This study identified the cellular compositions of the gills in molly fish and their role
in immunity using light-, electron- microscopy, and immunohistochemistry. The molly fish gills
consisted of four holobranchs spaced between five branchial slits. Each hemibranch carried many
fine primary and secondary gill lamellae. The gill arch was a curved cartilaginous structure, from
which radiated the bony supports of the primary lamellae. The gill arch contained the afferent and
efferent brachial arteries. The gill arch was covered by epidermal tissue rich with mucous cells. The
primary lamella had a central cartilaginous support and efferent and afferent arterioles and was
covered with pavement cells (PVC), salt-secreting chloride cells, and pale-staining mucous cells.
These chloride cells contained abundant mitochondria and tubulovesicular system and are involved
in ionic transport with a potential role in detoxification. The surface of the secondary lamellae (site of
gaseous exchange) consisted of overlapping or interdigitating PVC supported and separated by pillar
cells. Other cells were found within the gill epithelium and interstitial connective tissues, including
lymphocytes, macrophages, monocytes, telocytes, stem cells, astrocytes, and neuroepithelial cells.
The immunohistochemical analysis revealed that APG-5, iNOS-2, IL-1, NF-B, and TGF-B showed
positive immunoreactivity in macrophages. The epithelium of the primary gill lamellae contained
positive-GFAP astrocytes and S100 protein—chloride cells. The stem cells expressed SOX9, myostatin,
and Nrf2. Neuroendocrine cells expressed S100 protein. In conclusion, the current work suggests
that the gills of molly fish are multifunctional organs and are involved in immune reactions.