR-mediated chloride/water secretion
- Mucous glands (airways, GI): Mucus produced but abnormally viscous, obstructing ducts
- Sweat glands (eccrine): Chloride secreted normally but can't be reabsorbed in duct, leading to salty sweat
Learning Points
- Exocrine glands depend on ion/water secretion: CFTR dysfunction affects the fluid component of glandular secretions
- Different glands, different manifestations:
- Pancreas: Ductal obstruction leads to exocrine insufficiency
- Lungs: Mucus stasis leads to infection
- Sweat glands: Elevated chloride (paradoxically, less affected functionally)
- Merocrine secretion requires proper hydration: The enzyme-rich pancreatic juice requires adequate water content to flow
- Histology shows end-stage changes: Ductal dilation, acinar atrophy, and fibrosis result from chronic obstruction
- Endocrine vs. exocrine preservation: Pancreatic islets (endocrine) survive longer than acini (exocrine) because they don't rely on ductal drainage
Comparison: Sjogren Syndrome vs. Cystic Fibrosis
| Feature | Sjogren Syndrome | Cystic Fibrosis |
|---|---|---|
| Mechanism | Autoimmune destruction | Ion channel defect |
| Target | Acinar cells | Ductal epithelium (secretion) |
| Histology | Lymphocytic infiltration | Ductal obstruction, fibrosis |
| Primary affected glands | Salivary, lacrimal | Pancreas, respiratory, sweat |
| Secretion problem | Reduced production | Normal production, abnormal viscosity |
| Age of onset | Adulthood | Childhood |
Discussion Questions
- Why does the sweat test work for diagnosing cystic fibrosis when sweat gland function is relatively preserved?
- How does understanding glandular duct structure help explain why CF causes more severe disease in some organs than others?
- Why are the islets of Langerhans relatively preserved in early CF but may be lost later (CF-related diabetes)?
Summary: Glandular Histology in Clinical Disease
Both cases demonstrate how understanding glandular structure - the organization of acini, ducts, secretory mechanisms, and supporting cells - directly explains disease pathophysiology:
- Sjogren syndrome: Immune cells target and destroy the secretory acini themselves
- Cystic fibrosis: Defective ion transport in ductal epithelium leads to inspissated secretions that obstruct and destroy glands
Recognition of normal glandular architecture is essential for interpreting pathological changes in biopsy specimens and understanding the clinical manifestations of exocrine diseases.