Fine Structure of the Outer Epidermis of the Viviparous Monogenean Gyrodactylus sp. from the Skin of Gasterosteus aculeatus

Abstract

The fine structure of the covering layer of Gyrodactylus sp., an unusual monogenean with a peculiar kind of viviparous development, was studied for comparison with the outer layer of more conventional monogeneans which have a ciliated oncomiracidium larva. The outer layer of Gyrodactylus was found to be a syncytial epidermis like that of other monogeneans, digeneans, and cestodes, but unlike these other parasitic platyhelminths the superficial epidermis of Gyrodactylus does not appear to connect with nucleated regions (inner tegument) situated in the parenchyma. As full-term but unborn embryo worms have a nucleated superficial epidermis it seems likely that the adult epidermis either contains very few nuclei which would be difficult to observe, or that nuclei once present may have degenerated. The adult epidermis contains few ribosomes but Golgi may be present. The nucleated epidermis of embryo Gyrodactylus is packed with ribosomes; and vesicles which may contain mucoor glycoproteins are already present in the epidermis of late embryos. Lamellate inclusions consisting of membrane stacks or whorls have been found in the epidermis of both embryos and adults. These could be membrane storage depots although the possibility of their being fixation artifacts cannot be completely eliminated. Work with ferritin suggests that the outer layer is extremely impermeable to this colloid although in the gut ferritin is actively pinocytosed. The covering layer of the marine ectoparasitic monogeneans, Entobdella soleae and Acanthocotyle elegans, has recently been shown to be a syncytial cytoplasmic layer which connects with parenchymally situated epidermal cell bodies (Lyons, 1970) and is very like the outer layer of the endoparasitic digeneans and cestodes. A survey of the representatives of various monogenean groups (Lyons, 1968) suggests that this kind of outer layer occurs throughout monogeneans. Gyrodactylus is an unusual monogenean because it reproduces in a specialized way. The free-swimming larval stage found in all other groups of monogeneans is absent. The worms are viviparous and reproduce by a peculiar form of polyembryony such that one fertilized ovum eventually gives rise to four individuals which develop within one another and within the adult worm at different rates (Bychowsky, 1957; Dogiel, 1964; Kathariner, 1904). Work on the fine structure of the outer layer of Gyrodactylus suggested that this was unlike that found in other groups of monogeneans and that the differences could be related to the specialized kind of reproduction found in this group. MATERIALS AND METHODS Specimens of Gyrodactylus sp. (as yet unnamed and undescribed) were obtained from the skin and Received for publication 3 March 1970. fins of three-spined sticklebacks from Emmanuel College pond, Cambridge. Worms fixed and flattened in 10% formalin for 2 days were stained with alcian blue for acid mucopolysaccharides, the periodic acid-Schiff method for mucopolysaccharides and carbohydrates (Pearse, 1961), and Mayer's mucicarmine for mucus (Gomori, 1952). Whole worms were fixed for 1 to 1.5 hr in 2.5% glutaraldehyde in cacodylate buffer at pH 7.2 and a total molarity of 0.1 M. Material was postfixed at 4 C for 1 hr in 1% osmium tetroxide (Rosenbluth, 1965). Following rapid dehydration in cold ethanol, material was embedded in Araldite (Richardson, Jarrett, and Finke, 1960). Sections cut with glass knives on a Cambridge Instrument Co. ultramicrotome were mounted on uncoated copper grids and stained with a saturated solution of uranyl acetate in 50% ethanol, or for low-power work, with 30 to 50% uranyl acetate in absolute methanol for 3 min followed by Reynolds' lead citrate. The sections were viewed on AEI EM 6B and Philips EM 200 electron microscopes. In order to compare the possible ability of the epidermis and gut respectively to transport ferritin, specimens of Gyrodactylus were incubated in a 2% ferritin solution in tap water for 0.5 hr before being prepared for electron microscopy.