Ultrastructural alterations in branchial chloride cells of Atlantic salmon, Salmo salar, during parr-smolt transformation and early development in sea water

The ultrastructure of the gill primary lamellae of juvenile Atlantic salmon was examined during the parr-smolt transformation and for 42 days after smolts were exposed to sea water. Scanning electron microscopy indicated that primary lamellae were characterized by rough convoluted surfaces that became rougher throughout the experimental period and that crypts did begin to form in freshwater fish. Crypt formation increased in sea water.
Transmission electron microscopy indicated that parr preadapt for life in sea water in part by changes in chloride cells. Chloride cells show an elaboration of rough endoplasmic reticulum in fresh water and a decline of rough endoplasmic reticulum after 42 days of sea water exposure. The tubular membrane system becomes well developed in fresh water, and apical vesicles become abundant only after seawater exposure. Mitochondria are both spherical and elongate through the period and contain well developed cristae. No evidence of mitochondrial rupture was observed. The junctions between chloride cells and adjacent cells were characterized in fresh water by long tight junctions with desmosomes. This type of junction continued in sea water and was the norm between chloride cells and accessory cells after 42 days of seawater exposure. While leaky junctions appeared to be forming, no evidence was found of membrane interdigitation between accessory cells and chloride cells after 42 days of seawater exposure. It also appeared that seawater exposure influenced the number of chloride cells exposed to the external milieu.
Pavement cells showed an elaboration in fresh water of free ribosomes and rough endoplasmic reticulum and these elements became less prominent after seawater exposure.