# Coelomocytes: Biology and Possible Immune Functions in Invertebrates with Special Remarks on Nematodes

Authors: Qudsia Tahseen
Journal: International Journal of Zoology
 (2009)

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## Abstract

All metazoans are exposed to a wide range of microbes and have evolved complex immune defenses used to repel infectious agents. Coelomocytes play a key role in the defense reactions of most invertebrates. They are involved in important immune functions, such as phagocytosis, encapsulation, graft rejection, and inflammation, as well as the synthesis and secretion of several humoral factors especially in annelids and echinoderms. Coelomocytes in nematodes are variable in shapes from round, ovoid, cuboidal, and spindle-shaped to stellate or branched cells that are found usually at fixed positions in the pseudocoelom. Their number usually varies from 2 to 6. The model nematode,C. elegans lacks an adaptive immune system and the coelomocytes are capable of endocytosis, but their involvement in phagocytosis of bacteria seems unlikely. The aim of this review is to evaluate current knowledge on coelomocytes of invertebrates with special reference to nematodes. The morphology and structure of these coelomocytes are discussed along with their origin. Their relative positions and diversity in different nematode groups have also been discussed and illustrated.


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## Body

## 1. Introduction

Invertebrate
organisms have developed a variety of defense reactions to fight invading
foreign agents. The invertebrates possess nonadaptive, innate, nonclonal,
nonanticipatory immune responses contrary to vertebrate responses which are
induced, adaptive, acquired, clonal, and anticipatory [1].
Invertebrates do not possess the immunoglobulins found in higher animals,
although proteins containing immunoglobulin-like domains have been identified
[2]. The common defense mechanisms used by most
invertebrates to protect themselves against infectious agents are the synthesis
and secretion of antibacterial and antifungal proteins, agglutination and
nodule formation, encapsulation of foreign objects, and phagocytosis. During defense
reactions, invariably the foreign organisms are found to be encapsulated and
melanized and enzymes (i.e., phenol oxidase) play a vital role in
defense reactions. Among the lower forms, protists are the prototypes of
macrophages; sponges distinguish between self and nonself; cnidarians have the
phagocytes and cnidoblasts while some species are also provided with C3-like
molecules.Careful
study of the phylogeny of the immune system has revealed the evolution of three
important components, namely, the macrophage, lymphatic, and hematopoietic
systems [3]. The most ancient is the macrophage system
(largely found in invertebrates) that arises in the coelomic cavity as
mesenchymal amoeboid cells or coelomocytes for recognition of self from nonself
and for ingestion of foreign particles. The lymphatic system in higher animals
develops from the endoderm of pharyngeal pouches, while the hematopoietic
system originates from the splanchnic mesoderm of the yolk sac as hematogenic
tissue, containing hemangioblasts.Coelomocytes
n. pl. (Gr.koilos, hollow; kytos, container) are cells that tend to be obscure but are apparently omnipresent in most
coelomates. The nonmuscle macrophage-like cells inhabit the body cavity or the
coelomic spaces of many invertebrates. In annelids, the cells found in coelom
are categorized as coelomocytes, chloragogen cells (eleocytes), and haemocytes
[4]. The molluscs such as gastropods have haemocytes in the body
cavity [5]. The coelomic cells are also
referred to as haemocytes in most arthropods [6]. In
echinoderms, the coelomocytes occupy perivisceral coelomic cavities, the
water-vascular system, and the haemal system besides the connective tissue and
tissues of various organs [7, 8]. Nematodes possess mesenchymatous coelomocytes in
their pseudocoelom, adjacent to the gonads or other internal organs in the
anterior or posterior body regions. These cells were assumed to be
phagocytic to purify the body fluid and, therefore, attributed different
functions by different workers and assigned different names such as amoebocytes,
elaeocytes, athrocytes, and phagocytes. Due to their small size and relatively
lesser number, these cells
were largely ignored in nematodes particularly in the early developmental stages [9].The
ability of nematodes to osmoregulate varies considerably; free-living forms
which are exposed to wide variations in osmotic pressure are extremely
efficient osmoregulators; the parasitic forms, on the other hand, may have
relatively limited osmoregulatory capacity. In animal parasitic nematodes, the stellate pseudocoelomocytes
have been suggested to maintain the pseudocoelomic fluid, either as phagocytic
cells removing bacteria and other pathogens or  collecting certain xenobiotics
molecules and effectively removing them from the coelomic fluid or as further playing a
role in haem metabolism [10–12].Although
the function of nematode pseudocoelomocytes (=coelomocytes) was not precisely
known for a long time, yet the coelomocytes were frequently studied.
Coelomocytes were first observed and reported in the form of four stellate
cells inParascaris equorum in anterior third region of the body by
Bojanus [13]. Later Bugnion [14] and von Linstow [15] regarded them as blood
corpuscles of nematodes. Jägerskiöld [16–18], Nassonov [19, 20],
and Shipley [21] also observed and referred to such cells in their studies.
Rauther [22] described them to be fixed in position and attached to the body
wall by fine processes. They were also reported to be present in oxyurids and ascarids
by Martini [23, 24] and Höeppli [25]. Stefanski [26] found them
existing between the base of pharynx and anterior end of ovary or testis in Rhabditella
axei while B.-G. Chitwood and M. B. Chitwood [27] reported two binucleated “X” bodies in Cephalobellus
papilliger. B. G. Chitwood and M. B. Chitwood [28],
Weinstein [11, 29],
Douvres et al. [30], Peregrine [12], Boghen and Davey [31],
Ishikawa [32], Poinar and Jansson [33], A. F. Bird and J. Bird
[34] described coelomocytes in free-living and animal parasitic nematodes.


