Twenty-four adult molly fish (Poecilia sphenops, Valenciennes 1846) were collected to study
the morphology and distribution of ganglia using histological, immunohistochemical, and electron
microscopy and focusing on their relation to the immune cells. The ganglia were classified spatially
into cranial and spinal, and functionally into sensory and autonomic. Spinal ganglia (dorsal root
ganglia, DRG) contained large close ganglionic cells, enclosed by satellite cells, as well as bundles of
both myelinated and non-myelinated nerve fibers. There are glial cells, immune cells and telocytes
close to the ganglion. In addition, oligodendrocytes were closely related to myelinated axons. Glial
fibrillary acidic protein (GFAP) expression was confined to the glia cells and the nerve fibers in
the cervical ganglia next to the gills, and surprisingly, in the large ganglionic cells of the DRG. The
vestibular ganglia were large, connected to the hind brain, and contained numerous neurons packed
in columns. The cervical ganglia were large and observed around the pseudobranch, head kidney,
and thymus. Their neurons are randomly distributed, and nerve fibers are peripherally situated.
CD3-positive T-lymphocytes, dendritic cells, and CD68-positive macrophages were in close contact
with the ganglia. Furthermore, the ganglia around the head kidney showed positive Iba1-expressing
cells. Most ganglion cells and nerve fibers in the DRG, autonomic, and vestibular ganglia showed
moderate to strong S-100 immunoreactivity. The enteric glia, CD68-expressing macrophages, and
acetylcholine (Ach)-expressing neurons were observed along the muscular layer of the intestinal wall.
In conclusion, different ganglia of molly fish displayed direct communication with immune cells
which support and maintain healthy ganglionic cells.