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Development of the endoplasmic reticulum in the rodlet cell of two teleost species.

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Development of the Endoplasmic
Reticulum in the Rodlet Cell of Two
Teleost Species
Center of Anatomy and Cell Biology, Institute for Histology and Embryology,
Medical University of Vienna, Vienna, Austria
The rodlet cell found in different tissues and the blood of teleosts is
distinguished by a thick capsule and bipartite rodlets, each consisting of
club-like sac and a dense protein core. The development of its rough endoplasmic reticulum (RER) was ultrastructurally investigated in gill and
intestinal epithelium of trout (Salmo trutta L., Oncorhynchus kisutch). The
RER showed signs of hypertrophy beginning already in immature cells. The
typical vesicular appearance noted in the mature cell as well as the apical
labyrinth, an accumulation of dislodged mitochondria and vesicles, derives
from RER dilatations shedding their ribosomes and pervading the cytoplasm. These dilatations extend also into extracapsular protrusions formerly supposed to be nutritive. A possible role of RER in the formation of
the rodlets is indicated by the close association of RER-derived vesicles with
rodlet sacs. Conspicuous undulations of the membranes of the sacs as well
as the occurrence of tubules (ø ⫽ 30 nm) in the dilated RER, rodlet sacs, and
along the core support the proposed hypothesis of hypertrophy. Both features signal the storage of (membrane) protein during altered conditions
ranging from slowed metabolism to viral infections. The differentiation of
apical microvillar projections replaced later by cytoplasmic blebbing and
disruption is compatible with surplus production resulting in discharge by
pressure. Together with the occasional presence of junctional complexes,
these observations argue for an aberrant cell differentiation due to an
unknown cause, representing an alternative to the current interpretation of
the rodlet cell as a migrating secretory or leucocytic cell with a defensive
function. © 2005 Wiley-Liss, Inc.
Key words: rodlet cell; endoplasmic reticulum; ultrastructure;
trout; Salmo trutta L.; Oncorhynchus kisutch
The rodlet cell is a baffling cell type found in many
teleostean families. Characterized by a thick fibrous capsule, a basal nucleus, and conspicuous inclusions, the
rodlets which are club-shaped and consist of a basal rodlet
sac and a dense central core. Over 100 years ago, the
rodlet cell was first discovered and considered as a parasitic stage due to its superficial resemblance to Sporozoa,
but shortly afterward interpreted as an endogenous and
secretory cell. This dispute has still not been resolved to
date (Leino, 1974, 1979, 1996; Desser and Lester, 1975;
Barber et al., 1979; Grünberg and Hager, 1979; Viehberger and Bielek, 1982; Bielek and Viehberger, 1983;
Barber and Mills Westermann, 1986a, b; Smith et al.,
1995; Dezfuli et al., 1998, 2000, 2003a, 2003b; Fishelson
and Becker, 1999; Bielek, 2002; Manera and Dezfuli,
2004). The rodlet cells are preferentially found in epithelia, including endothelia, but also in the blood, hemopoietic organs, and connective tissue. Their number and tissue specificity seem to be varying both among species and
*Correspondence to: Edith Bielek, Center of Anatomy and Cell
Biology, Institute for Histology and Embryology, Medical University
of Vienna, Schwarzspanierstrasse 17, A-1090 Vienna, Austria. Fax:
0043-01-4277-9613. E-mail:
Received 29 July 2004; Accepted 5 November 2004
DOI 10.1002/ar.a.20162
Published online 26 January 2005 in Wiley InterScience
among individuals (Mattey et al., 1979; Sulimanović et al.,
1996; Reite, 1997; Fishelson and Becker, 1999). Various
interpretations of their possible function range from a
sensory or endocrine cell (Wilson and Westerman, 1967) to
a migrating regulatory element responsible for osmoregulation, pH, or some defensive secretion (Leino, 1974,
1979, 1996). As the only secretory cells known to migrate
into the blood are leucocytes, the rodlet cell was consequently considered as blood cell, probably a granulocyte or
an inflammatory cell derived from a circulating stem cell
(Duthie, 1939; Catton, 1951; Smith et al., 1995; El-Habback et al., 1997; Dezfuli et al., 1998, 2000, 2003a, 2003b,
2004; Manera et al., 2001). The secretion of antibiotic
substances was proposed by Leino (1974, 1996), although
the number of rodlet cells is not decreasing after the end of
an infection as expected (Leino, 1996; Koponen and Myers,
2000). Moreover, some inconsistencies remain due to the
fact that rodlet cells are absent under pathological conditions (Fishelson and Becker, 1999) or, by contrast, present
in apparently sound fishes (Mattey et al., 1979; Smith et
al., 1995).