## 2. Evaluation of Morphology of Coelomocytes

### 2.1. Shape

The
pseudocoelomic body cavity of the rotiferAsplanchna spp.
contains free cells (coelomocytes) that form a highly dynamic,
three-dimensional polygonal network of filopodia. Despite their morphological
heterogeneity, coelomocytes have not been categorized for systematic comparison
of nematodes. Members of the Secernentea usually possess prominent, unbranched
or branched coelomocytes [27] where large vertebrate
parasites like Ascaris and Strongylus have
branched coelomocytes [27]. The greater size of the coelomocytes
and their numerous branches bearing many cytoplasmic swellings increase
cellular surface area and metabolic efficiency in Ascaris. The groups, Torquentia and
Penetrantia, appear to have ovoid and usually unbranched pseudocoelomocytes. In general,
pseudocoelomocytes of nematodes tend to be variable in shapes from rounded, ovoid,
cuboidal, spindle-shaped to stellate or branched cells (Figures 1 and 2) with relatively fixed positions [34]. In Caenorhabditis elegans, they are
small rounded cells with small yet discernible nuclei (Figures 3(a), 3(b), and 3(g)). Some coelomocytes may have a
reticulate network (Figure 3(l)) while others may have several small vacuoles
(Figures 3(a), 3(d), and 3(h)) contained within. In ascarids, there are two or four large branched cells with a
central nucleated body and numerous radiating branches. The amoeboid extensions
of the coelomocytes
of Ascaris suum and Parascaris equorum show terminal swellings [10].  In free-living rhabditid nematodes, pseudocoelomocytes are usually
small spherical (Figures 3(a)–3(g), and 3(p)) or ovoid cells (Figures 3(i) and 3(o)) with rather
obscure nuclei. However, in an insect-associate rhabditid, Distolabrellus veechi  the coelomocytes tend
to be larger and prominent (Figure 3(o)). Earlier B.-G. Chitwood and M. B. Chitwood [27] reported
two round binucleated coelomocytes-like bodies from Cephalobellus papilliger and four
rounded bodies in Blatticola blattae. Ancylostoma duodenale possesses strand-like organs with refractive particles, in the body cavity
[20]. Unlike Secernenteans (Figures 4(c) and 4(g)), the coelomocytes are
not conspicuous in Adenophoreans, however, the monhysterids (Figure 4(d)),
araeolaimids (Figure 4(i)), chromadorids (Figure 4(h)) and few enoplids (Figures 4(a) and 4(b)) show prominent ovoid coelomocytes with relatively fixed positions.
Coelomocytes could also be detected in few species of dorylaims (Figure 4(f))
and mononchs (Figure 4(e)).Figure 1
Coelomocytes in free-living and parasitic nematodes. (a)
Stellate cell inStrongylus sp. (b) Spindle-shaped coelomocytes in Blatticola sp. (c), (f), (l) Round coelomocytes
in rhabditids. (d) Coelomocyte close to developing genital primordium. (e) Paired
coelomocytes in Panagrolaimus sp. (g) Branched cell in Ascaris sp. (h) Cuboidal coelomocyte in Monhystera sp. (i), (j) Angular coelomocytes in rhabditids. (k) Bilobed coelomocyte in Curviditis sp. (m), (o) Coelomocyte in strongylid. (n) Reniform coelomocyte in Plectus sp. (p) Bipolar
