Metaplasia

Development mechanism and role in pathology

Metaplasia belongs to pathological regeneration. Normally, reparative regeneration aims to fully restore organ function after injury (restitution) or at least replace the defect with a scar and hypertrophy of the remaining tissue (substitution). In dysregeneration, this mechanism becomes aberrant.

The primary trigger of metaplasia is prolonged chronic inflammation or injury. It alters the tissue microenvironment, which normally regulates the proliferation and correct differentiation of stem cells. As a result, cells begin to differentiate along an alternative pathway, creating a tissue that withstands current aggressive conditions better, but is incapable of performing the organ's original work.

Squamous metaplasia

This is one of the most frequent variants, in which specialized epithelium (ciliated, columnar, or transitional) is replaced by stratified squamous epithelium, sometimes with keratinization.

Examples from clinical practice:

• Respiratory system: in chronic bronchitis, COPD, or respiratory bronchiolitis, the ciliated epithelium of the bronchi transforms into stratified squamous epithelium. This is an important stage in the morphogenesis of COPD. Foci of squamous epithelium are also found in lung tissue in the late stages of idiopathic fibrosing alveolitis (usual interstitial pneumonia).
• Urinary bladder: against the background of chronic cystitis, the transitional epithelium (urothelium) transforms into non-keratinizing stratified squamous epithelium.
• Reproductive system: in gonococcal infection, the columnar epithelium of the cervical canal is replaced by squamous epithelium, often with keratinization and progression to cicatricial atrophy.
• Salivary glands: in cystic fibrosis, the gland ducts undergo progressive dilation and squamous epithelial transformation amidst fibrosis.

Metaplasia in the digestive and urinary systems

Depending on the direction of cell differentiation, several other types of the process are distinguished:

1. Glandular (columnar) — replacement of the urothelium or other lining with glandular columnar epithelium. Found, for example, in tumor growth within the urinary bladder (grade G2 transitional cell carcinoma).
2. Gastric — appearance of foveolar (gastric) epithelium in unusual locations. A classic example: the duodenal mucosa in chronic duodenitis.
3. Intestinal — appearance of intestinal-type epithelium (goblet cells, brush-border enterocytes, Paneth cells) in the mucosa of other organs.

Examples of intestinal metaplasia:

• Barrett's esophagus: a zone of acquired intestinal epithelium with villous structures and mandatory goblet cells (at least 3 cm in length) appears above the esophageal Z-line.
• Chronic atrophic gastritis: incomplete intestinal metaplasia develops in the stomach.

Intestinal metaplasia is divided into histochemical phenotypes: type I (complete), where all types of intestinal cells are present, and type II (incomplete), where a single cell type predominates (usually mucin-producing columnar cells).

Mesenchymal (connective tissue) metaplasia

In addition to epithelium, tissues of mesenchymal origin also undergo pathological transformation. The essence of the process is the formation of bone or cartilage tissue in areas where it should not be present.

Usually, this occurs in zones of chronically damaged connective tissue. For example, bone formation is possible within tendons, in myositis ossificans, or in the walls of blood vessels (foci of osteogenesis appear within calcified atherosclerotic plaques).

Class and exam questions

What else to study on this topic

• Dysplasia as an outcome of metaplasia: grades of severity and cellular atypia
• Morphogenesis of chronic obstructive pulmonary disease (COPD)
• Types of reparative regeneration: restitution and substitution
• Forms of dysregeneration: hyporegeneration and hyperregeneration
• Pathological anatomy of cystic fibrosis (changes in bronchi, liver, and pancreas)

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.