Some other open questions pertain for several morphologic features, which are difficult to fit into the general
scheme of a blood cell with defensive function, e.g., the
thick capsule that has been interpreted as protective
structure (Iger and Abraham, 1997) or as contractile
mechanism for the discharge of the rodlets (Leino, 1974,
1979; Barber et al., 1979), the latter opinion frequently
taken over by other authors. Besides different opinions on
the occurrence of cell junctions and the mode of discharge
and possible solubility of the rodlets, additional open questions remain with respect to the secretory cycle. Most
authors assume that the development of rodlets follows
the classical secretory pathway, i.e., increase of RER and
Golgi apparatus and the appearance of secretory vacuoles
at the trans face of the Golgi stacks, which develop into
the rodlets (Leino, 1974, 1979; Desser and Lester, 1975;
Flood et al., 1975; Barber et al., 1979). A possible direct
involvement of the RER in rodlet formation was only mentioned by Desser and Lester (1975), Flood et al. (1975),
and, more recently, Kramer and Potter (2002). The last
described the rodlets arising within the dilated cisternae
of the endoplasmic reticulum (ER) without discussing this
conflicting evidence. For this reason, further ultrastructural examination with the aim of clearing up some of
these discrepancies was performed on rodlet cells in gill
Fig. 1. Rodlet cell stages. Light microscopic survey of intestinal
epithelium containing rodlet cells with typical inclusions and thick capsule. Part of the cells show basal vacuoles at the outside of the capsule.
Narrow dense rodlet cells represent degenerating compressed forms
(arrowheads), while migrating leucocytes (mostly lymphocytes) are seen
at the base of the epithelium and in a damaged area (X). Semithin section
(0.5 ␮m). Toluidine blue staining.
Fig. 2. Rodlet cell stages. Transmission electron microscopical
(TEM) survey of intestinal epithelium with different rodlet cell stages: (A)
immature cell without capsule, identified by its dilated endoplasmic
reticulum; (b) immature cell with capsule; (C) rodlets apparently ejected
into normal epithelial cells; and (D) dense rodlet cells probably degenerating. The cell to the left displays only a large condensing vacuole and
an external vacuolar projection at its side, while the cell to the upper right
appears compressed.
Fig. 3. Rodlet cell stages. Immature rodlet cell from the base of the
intestinal epithelium displaying smooth dilatations of rough endoplasmic
reticulum, which extend through the developing capsular zone (boxed
and intestinal epithelium of trout (Salmo trutta L. and
Oncorhynchus kisutch). The study focused on the development of the endoplasmic reticulum and showed evidence
for signs of hypertrophy at all stages supporting the concept of a transformed cell rather than a normal secretory
In the course of diverse ultrastructural studies, adult
specimen of rainbow trout (Oncorhynchus mykiss) and
brown trout (Salmo trutta L.) were obtained either from
commercial suppliers or caught wild in different regions of
lower Austria. The fishes showed no pathological signs but
minor infections (e.g., protozoan parasites could not entirely be ruled out). Three specimen of wild Salmo trutta
L. (measuring 26, 29, and 50 cm) contained numerous
rodlet cells in gill and intestine. In eight adult specimen of
Oncorhynchus mykiss (30 –35 cm; three of them wild
catches) and two immature cultured ones (8 and 11 cm),
only one wild fish was rich in rodlet cells, while the probes
of the others contained only few or none. Small blocs (1–2
mm3) from different organs including intestine and gills
were dissected out and processed after standard electron
microscopic methods, i.e., fixation in 2% glutaraldehyde
buffered in phosphate buffer or cacodylate buffer (0.1 M,
pH 7.2–7.4, at 4°C) for 1–2 hr, postfixation in 1% OsO4 in
the same buffer for 1 hr, followed by dehydration in a
graded ethanol series and embedding in epoxy resin (Epon
812 or Glycidether). Semi- and ultrathin sections (0.5 ␮m
and ⬃ 70 nm, respectively) were cut on a Reichert ultramicrotome (OM U3, Ultracut S), stained with Toluidine
blue or uranyl acetate/lead citrate, respectively, and examined on a Jeol 1200 EXII electron microscope (for LM
survey: Leitz DMRB). Samples of gill and intestine from
fishes especially rich in rodlet cells (e.g., in focal accumulations) were selected for the ultrastructural analysis. No
essential differences between the rodlet cells of rainbow or
brown trout were observed.
