The secondary lamellae of the gill filaments are the sites of gas exchange in fish gills. They are mainly composed of 2 epithelial sheets joined together by pillar cells. These cells are characterized by collagen columns contained in infoldings of the cell membrane and oriented perpendicular to the epithelial sheets. The gill is the first organ to which the blood flows from the heart and within the secondary lamellae it flows through channels between the pillar cells.

The presence in the pillar cells of fine cytoplasmic filaments situated parallel to the collagen columns has now been observed in many fishes and the hypothesis has been advanced that they constitute a contractile system.

This paper describes how gill filaments were treated in a way similar to that used for other non-muscular cells in order to demonstrate in situ the presence of contractile proteins of the actomyosin type. Gill filaments were extracted in glycerol-containing solutions of low ionic strength, and then incubated with and without ATP. After incubation with ATP examination in the electron microscope showed that in the vicinity of the collagen columns, the pillar cells contain clusters of disordered thin filaments intermingled with spindle-shaped needles. This structure is characteristic of muscle actomyosin, as well as actomyosin-like proteins extracted from cells of non-muscular origin and fixed in a contracted state. It is deduced that the thin cytoplasmic filaments surrounding the collagen columns consist of an actomyosin-like con- tractile protein. On incubation of extracted gill filaments without ATP, the expected negative result was obtained, i.e. the filaments retain their orientation parallel to the columns. The function of the contractile filaments within the pillar cells is discussed in relation to the control of blood flow through the secondary lamellae and the reduction of pressure drop during the flow of blood across the gills of fish.

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