Density and maturation of rodlet cells in brain tissue of fathead minnows (Pimephales promelas) exposed to trematode cercariae

doi:10.1016/j.ijpara.2009.08.013

International Journal for Parasitology

Volume 40, Issue 3, 1 March 2010, Pages 307-312

https://doi.org/10.1016/j.ijpara.2009.08.013 Get rights and content

Abstract

Evidence for the presumed linkage between the enigmatic rodlet cells of fish and exposure to helminths is anecdotal and indirect. We evaluated the proliferation and development of rodlet cells in the optic lobes of fathead minnows exposed to cercariae of Ornithodiplostomum ptychocheilus. Mean rodlet cell densities (ca. 10/mm²) in the optic lobes were similar between unexposed controls and minnows with 1- and 2-week old infections. Rodlet cell densities increased at 4 weeks p.i., reaching maxima (ca. 200/mm²) at 6 weeks p.i., followed by a decline at 9 weeks. This temporal pattern of proliferation and maturation paralleled the development of metacercariae within the optic lobes. Unencysted metacercariae develop rapidly within tissues of the optic lobes for approximately 4 weeks after penetration by cercariae, then shift to the adjacent meninges to encyst. The former stage is associated with tissue damage, the latter with massive inflammation of the meninges. Thus, peak densities and maturation of rodlet cells correspond to the period when inflammation of the meninges caused by the large metacercarial cysts is at a maximum. Our results support recent contentions that rodlet cells comprise part of the host inflammatory defence response.

Graphical abstract

Introduction

The presence of distinctive pear-shaped cells, known as ‘rodlet cells’, within the epithelia of tissues of most species of teleost fish, has been known for almost 120 years. Their widespread distribution within and across most species of freshwater and marine fish is well established, as is their characteristic ultrastructure, although their function continues to be hotly debated (reviews by Manera and Dezfuli, 2004, Reite and Evenson, 2006). While definitive studies by cell and molecular biologists are lacking, the current view is that rodlet cells are a component of a generalised host response to a variety of external stressors, especially parasitic infection (Leino, 1996, Iger and Abraham, 1997; Dezfuli et al., 2008). Evidence for the linkage between rodlet cells and infection comes from observational studies that consistently report an association between rodlet cell proliferation and the presence of a variety of types of parasites (Reite and Evenson, 2006). Thus, high densities of rodlet cells tend to occur in close proximity to attached and/or feeding myxozoans, trematodes, cestodes, nematodes and acanthocephalans of fishes (Leino, 1996, Reite, 1997; Dezfuli et al., 2007a, Dezfuli et al., 2008).

Direct evidence for a causal linkage between rodlet cell densities and the presence or number of parasites is generally lacking, especially for interactions involving helminths. Most simply, comparisons of rodlet cell development and maturation in the tissues between exposed versus unexposed fish have not been completed. Nor do we understand the temporal pattern of rodlet cell proliferation and development following initial exposure to helminth infective stages. Lastly, the view that rodlet cells are associated with host defence against parasites (Reite and Evenson, 2006, Mazon et al., 2007), with parasite-induced tissue damage a likely cue for recruitment and proliferation (Leino, 1996, Reite, 2005), is based also on observational studies. These limitations are due to the lack of appropriate model systems that are amenable to experimental manipulation.

Metacercariae of Ornithodiplostomum ptychocheilus encyst in the brain of fathead minnows, Pimephales promelas. Experimental studies have indicated a complex bi-phasic pattern of metacercarial development following exposure of fish to cercariae (Sandland and Goater, 2000, Goater et al., 2005, Conn et al., 2008). Migrating diplostomules navigate to the outer layer of the tissues of the optic lobes within 24 h of exposure, after which they enter a 4-week period of rapid growth and development. During this phase, the tegument is enveloped by elaborate microvilli that extend outwards into adjacent host tissue (Goater et al., 2005). This phase is associated with transient tissue damage and impaired visual performance (Shirakashi and Goater, 2005). At approximately 4 weeks, metacercariae enter a characteristic encystment and resting phase (Goater et al., 2005). Altered visual performance was absent during this phase, but encystment is associated with inflammation of the meninges that leads to cranial distortion (Sandland and Goater, 2001). Over the course of these studies, rodlet cells were frequently observed in close proximity to metacercariae at various stages of development (Goater et al., 2005), similar to the descriptions of rodlet cells in the brains of European minnows infected with metacercariae of Diplostomum phoxini (Dezfuli et al., 2007b).

