The pseudobranch is a gill-like structure that exhibits great variations in structure and function
among fish species, and therefore, it has remained a topic of investigation for a long time. This
study was conducted on adult Molly fish (Poecilia sphenops) to investigate the potential functions of
their pseudobranch using histological, histochemical, immunohistochemical analysis, and scanning
electron microscopy. The pseudobranch of Molly fish was of embedded type. It comprised many rows
of parallel lamellae that were fused completely throughout their length by a thin connective tissue.
These lamellae consisted of a central blood capillary, surrounded by large secretory pseudobranch
cells (PSCs). Immunohistochemical analysis revealed the expression of PSCs for CD3, CD45,
iNOS-2, and NF-κB, confirming their role in immunity. Furthermore, T-lymphocytes-positive CD3,
leucocytes-positive CD45, and dendritic cells-positive CD-8 and macrophage- positive APG-5 could
be distinguished. Moreover, myogenin and TGF-β-positive PSCs were identified, in addition to nests
of stem cells- positive SOX-9 were detected. Melanocytes, telocytes, and GFAP-positive astrocytes
were also demonstrated. Scanning electron microscopy revealed that the PSCs were covered by
microridges, which may increase the surface area for ionic exchange. In conclusion, pseudobranch
is a highly specialized structure that may be involved in immune response, ion transport, acid–base
balance, as well as cell proliferation and regeneration.