coelomocyte in Oscheius sp.(a)–(h) Light micrograph (DIC) of coelomocytes. (a) Round
granular type. (b) Reticulate type. (c) Ovoid type with distinct nucleus. (d)
Round vacuolated type. (e), (g) Bipolar type. (f) Bilobed type. (h) Ovoid type.
(i) Transmission electron micrograph of coelomocyte: g = granule; n = nucleus;
nl = nucleolus; m = mitochondria; v = vesicle (scale bar A–H = 5μm;  I=0.5μm).
(a)(b)(c)(d)(e)(f)(g)(h)(i)Figure 3
Position of coelomocytes with respect to developing
gonad indicated by arrows. (a) Coelomocytes in second stage juvenile. (b) Coelomocyte in third
stage female juvenile. (c) Coelomocyte in fourth stage female juvenile. (d), (e) Coelomocyte
in adult female. (f), (g) Coelomocytes adjacent to testis in adult male. (h)–(j), (o), (p)
Coelomocytes between cardia and anterior reflexed ovary. (l)–(n), (q) Coelomocytes
with varied morphologies close to genital system in posterior body region
(scale bar = 5μm).Figure 4
Coelomocytes in different nematode groups indicated by
arrows. (a), (b) Ovoid coelomocyte in an enoplid. (c) Bipolar coelomocyte in a rhabditid. (d)
Stellate coelomocyte in a monhysterid. (e) Rounded coelomocyte in a mononchid.
(f) Inconspicuous coelomocyte in a dorylaim. (g) Paired spindle-shaped
coelomocytes in a panagrolaimid. (h) Elongate coelomocyte in a chromadorid. (i)
Binucleate coelomocyte in an araeolaimid (scale bar = 5μm).


### 2.2. Size

The coelomocytes in earthworms tend to be of two sizes: small
(cytotoxic) and large (phagocytic) [35]. In sea urchins,
coelomocytes are free-wandering cells that populate the coelomic cavity of which two-thirds are phagocytic, and the rest
are vibratile cells, or colorless and red spherule cells [36].Most descriptive studies [9, 37] have indicated
that nematode coelomocytes frequently become giant cells during the parasitic phase of
development, whereas in free-living species the cells remain relatively small. In C. elegans a coelomocyte is about 10–20 μm in diameter whereas the large stellate pseudocoelomocyte of adult parasite, P. equorum, measures approximately 5mmlong×3mmwide and 250 μm thick.


### 2.3. Number

In the purple sea urchin (Strongylocentrotus
purpuratus), there are on average 7.5×106  putative coelomocytes per mL of coelomic fluid [38]. However, in
nematodes their number is considerably smaller and varies from 2 to 6. There
are five coelomocytes in males and six in adult hermaphrodites of C. elegans [39]. In Thelastomatidae,
the voluminous pseudocoel contains 3 large pseudocoelomocytes. In Sphaerolaimus gracilis, two pairs
of coelomocytes occurred laterally posterior to the pharyngeointestinal junction
[40]. The anterior pair of the coelomocytes between the renette
cell and gonad lies on the left or the right lateral side of the body. The
posterior pair of coelomocytes lies on the opposite side [41].