In the comparatively high intestinal epithelium, the
different developmental stages could be easily observed
(Figs. 1 and 2). Immature ovoid stages without or with
only an indistinct capsule were identified by their dilated
cisternae of rough endoplasmic reticulum (RER). Mature
area). The apically located rodlet sacs (s) show varying condensation
and the central dense core. Note prominent nucleolus in the uncondensed nucleus and desmosomes (arrowhead). bm, basal membrane; f,
capsular zone; L, migratory lymphocyte; s, rodlet sac. a: Detail. The
cisternae of the rough endoplasmic reticulum are irregularly distended
and project into neighboring cisternae, mitochondria (arrowhead), and
through the developing basal capsule (f), thereby losing their ribosomes.
At the right border contact with a lymphocyte (L) and a desmosome-like
junction with an adjacent cell process.
Fig. 4. Rodlet cell stages. Apex (to the left) of an immature rodlet cell
slightly more developed than that in Figure 3, exhibiting numerous cores,
dilated ER partly invaginating the aggregated mitochondria (arrowheads), and three desmosomal junctions with neighboring cells. The first
traces of the capsule appear as marginal zone free of organelles (f). a:
Similar stage with RER dilatations shedding their ribosomes (asterisk)
and invaginating into mitochondria (m; arrowhead). f, capsular zone; s,
rodlet sac.
Figures 1– 4.
elongated cells with the conspicuous capsule were packed
with rodlets and situated near or in contact with the
epithelial surface (Fig. 1). Smaller or narrower dense cells
represented probably degenerating stages and often had a
compressed appearance (compare Figs. 1, 2, and 19). Vesicular protrusions at the cell base were large enough to be
discernible by light microscopy (Fig. 1). Ejected intact
rodlets (Figs. 14, 16, gill, and 17) or whole rodlet cells (not
shown) were found near the surface, although occasionally
rodlet sacs protruded from mature cells situated in the
middle zone of the epithelium into the neighboring intestinal cells (Fig. 2). In the gill, similar stages occurred but
were more scattered. Their axis was usually oriented not
perpendicularly but obliquely due to the lower height of
the squamous epithelium.
Immature Stage
The most immature rodlet cells were located near the
basal membrane, showed a basal euchromatic nucleus
with great nucleolus, and were identified by an increase
and dilatation of RER. Simultaneously, the first traces of
the fibrous capsule or at least a marginal cytoplasmic zone
free of organelles were discernible (Fig. 3). Centrioles,
Golgi complex, and associated vacuoles with flocculent
content sometimes already displaying the typical dense
core were found in a supranuclear position. Already, at
this early stage, the RER cisternae often showed vesicular
dilatations losing their ribosomes and penetrating the
forming capsule, usually at the base of the cell but occasionally also at the lateral sides, thereby causing vesicular
protrusions (Fig. 3). The enlarging and apparent fusion of
these vesicles led to the characteristic basal or lateral
processes found in more mature stages (Fig. 8 and Figs. 18
and 19, respectively) mentioned above and visible by LM
(Fig. 1). Smaller vesicular dilations invaginated the mitochondria, especially in the more apical cell area (Fig. 4),
thus deforming them into crescent shapes. Rarely even
the nuclear envelope was seen bulging into mitochondria.
Desmosomal junctions with neighboring cells were found
in varying numbers (Figs. 3 and 4). In the intestinal epithelium, intraepithelial migrating leucocytes (mostly lymphocytes) were often seen in the vicinity or in contact with
rodlet cells (Fig. 3; see also Fig. 1).
