Novel Identification and Microscopy of the Intestinal Bulb of Molly Fish (Poecilia sphenops) with a Focus on Its Role in Immunity

Abstract

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.

Structural and immunohistochemical characterization of pancreas of Molly fish (Poecilia sphenops), with a special reference to its immune role

“…Dendritic cells were found in a close association to the exocrine pancreatic acini of the Molly fish. These cells were previously reported in the spleen and intestinal bulb of Molly (Mokhtar, Hussein, & Sayed, 2022; Sayed et al, 2022). Functional dendritic cells have been identified in various tissues including kidney, spleen and gills of some teleosts such as zebrafish, rainbow trout, grass carp, and catfish (Mokhtar, 2021).…”

“…They are interstitial cells having a relation with various cellular types of cells and were identified in various tissues and organs of humans, animals, birds, and fish, performing a wide variety of biological functions (Huang et al, 2021; Hussein & Mokhtar, 2018). These cells were previously detected in various tissues of Molly fish including intestinal bulb (Mokhtar, Hussein, & Sayed, 2022) and liver (Hussein et al, 2023). Moreover, these cells play a vital role in the in the regeneration process of some organs (Cretoiu & Popescu, 2014).…”

“…In addition, the pancreatic acinar cell of intrahepatic pancreas and disseminated pancreas showed immunoreactivity to IL‐1β, NF‐κB, and Nrf2. Our previous studies reported the expression of these proteins in the intestinal bulb, spleen, liver, and ependymal cells of the Molly fish (Hussein et al, 2023, Mokhtar, Hussein, & Sayed, 2022, Mokhtar, Sayed, et al, 2022, Sayed et al, 2022). IL‐1β performs a variety of physiological functions, and fish retain its ability to use IL‐1β to regulate inflammatory process (Secombes et al, 2011), as it has a pivotal role in the local and systemic responses initiation to different stimuli through T and B lymphocytes, natural killer cells, and activating macrophages (Dinarello, 2011).…”

“…Among the striking findings in this study is the expression of SOX9 in the pancreatic acinar cell of intrahepatic pancreas and disseminated pancreas. We previously reported expression of the SOX9 expression in different tissues of Molly fish (Hussein et al, 2023, Mokhtar, Hussein, & Sayed, 2022, Mokhtar, Sayed, et al, 2022, Sayed et al, 2022). SOX9 is a member of the SOX family, which play an important role in the regulatory mechanisms of cell proliferation and cell fate during embryogenesis (Bastide et al, 2007).…”

“…Interestingly, strong immunoreactivity to APG5 and TGF‐β was detected in the pancreatic acinar cells of both hepatopancreas and disseminated pancreas of Molly fish. APG5 and TGF‐β expressions were reported in different tissues of Molly fish (Hussein et al, 2023; Mokhtar, Hussein, & Sayed, 2022; Mokhtar, Sayed, et al, 2022; Sayed et al, 2022). APG5 is one of the principal players in the autophagy process, where it plays a fundamental role in formation of autophagic vesicles, development and proliferation of lymphocyte, differentiation of adipocyte, and apoptosis (Pierdominici et al, 2012).…”