Ultrastructural reconstruction of Taenia ovis oncospheres from serial sections

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Abstract

The cellular organisation of Taenia ovis oncospheres is interpreted from ultrathin serial sections and transmission electron microscopy following high pressure freezing and freeze-substitution. The surface of a hatched, non-activated T. ovis oncosphere is covered by an oncospheral membrane below which is the tegument bearing microvilli. The basal lamina of the tegument is underlain by broad bands of peripheral somatic musculature. Three pairs of hooks and associated muscles are present in the somatophoric third of the oncosphere. Approximately 19 cells of seven different types were identified which include: (i) a quadri-nucleated syncytium of penetration gland type 1 containing two lateral pairs of cell bodies interconnected by narrow cytoplasmic bridges (PG1); (ii) a quadri-nucleated syncytium of penetration gland type 2 (PG2); (iii) a single-nucleated median mesophoric gland cell; (iv) 10 somatic cells; (v) two germinative cells; (vi) two nerve cells; and (vii) a pair of median somatophoric cells. This study provides a clear understanding of the morphology of T. ovis oncospheres and forms the basis for further investigations into the biology of taeniid oncospheres.

Introduction

Taenia ovis is a taeniid cestode parasite infecting sheep and goats as intermediate hosts and canids as definitive hosts. Taenia ovis has played a prominent role in the study of immunity to cestode infections (Gemmell, 1962, Gemmell, 1964), particularly following its use in the development of the first recombinant anti-parasite vaccine (Johnson et al., 1989, Lightowlers, 2006). The host-protective antigens of T. ovis are uniquely associated with the oncosphere, which is the infective life cycle stage present within the parasite’s eggs (Lightowlers, 2006). The ultrastructural location of the host-protective antigens is unknown and the available information on the structure of the T. ovis oncospheres is insufficient to allow accurate interpretation of the antigens’ localisation (Jabbar et al., 2010). The studies reported here sought to improve our understanding of the biology of T. ovis oncospheres by providing a detailed description of the parasite’s ultrastructure.

A number of studies have been undertaken which describe the structure of taeniid (Platyhelminthes: Cestoda: Taeniidae) eggs and/or oncospheres using light microscopic methods, including Taenia pisiformis (Janicki, 1907, Heath and Smyth, 1970); Taenia saginata (Skvortzov, 1942, Silverman, 1954), Taenia taeniaeformis (Wardle and McLeod, 1952, Heath and Smyth, 1970), T. ovis (Heath and Smyth, 1970), Taenia hydatigena (Heath and Smyth, 1970) and Echinococcus granulosus (Heath and Smyth, 1970, Heath and Lawrence, 1976). Following the advent of electron microscopy, further investigations were undertaken on oncosphere structure in taeniid cestodes including Taenia crassiceps (Chew, 1983), Taenia multiceps (Race et al., 1966), T. ovis (Harris et al., 1987), Taenia parva (Świderski et al., 2007), T. saginata (Schramlova and Blazek, 1982, Schramlova et al., 1984), T. taeniaeformis (Nieland, 1968, Engelkirk and Williams, 1982, Engelkirk and Williams, 1983) and E. granulosus (Świderski, 1983, Harris et al., 1989, Holcman et al., 1994). The covering of the taeniid egg after it leaves the proglottid, the embryophore, has also been the subject of interest in a number of investigations (Inatomi, 1962, Morseth, 1965, Race et al., 1966). All of these studies describe the structure of oncosphere of the respective taeniid species based on sections taken at random, with the exception of Świderski (1983) who described the structure of E. granulosus oncospheres based on ultrathin serial sections dealing mainly with the hook–muscle system. The cellular composition of oncospheres in other cestodes has also been studied including Hymenolepis citelli (Collin, 1969), Catenotaenia pusilla (Świderski, 1972), Nematotaenia dispar (Świderski and Tkach, 1997), Staphylocystoides stefanskii (Świderski and Tkach, 1999), Inermicapsifer madagascariensis (Świderski and Tkach, 2002), Bothriocephalus clavibothrium (Swiderski and Mackiewicz, 2004) and Mosgovoyia ctenoides (Młocicki et al., 2006) based on ultrathin serial sectioning.

The only published report on the ultrastructure of T. ovis oncospheres is that of Harris et al. (1987). These authors investigated activated hexacanths (terminology adopted after Conn and Świderski, 2008) and metacestodes cultured in vitro, and observed that there were four lobes and four nuclei in the penetration glands and that each of these lobes was filled with granules of varying electron densities. The study utilised sections taken at random rather than serial sections. Two reports describe the ultrastructure of activated hexacanths of E. granulosus and metacestodes cultured in vitro (Harris et al., 1989, Holcman et al., 1994), again based on examination of random sections. Although random sections allow general features of oncospheres to be described, reconstructions based on serial ultrathin sections are required to obtain comprehensive ultrastructural details about the relationship between various structures and the number of different cell types. Here we describe a comprehensive study using ultrathin serial sections and reconstruction of hatched, non-activated oncospheres of T. ovis.

Section snippets

Collection of parasite material

Taenia ovis was maintained in dogs and sheep as described by Coman and Rickard (1975) and Kyngdon et al. (2006). All of the procedures were performed in accordance with the requirements of the University of Melbourne Animal Ethics Committee.

Collection and hatching of oncospheres

Mature eggs were collected from the terminal gravid proglottids of the adult worm which was obtained by purging an experimentally infected dog. Non-activated oncospheres were collected following hatching of the mature eggs as described by Lightowlers et al.

Cellular composition of the oncospheres

The general topography and symmetry of cellular organisation of the hatched non-activated oncospheres of T. ovis is revealed by the serial sections shown in Fig. 1. Taenia ovis oncospheres consist of approximately 19 cells and their arrangement was bilaterally symmetrical in a plane running in an anterio-posterior direction and through the dorsal and ventral poles. Based on the analysis of ultrathin serial sections of 15 T. ovis oncospheres, seven different cell types were distinguished: (i) a

Discussion

A reconstruction of a T. ovis oncosphere is summarised and shown in Fig. 8. Many features seen in T. ovis were consistent with the ultrastructure of other cestode oncospheres, but a number of characteristics of T. ovis oncospheres were identified which clearly differed from previous descriptions of oncosphere structures. A single cell not previously described was found in T. ovis and was designated a median mesophoric gland cell. In addition, the characteristics of a previously described

Acknowledgement

Funding from National Health and Medical Research Council, Australia (Grants 350279, 400109 and 628320) is acknowledged.

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