Advanced Immature Stage
With progressive maturation, dilatations of the RER
cisternae and the Golgi stacks grew more prominent. With
respect to the development of the rodlets, small evaginations of the RER without ribosomes were observed, apparently giving rise to small vesicles showing increasing size
and density and even forming rows. Their content was
comparable to the structure of the adjacent rodlets and
cores (Fig. 5). Dilated cisternae of RER were apposed to
the membranes of the rodlet sacs, shedding their ribosomes (Figs. 5–7) and often indenting the sacs (Fig. 6).
Openings between the sacs occurred, as well as indications of occasional fusion between the membranes of the
sacs and indenting vesicular elements possibly derived
from the RER (Fig. 6). In rare cases, the regular scalloping
of the membranes of the developing rodlet sacs seemed
also to give rise to tubular elements found in the sacs (Fig.
7). The rodlets showed varying condensation of their flocculent contents, with one or rarely several (Fig. 6) dense
cores in the center. Apically, the membrane-bounded cores
extended between the dislocated mitochondria that aggregated in the cell apex together with the mentioned vesicular ER elements indenting them.
The basal endings of the cores were found in the Golgi
area (Fig. 11) but were also stretched either to the nuclear
membrane (Fig. 8) or to marginal RER cisternae under the
capsule (Fig. 8). With the increasing development of the
rodlet sacs, the Golgi apparatus became often dislocated
from its supranuclear position (Fig. 9) to the cell base,
while the nucleus assumed a bipartite or irregular shape
(Figs. 8 and 10). At this stage, accumulations of free ribosomes and glycogen, as well as vacuoles similar to dense
primary or heterogeneous secondary lysosomes, myelinand multivesicular bodies, could be detected in this region.
In the trans and lateral zone of the Golgi apparatus, the
expected variety of vesicles and vacuoles occurred (Figs. 8
and 11), including vacuoles with flocculent contents representing developing rodlet sacs (Fig. 9). In some cases,
only a single large condensing vacuole was found. These
vacuoles fused with small vesicles derived from the trans
Golgi face but were also in close association with vesicles
and RER profiles from the intermediate compartment, i.e.,
the cis region of the Golgi stack. The RER elements were
often dilated; the RER cisternae formed large evaginations around the whole area (Figs. 9 –11). The condensing
vacuole sometimes contained tubular elements associated
again with the membrane (Fig. 11) or the core. The tubules displayed a diameter about 30 nm, some of them
with a distinct dense membrane, and represented possibly
different elements. At times, similar straight tubules were
also seen in extremely hypertrophying RER (Fig. 17).
Mature Stage
The fully developed and usually elongated cells near or
in contact with the epithelial surface were packed with
rodlets converging toward the apex, with microtubules
running between them to the centrioles sometimes visible
in the neck region of the cell. The terminal stage of the
dilatation of the ER resulted in a vesicular appearance of
the cytoplasm. In some cases, especially in the gill epithelium, extended ER lacunae surrounded the basally or apically dislodged organelles (Figs. 17). The nucleus of these
rodlet cells showed occasionally dense accumulations (not
With respect to the extrusion of rodlets, the rodlet cells
reaching the epithelial surface showed the known thinning and later loss of the capsule in the apical area.
Junctional complexes with the adjacent epithelial cells
were found often but not always at the lateral sides of the
neck region and revealed varying differentiation, ranging
from well-developed tight or desmosomal junctions to reduced desmosome-like connections (Figs. 12–15). In the
high intestinal epithelium with its typical microvillar border, the apex of the rodlet cells displayed varying cytoplasmic formations. A smooth contour was found combined
with intact organelles, i.e., mitochondria (Fig. 15), while
cytoplasm filled with rodlets and apparently degenerating
organelles displayed either a microvillar tuft (Fig. 12), a
swollen cytoplasmic protrusion (a bleb: Fig. 13), or a disrupted zone with the organelles spilled into the intestinal
lumen (Fig. 14). A few dense projections (Fig. 19) or a
single thicker microvillar process (not shown) were visible
in dense, probably degenerating or apoptotic cells. The
discharged rodlet cores were found not only in the intestinal lumen but also near or between the microvilli, some-
Fig. 5. Development of rodlet sacs. Immature rodlet cell in gill epithelium showing slightly condensed nucleus with large nucleolus, dilated
ER cisternae, several rodlet sacs, and a well-developed capsule. bm,
basal membrane. a: Detail showing series of small vesicles with flocculent contents similar to the adjoining rodlet sac (s) with a projection from
the dilated RER, which has shed its ribosomes between them (arrowhead).
