Fig. 1. Normal developmental pathways in the gastric unit of the adult stomach. (A) The epithelium of the corpus (body) consists of repeating, invaginating units that can be further subdivided into four zones: the pit, composed of mucus-secreting pit cells; the isthmus, where the multipotent stem cell resides; the neck where the mucous neck cells and the vast majority of the acid-secreting parietal cells reside; and the base, filled with digestive-enzyme-secreting zymogenic cells. (B) Key differentiation pathways in the gastric unit are depicted, with the gastric lumen to the left. The parietal cell arises within the isthmus. Pit and neck or zymogenic cells develop along well-defined, spatiotemporally organized developmental gradients. Neck cells are thought to enter the base and then become ZCs. (C) The neck and base of a single unit are depicted (unit delimited by white dashed line) in this Toluidine Blue-stained, 1 µm plastic-embedded section, oriented as in the cartoon in B. Inset: shaded rectangle in the neck zone of the photomicrograph at left with the gastric unit lumen outlined in gray. (D) TEM showing a typical neck cell (left) and a typical basal (i.e. mature) zymogenic cell (right). Inset: cytoplasmic projection of neck cell stretching between adjacent parietal cells (one labeled PC); note the nascent network of rER (arrow), which must become the extensive lamellar network in the mature zymogenic cell.