Characterization of morphology and physiological actions of scale osteoclasts in the rainbow trout

Abstract

The morphology of scale osteoclasts in rainbow trout Oncorhynchus mykiss was characterized by light and scanning electron microscopy, and the effects of oestradiol‐17β‐treatment and sexual maturation on scale osteoclast morphology were investigated. The cells associated with resorption cavities could be distinguished morphologically as two types: symmetrical, compact cells lacking or having only a few cell processes, termed type 1 cells, and asymmetrical cells covered with folds and having several cell processes, termed type 2 cells. In adult sexually maturing fish, where scale resorption was high, type 1 cells were predominant. In juveniles and spawned adults where scale resorption was assumed to be relatively low, mostly type 2 cells were present. Oestradiol 17‐β‐treatment of juvenile rainbow trout increased the osteoclast activity, but did not affect the osteoclast morphology. Using light microscopy, the majority of the cells observed in, and closely associated with, the resorption cavities were mononucleated in both maturing and spawned fish. Occasionally, bi‐ and multinucleated osteoclasts were observed in the maturing, but not in the spawned fish. Light microscopic enzyme‐histochemistry showed that the majority of the mononucleated cells, as well as the bi‐ and multinucleated ones, were tartrate resistant acid phosphatase positive in both groups of fish, thus implying that both type 1 and type 2 cells were osteoclasts. It is thus apparent that scale resorption in rainbow trout is carried out by two morphologically distinct osteoclast populations, representing different stages of osteoclast activity and/or different stages of osteoclast differentiation.