Types:
   I  G-cell hyperplasia: Associated with increased number of G -cells in the antrum,hypergastrinemia and ulcerogenic syndrome resembling Zollinger Ellison syndrome.
   II ECL cell hyperplasia:

Types of gastric carcinoids:
    I  Tumour composed of G-cells (gastrinoma)
   II  Tumour cells are composed of enterochromaffin-like (ECL) cells in the oxyntic  mucosa. 

Hypergastrinemia plays an important role in the pathogenesis of gastric carcinoids.

MEN type 1 gene locus may also be involved in the pathogenesis.

G-CELL HYPERPLASIA AND G-CELL CARCINOIDS:

- G-cell hyperplasia (increased number of G-cells in the middle and lower third of antral glands)
- Progress to form micronodular G-cell clusters.
- Finally form  G-cell carcinoids (Gastrinoma) .
- These are usually solitary lesions.
- Located in the gastric antrum.
- G-cell hyperplasia should be distinguished from G-cell carcinoids.
- Gastrinomas are commonly present in the pancreas and duodenum.

HYPERPLASIA OF ENTEROCHROMAFFIN - LIKE (ECL) CELLS AND CARCINOIDS COMPOSED OF ECL CELLS.

- There is corpus ECL cell hyperplasia following antral G- cell hyperplasia. Gastrin has a stimulatory action on ECL cells.
-  ECL cell are scattered singly or in clusters.(upto 3 cells / glands).
-  Linear hyperplasia along glandular basement
membrane.
-  Followed by formation of solid micronodular endocrine cell nests ( microcarcinoids ). These are
100 - 150 mm in diameter.
-  Micronodules enlarge show cytological atypia  and break basement membrane.
-  Finally these nodules form overt carcinoid tumour. These have a lobular or trabecular pattern and are more than 0.5 mm in diameter. 

FEATURES OF  ECL CELL CARCINOIDS:

- Most common type of gastric carcinoid.
- These are usually multiple lesions.
- Distributed  throughout  the fundus.
- Non- argentaffin but strongly argyrophilic.
- ECL cell carcinoid  in chronic corpus gastritis (autoimmune) are usually small (less than 1cm in diameter)  and rarely metastasize.
- Virtually never lethal.
- Never produce clinical hypersecretion syndrome.
- May regress spontaneously or following antrectomy.
- Treated by local excision or endoscopic polypectomy
- Gastrectomy is indicated in rare cases where tumours are larger (more than 2cm) and numerous or small tumours with angioinvasion.

FEATURES OF ECL CELL CARCINOIDS:

- Most common type of gastric carcinoid.
- These are usually multiple lesions.
- Distributed  throughout  the fundus.
- Non- argentaffin but strongly argyrophilic.
- ECL cell carcinoid  in chronic corpus gastritis (autoimmune) are usually small (less than 1cm in diameter)  and rarely metastasize.
- Virtually never lethal.
- Never produce clinical hypersecretion syndrome.
- May regress spontaneously or following antrectomy.
- Treated by local excision or endoscopic polypectomy
- Gastrectomy is indicated in rare cases where tumours are larger (more than 2cm) and numerous  or small tumours with angioinvasion.

Gross features of gastric carcinoids:

- Small polypoid, well circumscribed raised lesion covered by mucosa.
- Larger lesions infiltrate through the full  thickness of the stomach wall.
- Cut surface has a yellow-gray appearance.
 
Microscopic features:

-Microglandular or trabecular growth pattern.
-Composed of small regular cells with finely granular cytoplasm.
-Clear cell  variant has been reported.
-Round or oval nuclei.
-Minimal pleomorphism.
-Mitoses are scanty. Necrosis is usually absent.
-Retraction artifact noted around nests of cells.