Fig. 6. Development of rodlet sacs. Developing rodlet sacs with
apposed membranes of dilated RER cisternae, which focally indent the
sacs (arrows). Rarely, openings between the sacs (arrowheads) or a
membrane-bounded cluster of several cores are seen. f, fibrillar capsule.
Fig. 7. Development of rodlet sacs. Dilated ER (ER) apposed to
rodlet sacs containing tubular elements (arrowheads) probably derived
from the strongly undulating membranes.
times showing a strongly undulating membrane apparently giving rise to tubular elements (Fig. 16).
Occasionally, rodlets were seen discharged into neighboring cells (compare Fig. 2) or penetrating through the lateral side of the capsule (Fig. 8). In the comparatively low
gill epithelium with only irregular microridges, the apical
openings of the rodlet cells showed similar disruption but
sparse microvillar projections (Fig. 17). A regular scalloping of the outer membrane of the rodlet cells indicating
possible contraction was lacking.
cases, the cells appeared narrow and compressed (Fig. 19;
compare also Figs. 1 and 2).
Degenerative Stages
In scattered cells belonging to different developmental
stages, an increase in cytoplasmic density indicated degenerative changes (Figs. 18 and 19). The secretory organelles (ER cisternae and Golgi stack) were reduced (Fig.
18), although a large condensing vacuole was occasionally
seen (Fig. 2). The number of rodlet sacs differed, and their
contents appeared often clear and swollen, revealing
again the mentioned tubular elements associated with
flocculent aggregations or the core (Fig. 18a). In other
As mentioned above, the controversy concerning the nature and function of the rodlet cell has been amply discussed
(Manera and Dezfuli, 2004). The thick fibrous capsule and
the peculiar club-like inclusions with their dense core are
reminiscent of a parasitic stage, whereas the electron microscopic investigation of the organelle development is compatible with secretory activity of an epithelial or a migrating
blood cell. The most immature rodlet cells can be identified
by dilated profiles of RER typical of the onset of protein
synthesis, with the first traces of the fibrillar capsule appearing simultaneously or shortly thereafter (Leino, 1974; Desser
and Lester, 1975; Flood et al., 1975; Barber et al., 1979). In
trout, the present data show already at this early stage that
part of the cells display vesicular protrusions of the RER
shedding their ribosomes, a process usually indicating degeneration. Some of these dilatations extending through the
forming capsule correspond to vesicular cytoplasmic exten-
Fig. 8. Mature rodlet cell not yet reaching the surface of the intestinal
microvillar border. The cell apex is filled with the mitochondrial labyrinth
and the tips of the rodlets, with one core piercing the capsule laterally
(arrowhead) and another ending near it (arrow). The basal part of the cell
contains the irregular nucleus, a large Golgi area, and vacuoles extending through the capsule. One core is seen contacting the nucleus (boxed
area). a: Detail with basally dislocated Golgi area displaying various
vesicles at its trans side and dilated intermediate elements at the cis
side. The ER cisternae (ER) and the nuclear envelope are partly dilated,
and the core of a rodlet sac (s) touches the nuclear envelope. G,
sions presumed to have nutritive function (Kimura, 1973;
Barber et al., 1979), a proposal often taken up but appearing
now rather unlikely in view of its origin.
Similarly, dilated protrusions of ER and rarely of the
nuclear membrane bulge also into mitochondria, thereby
deforming them into crescent-shaped forms. As these
evaginations shed their ribosomes, an artifact due to fixation and/or contraction of fibrils seems improbable. The
intermingling vesicular elements and degenerating mitochondria get apically displaced by the developing rodlet
sacs, thereby forming a complex known in literature as
whorled structures of the membranous labyrinth. The continuous formation of RER dilatations shedding their ribosomes proves the early assumption (Leino, 1974, 1979;
Barber et al., 1979; Kramer and Potter, 2002) that the
typical vesiculation in mature rodlet cells originates from
the disappearing RER. As a consequence, accumulations
of free ribosomes are found together with glycogen in the
basal region of mature rodlet cells.
