Membrane Transformations in Degenerating Rodlet Cells in Fishes of Two Teleostean Families (Salmonidae, Cyprinidae)

Abstract

Rodlet cells (RCs) of teleosts are identified by their fibrillar capsule and peculiar inclusions, the rodlets, consisting of a club-like sac and a central dense core. Former ultrastructural studies showing signs of hypertrophy of endoplasmic reticulum (ER) were followed up in Salmonids (Oncorhynchus mykiss, Salmo trutta L.) and compared with Cyprinids (Cyprinus carpio L., Carassius auratus L., Alburnus alburnus). Focusing on membrane transformations, unusual undulations of the membranes of rodlet sacs and often apposed ER-membranes, which were observed in mature or discharging cells, increased continuously in degenerating stages and ejected cytoplasmic packages or rodlets. Tubular elements (ø 25-30 nm or 30-50 nm) or small vesicles appeared partly derived from them. Terminal stages of this development were represented by RCs retained in the epithelium, which were completely filled by stacks of tubules and cores. Convoluted membranes were also found persisting between mostly undissolved rodlets at the epithelial surfaces. In Cyprinid species, the membrane changes were less conspicuous but essentially similar, including stages with confluent ER reported only in trout up to now. The membrane transformations resemble structures known as "crystalloid ER" indicating a disturbance in the protein production. The positive immunocytochemical reaction for calreticulin in the rodlet sacs, a luminal ER chaperone mediating recycling of misfolded proteins and upregulated during stress, supports this interpretation. The ER stress-reaction is an evolutionary conservative cytoprotective mechanism during physiological, environmental, and genetic aberrations and fits the increase of RCs reported in quite different situations, although details of its triggering need further investigation.