Parasitol Res (1987) 73:472-474
Parasitology
gesearcn : o.
9 Springer-Verlag 1987
Short communications
Exocytosis of dense granules of cyst merozoites (cystozoites)
of Sarcocystis cuniculi (Protozoa, Apicomplexa) in cell cultures
B. Jantzen and R. Entzeroth
Zoologisches Institut der Universit/it Bonn, Poppelsdorfer Schlog, D-5300 Bonn 1, Federal Republic of Germany
Host-parasite interactions play a considerable part
in the development of parasitic protozoa. Numerous studies analyse the invasion mechanism of coccidia. The invasion of the host cell, for example,
was studied in Eimeria larimensis (Roberts et al.
1971), Plasmodium knowlesi (Aikawa et al. 1978)
and Sarcocystis muris (Entzeroth 1985). Three
types of characteristic organelles appear to be involved in host-parasite interactions during and
after invasion: dense granules, rhoptries and micronemes (Dubremetz and Dissous 1980; Entzeroth et al. 1986).
Sarcocystis cuniculi, an intracellular parasite
with the cat (Fells catus) as final host and the rabbit (Oryctolagus coniculus) as intermediate host
was used for tissue culture experiments. Monolayers of feline embryonic fibroblasts were cultured
then inoculated with a purified suspension of
S. cuniculi cyst merozoites. The parasites were obtained by treating rabbit muscle cysts with trypsin.
Then 35 min after inoculation, the cultured cells
were fixed in 2.5% glutaraldehyde in sodium cacodylate buffer, pH 7.4, embedded in Araldite, sectioned and examined by a Zeiss EM 9 $2 electron
microscope. On light micrographs of cyst merozoites before inoculation, one of the three structures which presumably play an important part in
host-parasite interactions, the dense granules, can
be seen (Fig. 1). On electron micrographs of intracellular cyst merozoites (gamonts), these memReprint requests to: R. Entzeroth
brane-bound, electron-dense globules were sometimes seen to discharge their contents by exocytosis
(Fig. 2). Similar observations were made in merozoites of Plasmodium knowlesi (Bannister et al.
1977) and Sarcocystis tenella (Dubremetz and Porchet 1978). In S. muris, the dense material is shed
by exocytosis into the lumen of the developing secondary parasitophorous vacuole (pv) (Entzeroth
1984, 1985). Theses pv's could also be found in
Sarcocystis cuniculi (Fig. 5). Dense granules emptying their contents by exocytosis were not only
seen in intracellular cyst-merozoites (gamonts) of
S. cuniculi but also in extracellular cyst-merozoites
(Figs. 3 and 4). At higher magnification, the membrane of the dense granule was continuous with
the parasite plasmalemma (Fig. 4). The inner membrane complex formed a thickened cylindrical
structure that surrounded the exocytotic pore (arrows). Thus the ultrastructure of a dense granule
during exocytosis resembles an active micropore
(Scholtyseck and Mehlhorn/970). An unusual micropore was described by Speer et al. (/985) in extracellular, second-generation merozoites of Eimeria tenella. These structures were induced by immune complexes and were active in endocytosing
extracellular immune complexes.
The dense granule contents in S. cuniculi seem
to be structured differently to those of Sarcocystis
muris (Entzeroth 1984, 1985). While the exocytosed dense material of S. muris is particulate, the
material of S. cuniculi is compact and is shed in
cord-like strings.
Fig. 1. Light micrograph of Sarcocystis cuniculi cyst merozoites (cm) after mechanical destruction of a muscle cyst, with numerous
dense granules (dg). x 1350
Fig. 2. Tangential section of cyst merozoite (gamont) with micronemes (mn), dense granules (dg) and a partially empty rhoptrie
(rh) inside a host cell (he). The intracellular parasite shows a dense granule emptying its contents by exocytosis (arrow). x 27 500
Fig. 3. Section through an extracellular cyst merozoite with dense granules (dg), micronemes (mn), mitochondrium (mi), nucleus
(n) and a dense granule during the process of exocytosis (arrow). x 15 300
B. Jantzen and R. Entzeroth: Exocytosis of dense granules in cell cultures
473
Fig. 4. Higher magnification of the dense granule from Fig. 3, emptying its dense material (dm) into the extracellular space (ex).
The micrograph shows the plasmalemma (p/) and inner membrane complex (im) of the parasite's pellicle. The arrow points
out the thickened cylindrical structure surrounding the exocytosis porus. • 52500
Fig. 5. Cyst merozoite of S. cuniculi inside an embryonic feline fibroblast cell, 35 min post inoculation. The section shows dense
material (din) which originates from dense granules inside a secondary parasitophorous vacuole (pv). x 20000
474
B. Jantzen and R. Entzeroth: Exocytosis of dense granules in cell cultures
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Accepted March 3, 1987