Light micrograph of the fundic region of the stomach of R. icterica. Note the mucosa, submucosa, muscular, and serosa layers. Gastric glands occupy the mucosa layer, being formed by mucous neck cells, predominantly at the upper portion, and oxynticopeptic cells, preferentially located at the lower portion. The muscular layer of smooth muscle is subdivided into longitudinal internal and circular external layers. HE staining.