Mast cell progenitors are present in mesenteric blood vessels 48 hours after IP injection of distilled water . (A) In animals not injected with distilled water, mast cells (arrows) can be seen along a blood vessel (V) and in the mesenteric windows. (B) The mast cells (arrows) are positive when immunolabeled with mAb AA4-FITC (arrows). (C) No mast cells are seen in the mesentery or along a mesenteric blood vessel (V) 48 hours after distilled water injection. Toluidine blue. (D) MCp (arrows), detected by mAb AA4-FITC, are associated with and inside blood vessels (arrows). (E) Superimposition of MCp in a blood vessel immunolabeled with mAb AA4-FITC (arrows) seen in F with corresponding DIC image. Other blood cells, which do not label with mAbAA4, are also present inside the blood vessel. (F) At this magnification, MCp labelled with mAb AA4-FITC are clearly seen inside a mesenteric blood vessel.