Histopathological examination of tissues from clinically ill juvenile angelfish (Pteropyhllum scalare scalare) revealed large numbers of rodlet cells in a variety of tissues. Numerous rod let cells were observed within the large postorbital vessel caudal to the eye and in the parenchyma of the cranial kidney, both areas which were closely associated with concurrent pathology. Smaller numbers of rodlet cells were observed without associated pathology in other tissues, such as the caudal kidney, gall bladder, gill and intestine. Free rodlet cells were also observed in the vessels of the angelfish, where they occurred either singly among the other blood elements, or in clusters of several rodlet cells associated with a variety of inflammatory cells.

Special staining of rodlet cells demonstrated the capsule of the rod let cell to be PAS negative, while the rod lets had a slightly positive PAS reaction. Ultrastructural examination of the rodlet cells revealed them to be similar to that which has already been described in the literature; an outer capsule consisting of numerous circularly-oriented, irregularly spaced layers of fibrillar-like elements, a large, eccentrically-located nucleus, and a variable number of longitudinally-oriented, club-shaped rodlets consisting of an electron-dense core surrounded by an amorphous, less-dense material.

Rollet cells have been described in both freshwater and marine teleosts; with some investigators believing them to be parasitic, while others consider them to be of endogenous origin. Comparison of healthy angelfish from six other sources suggests that the rodlet cells observed in the clinically ill fish are not part of the normal histology of healthy individuals, but rather are either acquired from an exogenous source or induced from an endogenous precursor cell. Since the only known contributory factor for the observed mortality in these fish was chronic poor water quality, these observations suggest that rodlet cells may be a stress-induced type of fish cell produced in response to sub-lethal environmental conditions.