A mode of incipient growth in chemically induced signet ring cell carcinoma of the canine stomach. 
A 31-month-old female mongrel dog was orally administered with 50 mg or 100 mg of N-nitrosobutylurea (NBU) in gelatin-capsule 3 times per week for 19 months with interposing periods of complete suspension.  Thirty-four foci of signet ring cell carcinoma were found in the antral region of the stomach.  The majority of the foci (31 foci) were early cancer, and the remaining foci were invasive cancer.  In addition to these lesions, there was "a single gland cancer" in which a row of cancer cells was confined to a single gland.  The whole gland was composed of two cell layers; the inner layer facing the lumen was normal gastric cells and the outer layer was atypical or neoplastic cells underlaid by the basement membrane.  Mitosis was frequently observed on the bottom of the gland.  Atypical or neoplastic cells seemed to mature gradually through a process of upward migration with increase in cytoplasmic Alcian blue-PAS and HID-AB positive mucin.  Some of the cells rich in mucin moved into the lamina propria.  The other cells remained in the flow of the regular cell renewal system of the normal gastric cells and reached the top of the gland.  This observation revealed a mode of incipient gastric cancer growth, which starts and spreads within a single gland, before it invades the surrounding lamina propria.