Development of the endoplasmic reticulum in the rodlet cell of two teleost species.

The rodlet cell found in different tissues and the blood of teleosts is distinguished by a thick capsule and bipartite rodlets, each consisting of club-like sac and a dense protein core. The development of its rough endoplasmic reticulum (RER) was ultrastructurally investigated in gill and intestinal epithelium of trout (Salmo trutta L., Oncorhynchus kisutch). The RER showed signs of hypertrophy beginning already in immature cells. The typical vesicular appearance noted in the mature cell as well as the apical labyrinth, an accumulation of dislodged mitochondria and vesicles, derives from RER dilatations shedding their ribosomes and pervading the cytoplasm. These dilatations extend also into extracapsular protrusions formerly supposed to be nutritive. A possible role of RER in the formation of the rodlets is indicated by the close association of RER-derived vesicles with rodlet sacs. Conspicuous undulations of the membranes of the sacs as well as the occurrence of tubules (ø = 30 nm) in the dilated RER, rodlet sacs, and along the core support the proposed hypothesis of hypertrophy. Both features signal the storage of (membrane) protein during altered conditions ranging from slowed metabolism to viral infections. The differentiation of apical microvillar projections replaced later by cytoplasmic blebbing and disruption is compatible with surplus production resulting in discharge by pressure. Together with the occasional presence of junctional complexes, these observations argue for an aberrant cell differentiation due to an unknown cause, representing an alternative to the current interpretation of the rodlet cell as a migrating secretory or leucocytic cell with a defensive function.