Scanning electron microscopic study on the saccus vasculosus of the rainbow trout

Both free and cracked surfaces of the saccus vasculosus in adult rainbow trout (Salmo gairdneri) have been investigated by scanning electron microscopy (SEM). The saccus vasculosus is the rich-folded epithelium composed of highly specialized coronet cells, supporting cells, and presumable liquor-contact neurons. The coronet cells are characterized by both numerous specialized cilia, so-called "globules" projecting into the saccus lumen and abundant smooth endoplasmic reticulum in the cytoplasm. By the appearance of these globules, coronet cells are roughly divided into two types: botryoidal coronet cells and flower-like. The former is supplied with grape-shaped globules, whose density is 60-80 pieces a cell. The latter contains club- or petal-shaped globules, which number 50-60 pieces a cell. The functional significance of these globules is additionally discussed. The free surface of the supporting cells is relatively smooth, and short microvilli lie scattered on the surface. The cells called liquor-contact neurons (the third type) possess a head-like protrusion where long solitary cilia can be recognized.