The final stage of the continued ER hypertrophy is represented by cells displaying enormously distended ER profiles surrounding the decreasing secretory organelles,
which are dislocated and packaged together against nucleus or apical capsule. This corresponds to uncommented
micrographs in studies of fishes in polluted water (Iger et
al., 1994; Iger and Abraham, 1997; Pawert et al., 1998).
Whether these extreme ER dilatations are typical for Salmonids or are a common reaction to adverse influences
has to be further investigated. However, the varying extent or termination of this process of vesiculation at different stages would explain the described variations of
maturing and degenerating rodlet cells in literature, as
Fig. 9. Association of RER and Golgi apparatus (from gill region).
Rodlet cell with immature uncondensed nucleus, large nucleolus, and
indistinct capsule (f). The Golgi apparatus is well developed with a large
condensing vacuole (cv) at its trans face and a row of vesicles and
rodlets in varying stages of condensation laterally localized. The RER
cisternae are dilated and confluent to wide lacunae (ER) at the cell apex
and show indentations toward the developing rodlets (arrows).
Fig. 10. Association of RER and Golgi apparatus (from gill region).
More mature rodlet cell than in Figure 9, displaying an irregular and
slightly condensed nucleus. The developing rodlets (s) have dislocated
the mitochondria to the apex and the Golgi stack with a condensing
vacuole (cv) to the base. Dilated RER lacunae (ER) surround the latter.
Fig. 11. Association of RER and Golgi apparatus (from gill region).
Detail from a Golgi stack demonstrating grossly dilated RER lacunae
(ER) bordering the Golgi area. The cis face displays dilatations (asterisk),
the trans face a multitude of vesicles, while a large rodlet sac (s) contains
tubular elements (diameter ⬃ 30 nm; arrows) running in various directions and possibly stemming from the membrane (arrowheads). The core
appears to originate from the lateral side of the Golgi apparatus.
well as possible species-specific differences. As an example, the rodlet cells of trout are characterized by an irregular nucleus and large vesicular distensions, while in Cyprinids and some other species a round nucleus and smaller
cytoplasmic vesicles are typical [compare, e.g., Bielek and
Viehberger (1983) and Dezfuli et al. (2004)]. As a possible
consequence, the apically dislocated mitochondria in Cyprinidae often appear not deformed but dense and elongated between clear vesicles [“dense tubular bodies,” after
Iger and Abraham (1997)].
With respect to the differentiation of the rodlets, many
authors assume them to originate in the classical way
from Golgi vesicles at the trans face. Corresponding morphological and cytochemical evidence indicating glycoprotein content in inner Golgi lamellae, associated vesicles,
and developing rodlet sacs has been reported using routine phosphotungstic acid (PTA) or silver staining (Leino,
1979, 1982; Mattey et al., 1979; Bielek and Viehberger;
The origin of the proteinaceous core seems less clear.
Although its condensation in the center of the sac has been
assumed, small vesicles of similar density can be found
scattered in the cytoplasm distant from either developing
sacs, condensing vesicles, or Golgi stack. Their exact ori-
Figures 12–19.
gin and nature have to be decided cytochemically, but in
the present study, small dense vesicles with contents similar to condensing core material were seen budding from
dilated parts of RER cisternae losing their ribosomes,
indicating the addition of a separate, probably proteinaceous component by direct participation of ER-derived
elements. The close association of rodlet sacs and RER
cisternae is very conspicuous also in more mature stages,
and a direct participation of ER elements was repeatedly
considered in literature. The fusion of the irregular marginal zones was proposed by Barber et al. (1979), and
Desser and Lester (1975) assumed the complete investment of the rodlet sacs only at a later stage. Kramer and
Potter (2002) described the development of the rodlets in
the ER without referring to the role of the Golgi apparatus
or the relevance of these unusual results. The direct sequestration of proteins from the RER is known from peroxisomes, a possibility discussed by Iger and Abraham
(1997) based on a positive peroxidase reaction in the sacs.
