Well-, moderately, and poorly differentiated adenocarcinomas (□ Fig. 10) are classified according to their degree of differentiation. Well-differentiated carcinomas grow at a faster rate than poorly differentiated ones [127].

Papillary adenocarcinoma. This is adenocarcinoma of tumor complexes in the form of fingerlike projections. The tumors are characterized by various degrees of cell and nucleus polymorphism; they can contain well-differentiated tubular structures, and in this case they are called papillotubular. Mac-roscopically, papillary adenocarcinoma is characterized mostly by exophytic growth.

Tubular adenocarcinoma. This is adenocarcino-ma consisting of branching tubules. Their diame-

□ Fig. 10. Papillo-tubular adenocarcinoma: adenocarcinoma of tumor complexes in the form of tubules and fingerlike projections with a fibrovascular base. Hematoxylin and eosin, x400

ters vary within a wide range, e.g., owing to the accumulation of mucus in their lumen.

Mucous adenocarcinoma. The term mucous ad-enocarcinoma is synonymous with colloid, muci-nous, and muconodular (use of the latter should be avoided). This adenocarcinoma involves the glands actively producing extracellular mucus. Two types of this adenocarcinoma are distinguished: one involving glands secreting mucus into the stomach lumen or stroma, and the other involving scattered or grouping signet-ring cells floating in the »lakes« of mucus.

Signet-ring cell carcinoma. This is adenocarcinoma, the dominant component of which is single or grouped tumor cells containing intracytoplasmic mucus. Characteristic of the latter cells are (a) the displacement of their nucleus towards the periphery and (b) the formation of fingerlike shapes. Four types of signet-ring cells are differentiated. The first type have the classical fingerlike shapes with optically light cytoplasm (D Fig. 11a) containing acid in-tracytoplasmic mucin (pH 2.5) which is stained violet with alcian blue. The second type is characterized by a less eccentrically located nucleus with eosinophilic granular cytoplasm (D Fig. 11b), presented by cytoplasmic granules containing neutral PAS-positive diastase-resistant mucin, which is stained pink. The third type is goblet-like cells, with cytoplasm containing granules of acid or neutral mucin. The nucleus of these cells is displaced towards the periphery, and in most cases it is rounded (D Fig. 11c).

D Fig. 11. b The second type is characterized by a less eccentrically located nucleus with eosinophilic granular cytoplasm. Hematoxylin and eosin, x400.
D Fig. 11. c The third type presents as goblet-like cells, in which the rounded nucleus is displaced towards the periphery. Hematoxylin and eosin, x100; d The fourth type presents as mucus-free cells. Hematoxylin and eosin, x400

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