The purpose of this study was to evaluate temporal changes in the proliferation and maturation of rodlet cells in the optic lobes of minnows exposed once to cercariae of O. ptychocheilus. Our ability to experimentally manipulate this host–parasite system provides an opportunity to test alternative hypotheses regarding the functional link between rodlet cell development and proliferation, and helminth infection. According to the ‘anti-parasite hypothesis’, rodlet cell secretions primarily function to reduce the development and survival of parasites (Leino, 1996). If this is so, peak rodlet cell densities should occur at approximately 1–2 weeks after infection, corresponding to the period of maximum damage to adjacent host tissue. If rodlet cells play a role in the inflammatory defence response of fish (‘inflammation hypothesis’; Manera and Dezfuli, 2004, Reite and Evenson, 2006), peak densities should be present at approximately 4 weeks after infection when metacercariae have entered the encystment phase and are causing severe inflammation of the adjacent meninges.

Section snippets

Infection procedure

The methods used to infect fathead minnows with O. ptychocheilus cercariae followed Sandland and Goater (2000). Trematode eggs were collected from the faeces of day-old chicks that were fed the brains of field collected infected minnows, and processed through a series of washes and filters. The F1 generation of field collected snails, Physa gyrina, reared in the laboratory in de-chlorinated water and fed boiled lettuce ad libitum, was exposed to the hatched miracidia in June 2007. Cercariae

Temporal changes in rodlet cell density

Rodlet cells were present in the optic lobes and in the adjacent meninges at 1 week p.i., coinciding with the period when diplostomules occupied the outermost layer of the optic lobes (Fig. 1A). At 6 weeks p.i., the fully encysted metacercariae had shifted location from the tissues of the optic lobes to the adjacent meninges (Fig. 1B). Our qualitative observations indicated that rodlet cell densities were higher at 6 weeks p.i. than at 1 week p.i. and more cells occurred within brain tissue.

Discussion

The 30-fold increase in rodlet cell density between uninfected and infected minnows at 6 weeks p.i. provides direct evidence that rodlet cell proliferation results from exposure to cercariae. The positive correlation between the total area occupied by metacercariae in the brain and rodlet cell density provides an additional line of evidence for an association between the presence of metacercariae and the presence of rodlet cells. Thus, a single exposure to trematode cercariae ultimately leads to

Acknowledgements

Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada to C.P.G. (Discovery Grants program) and C.E.M. (Postgraduate Scholarships program).

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Density and maturation of rodlet cells in brain tissue of fathead minnows (Pimephales promelas) exposed to trematode cercariae

Abstract

Graphical abstract

Introduction

Section snippets

Infection procedure

Temporal changes in rodlet cell density

Discussion

Acknowledgements

Fish Shellfish Immunol.

Fish Shellfish Immunol.

Int. Rev. Cytol.

Tissue Cell.

Fish Shellfish Immunol.

Fish Shellfish Immunol.

Fish Shellfish Immunol.

Fish Shellfish Immunol.

Rodlet cells in teleosts: new ultrastructural observation on the distribution of cores in trout (Oncorhynchus mykiss, Salmo trutta L.)

J. Submicrosc. Cytol. Pathol.

Parasitology meets ecology on its own terms: Margolis et al. revisited

J. Parasitol.

Developmental and functional ultrastructure of Ornithodiplostomum ptychocheilus diplostomula (Trematoda: Strigeoidea) during invasion of the brain of the fish intermediate host, Pimephales promelas

J. Parasitol.

An ultrastructural study of the enigmatic “rodlet cells” in the white sucker, Catostomus commersoni (Lacepède) (Pisces: Catastomidae)

Can. J. Zool.