Cell Differentiation and Morphologic Heterogeneity of Gastric Carcinoma : Insights from the Immunocytochemical and Molecular Study of Trefoil Peptides and E-Cadherin = Diferenciação Celular e Heterogeneidade Morfológica do Carcinoma Gástrico : Estudo Imunocitoquímico e Molecular de "Trefoil Peptides" e da Caderina-E