It occurs also in certain examples of intense protein production or various pathological conditions (Ghadially,
1997; Kopito and Sitia, 2000), but without specific markers the involvement of RER-derived elements must remain hypothetical. However, as an additional possibility,
the participation of vesicles of the ER Golgi intermediate
compartment (ERGIC) is implied by the very close association of ER cisternae, lateral side of the Golgi stack, and
condensing vacuole, an observation leading Leino (1979)
in an early study to consider the extra-Golgi pathway as
an alternative to the classical secretion mode. An interesting parallel could be the hijacking of vesicles from the
retrograde pathway from the Golgi trans face to the RER
found in certain viral infections (Mackenzie et al., 1999).
Another aspect of this problem is the observation of
cores ending not only in the Golgi region but also near the
marginal RER cisternae under the capsule and frequently
Fig. 12. Release of rodlets and degenerating forms. Apical part of a
rodlet cell showing microvillar border similar to the adjacent intestinal
cells, a cell junctional complex in the neck region at the end of the
capsule, and an accumulation of vesicles and deformed mitochondria.
Fig. 13. Release of rodlets and degenerating forms. Similar region as
in Figure 12, again with cell junctions. The apical cytoplasm is bulging
into the lumen, revealing distended vesicles, cores, and the mitochondrial labyrinth.
Fig. 14. Release of rodlets and degenerating forms. Apical region
with apparently disrupted border, ejecting vesicles, and rodlets.
Fig. 15. Release of rodlets and degenerating forms. Apical region
with cell junctions, dense mitochondria, and smooth border.
Fig. 16. Release of rodlets and degenerating forms. Released tip of
rodlet caught in microvillar border with its limiting membrane displaying
undulations forming tubular elements.
Fig. 17. Release of rodlets and degenerating forms. Rodlet cell in gill
epithelium showing apical disruption and expelled intact rodlet near the
gill surface. In the cell, rodlets and organelles are again dislocated by
RER lacunae (ER) containing tubules (arrowheads).
Fig. 18. Release of rodlets and degenerating forms. Rodlet cell in the
intestinal epithelium showing dense cytoplasm, swollen rodlets, and
lateral vesicular protrusions. a and b: Details (boxed areas) showing sac
and core associated with tubules (arrowheads) and a possible RER
dilatation containing a core (arrowhead).
Fig. 19. Release of rodlets and degenerating forms. Rodlet cell
reaching the microvillar border but appearing compressed and displaying dense capsule, vacuolar cytoplasm, and vesicular extensions at the
outer side of the capsule (arrows).
in contact with the nuclear envelope. Only Barber et al.
(1979) mentioned “the stretching of the cores to the proximal side of the nucleus.” This observation as well as core
projections from the base of mature sacs (Dezfuli et al.,
2003b) could be explained by continuous growth of the
core, but the finding of small cores identified by their
crystalloid content in the immediate vicinity of the nucleus (Bielek, 2002) indicates again the derivation from
ER (or nuclear envelope).
The assumption of ER hypertrophy indicating storage of
overproduction fits with several other peculiar features,
e.g., the crystalloid or at least striated structure of the
cores reported in several studies (Leino, 1974; Desser and
Lester, 1975; Della Salda et al., 1998; Fishelson and
Becker, 1999; Bielek, 2002). Other features are the occasional strong undulation of apposed ER and limiting membranes of rodlet sacs as well as the occurrence of tubular
elements partly derived from them (Bielek, 2002) but
found also in dilated ER and condensing vacuoles.
Straight or curved tubuli in the rodlet sacs were described
only by Leino (1974, 1979), who interpreted them as a
species-specific differentiation. As comparable tubules as
well as strongly undulating ER membranes are associated
with overproduction of a single or a few membrane proteins, forming sometimes a crystalloid or microtubular
reticular complex (TRS) (Ghadially, 1997), they might also
be taken as support for a pathological transformation or
some physiological disturbance. These typical aggregates
have been found in a variety of cells, either signaling
storage of membranes due to slowing metabolism, genetic
disorders, viral infections, or, more recently, the elevation,
e.g., of interferon (Ghadially, 1997), an interesting link to
the proposed defensive effect of rodlet cells in infections.
