The intestinal bulb is a simple dilatation in the anterior part of the intestine of agastric fish. This study was conducted on 18 adult specimens
of molly fish (Poecilia sphenops) and demonstrated the presence of an intestinal bulb. The intestinal epithelium was composed of enterocytes
covered with microvilli, many mucous goblet cells, and enteroendocrine cells. Numerous intraepithelial lymphocytes, neutrophils,
plasma cells, dendritic cells, stem cells, rodlet cells, and macrophages were identified in the epithelial layer. Interestingly, this study recorded
the process of autophagy and formation of autophagosomes, multivesicular bodies, and dense bodies. The intestinal epithelium extended
into the intestinal gland that consisted of simple columnar epithelium, mucous cells, stem cells, enteroendocrine cells, and basal cells. These
glands opened to the lumen of the bulb and were surrounded by a network of telocytes. Moreover, immunohistochemistry revealed that the
intestinal epithelium expressed APG5, myostatin, TGF-β, IL-1β, NF-κB, Nrf2, and SOX9. Leukocytes in the lamina propria-submucosa
expressed APG5. The inflammatory cells in the connective tissue showed strong immunoreactivity to myostatin and TGF-β. The smooth
muscular layer also expressed myostatin. Both IL-1β and NF-κB showed immunoreactivity in macrophages in the lamina propria-submucosa.
Stem cells expressed Sox-9 and telocytes expressed NF-κB and SOX9; while astrocytes in the tunica muscularis expressed GFAP. The
high frequency of immune cells in the intestinal bulb suggested an immune role of this organ. This is the first study demonstrating the
absence of the stomach and its replacement with an intestinal bulb in molly fish, and consequently, this species could be reclassified as
agastric fish according to this study.