Macroscopic and Microscopic Classification
Malignant tumors of the esophagus exhibit a variety of growth patterns. Gross examination identifies three main forms: exophytic-polypoid carcinoma (growing into the lumen of the organ), flat ulcerated carcinoma (forming an ulcer defect), and infiltrative carcinoma (growing through the thickness of the wall).
At the microscopic level, two major forms dominate:
- Squamous cell carcinoma. This is the primary and most common form. Based on the degree of cellular differentiation, it is classified as well-differentiated, moderately differentiated, poorly differentiated, and undifferentiated. Histological variants include verrucous, spindle cell, and basaloid squamous cell carcinoma. Its favored localization is the middle and lower thirds of the esophagus, whereas it rarely arises in the upper third. Hematoxylin and eosin staining reveals microscopic features demonstrating invasive complexes.
- Adenocarcinoma. Microscopically, this form presents as tubular-papillary carcinoma. Unlike squamous cell carcinoma, adenocarcinoma is strictly localized to the lower third of the esophagus and arises within the zone of Barrett esophagus.
Important feature: Intra-organ spread of metastases is extremely typical for both forms of esophageal carcinoma.
Etiology and Morphological Progression
The main triggering factor for the development of adenocarcinoma is chronic exposure to gastroesophageal reflux. The reflux of gastric contents leads to continuous chemical damage to the mucosal lining. The primary aggressive agents are bile acids, low pH (acidic environment), the enzyme pepsin, and nitric oxide (NO).
Under the influence of these factors, the normal stratified squamous epithelium undergoes sequential morphological transformation. This process is divided into six distinct stages:
- Normal stratified epithelium.
- Esophagitis (mucosal inflammation).
- Barrett esophagus (replacement of squamous epithelium).
- Low-grade dysplasia.
- High-grade dysplasia (severe atypia).
- Barrett adenocarcinoma (invasive tumor).
Molecular and Genetic Alterations
Morphological progression is accompanied by profound molecular-level alterations. These disruptions are strictly linked to the stages of disease development.
Changes in Cell Function: Already at the Barrett metaplasia stage, cells acquire resistance to apoptosis (programmed cell death), and this property persists thereafter. Later, at the high-grade dysplasia stage, hyperproliferation joins the process.
Mutations and Loss of Allelic Heterozygosity:
- The earliest event is chromosomal instability, which is detected as early as the Barrett metaplasia stage.
- During the transition to low-grade dysplasia, mutations appear in the DCC and CDKN2 genes. This stage is also characterized by APC gene mutations.
- A late event is the p53 gene mutation, which is associated with high-grade dysplasia and direct transition to carcinoma.
Epimutations and Gene Overexpression: Epigenetic alterations of the CDKN2 gene are recorded at the low-grade dysplasia stage. Concurrently, an increase in activity (overexpression) of various genes occurs:
- Markers of early changes (NOS 2, COX 2, Src, Bcl-2) begin to be actively expressed during Barrett metaplasia.
- At the low-grade dysplasia stage, the expression of fibroblast growth factor and the enzyme telomerase increases.
- A marker of late progression is the overexpression of Rab11, characteristic of high-grade dysplasia.
Pathomorphological Diagnostics
Diagnosis of esophageal carcinoma is based on the study of gross specimens and micropreparations.
When examining the macroscopic specimen of a resected (removed) esophagus, the pathologist visually evaluates the tumor node. In the case of adenocarcinoma, its association with the background disease—Barrett esophagus—is clearly traceable.
For microscopic examination, tissue is stained using the standard hematoxylin and eosin method. Under the microscope, the physician verifies the histological picture: the presence of invasive complexes in squamous cell carcinoma or the formation of tubular-papillary structures against the background of metaplastic epithelium in adenocarcinoma.