With respect to the discharge of the rodlets, the observed increase of ER supports a release by pressure (Bannister, 1966; Richards et al., 1994; Smith et al., 1995), an
opinion contrary to the widespread theory of an expulsion
by active and triggered contraction of the capsule in response, e.g., to a parasitic infection (Leino, 1974, 1979;
Desser and Lester, 1975; Barber et al., 1979; Mattey et al.,
1979; Kramer and Potter, 2002). The apical cytoplasmic
corona-like structure observed in numerous studies was
assumed to have a function during expulsion (Grünberg
and Hager, 1978) or in the movement of the cells (Smith et
al., 1995; Fishelson and Becker, 1999). Manera et al.
(2001) described bleb-like and corona-like processes, the
former increasing strongly during toxic conditions. A single thick microvillus was interpreted as anchoring structure between endothelial cells (Smith et al., 1995) or a
chemoreceptor (Leino, 1996). The present observations
show a sequence from more (intestinal epithelium) or less
(gill) developed microvillar projections to varying stages of
cytoplasmic blebbing, leading apparently to the rupture of
the cell and the spilling of rodlets and organelles, which
can be observed occasionally also in the middle of the
comparatively high intestinal epithelium or at the lateral
side as shown here and previously (Dezfuli et al., 2000;
Bielek, 2002; Kramer and Potter, 2002).
So while all the described features are difficult to fit into
the pattern of a normal secretory cell, either epithelial or
blood-borne, a possible malfunction of ER and/or transport
mechanisms is compatible with the observed dilatations of
the ER cisternae and the intermediate compartment at
the cis side of the Golgi apparatus. The existence of immature rodlet cells at the base of the diverse epithelia, the
occurrence of cell junctions, and a microvillar tuft after
reaching the epithelial surface speak for an endogenous,
probably transformed epithelial cell, but leaves open the
cause for the proposed transformation and the function of
capsule and presumed migration. The reason for the long
controversy concerning the rodlet cell is the problem that
each of the hypotheses (parasite, secretory cell, blood cell)
is only covered by part of the observations and contradicted by others. The thesis of a defensive function has
been supported by recent statistical studies showing an
increase of rodlet cells under the influence of quite different stressors, ranging from parasites, toxins, neoplasm,
viral infections, general tissue damage, to adverse environmental influences (Manera and Dezfuli, 2004) and
even different depth of water (Winckler and Portela-Gomez, 1962). Therefore, most of these studies interpret the
rodlet cell as regular element of the defense system, appearing in direct association with stressors (Iger and
Abraham, 1997) and comparable to inflammatory cells
(Dezfuli et al., 1998, 2000, 2003a, 2003b, 2004; Manera et
al., 2001; Manera and Dezfuli, 2004). Nevertheless, there
seem to exist convincing exceptions to this rule (Fishelson
and Becker, 1999). As the rodlet cells are definitely found
scattered in completely differing tissue compartments, including the blood stream, this distribution is usually only
compatible with a leucocyte with defensive function. On
the other side, while the question of junctions is just
debatable for parasites, junctional complexes are without
precedent in blood cells. Moreover, protection against aggressive granule components, a function for the capsule
proposed by Iger and Abraham (1997), is managed by
enzymatic masking (Rowley et al., 1988; Zucker-Franklin
et al., 1988; Imagawa et al., 1998). The early suggestion of
a modified epithelial secretory cell comparable, e.g., to
goblet cells (Al-Hussaini, 1949), which also occur and increase at sites of epithelial infection, is not consistent with
capsule or migration into connective tissue or blood.
Searching again for a parallel under pathological conditions, an example might be found in neoplasm or virally
infected cells, which are known to round up, change adhesion, reduce number and types of junctions, and migrate
depending on the surrounding tissue (Gadhially, 1997).
Some inconsistencies in literature with respect to number
and type or complete absence of cell junctions could be due
to the resulting variations. Considering the numerous
open questions, the proposition of some submicroscopic
infection (Fishelson and Becker, 1999) causing possibly an
opportunistic reaction under stress conditions seems
worth following up. Further investigations concerning cytochemical nature and distribution of the different components and the possible causative agents are needed to
clear up these questions.
The author is grateful to E. Vanyek-Zavadil for excellent technical support, Dr. E. Noisser for helping to obtain
wild specimen, Dr. R. Czaker and Professor G. Viehberger
for helpful comments, and Mag. R. Schulz for English
revision of the final draft.
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development, species, two, rodlet, endoplasmic, reticulum, teleost, cells
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