### 2.4. Structural Details

The sipunculids were investigated to possess coelomic cell
complexes in the coelomic fluid, using Transmission Electron Microscopy [42]. Each complex was made of a
central glandular cell and the outer layer of podocytes. Peculiar cell
complexes (“urns”), comprising ciliary and granular cells, were described in Thysanocardia nigra [42, 43].In
pseudocoelomate phylum Rotifera, video-enhanced differential interference
contrast microscopy has
revealed a network of filopodia in the pseudocoelomocytes that show motility
[44].
The filopodial junctions are regularly displaced. Thereafter, the free-ending
filopodia form and
extend, and further show retraction. There occur  enlargements, diminutions,
and extinctions of filopodial
polygons, and the formation of new polygons in these rotifers.In
the group Priapulida, the mesenteries that suspend organs are reported to be
formed from extracellular material produced by coelomocytes or amoebocytes that
move freely in the body cavity. These amoebocytes produce fibers that suspend
the digestive system and other organs in pseudocoelom.
Two types of coelomocytes have been observed in the body cavity ofPriapulus caudatus by Mattisson and Fänge [45]. The “erythrocytes” are
nucleated and contain marginal bands,
vacuoles and occasionally crystals. The cytoplasm has few organelles. The
“leucocytes” are amoeboid,
motile cells, the cytoplasm of which contains numerous organelles.In
nematodes, the coelomocyte is a highly specialized cell containing a swollen,
ramifying rough endoplasmic
reticulum, which is filled with an amorphous material, and is associated with
many large
complex Golgi bodies. A heterogeneous population of electron-dense bodies
constitutes the major
cellular inclusion. These structures may represent either a product of the cell
and/or material taken
up by the coelomocytes [46].In
animal parasitic nematode,P. equorum, a large stellate
cell lies in the pseudocoelom, firmly attached
to the surrounding tissue [10] with a large nucleus and cytoplasm
containing numerous granules and vesicles. A. suum like P. equorum contains a large irregular nucleus lying in a central
cytoplasmic mass which branches into many amoeboid extensions [10, 20, 47–49]. These extensions terminate in
spherical bodies of cytoplasm that have been referred to as “cytoplasmic
pearls”  (see Hurlaux [10]). Fukuda [49] performed detailed
study on the branched cell of A. suum. The transmission
and scanning electron microscopic studies have elucidated finer details of
coelomocytes of adult A. suum. The extraordinarily large cell with a
main body of cytoplasm, contains a centrally located nucleus including many
large nucleoli. The central cytoplasm branches extensively, and each branch
ends in small bulbs composed of a fibrous core surrounded by a region
containing most of the cell organelles. The branches of the coelomocytes
increase the cell surface area and extend over the gut, lateral line, and body
muscles. The free-floating spherical coelomocytes lying in the pseudocoelomic
cavity of larvae and adult
C. elegans
have been found to
endocytose many compounds, possibly for immune surveillance. The coelomocytes
in adult hermaphrodites, often lying pairwise together, display prominent
cytoplasmic inclusions and vacuoles. These coelomocytes become more granulated
and vacuolated during postembryonic development of C. elegans [41].


## 2.1. Shape

The
pseudocoelomic body cavity of the rotiferAsplanchna spp.
contains free cells (coelomocytes) that form a highly dynamic,
three-dimensional polygonal network of filopodia. Despite their morphological
heterogeneity, coelomocytes have not been categorized for systematic comparison
of nematodes. Members of the Secernentea usually possess prominent, unbranched
or branched coelomocytes [27] where large vertebrate
parasites like Ascaris and Strongylus have
branched coelomocytes [27]. The greater size of the coelomocytes
and their numerous branches bearing many cytoplasmic swellings increase
cellular surface area and metabolic efficiency in Ascaris. The groups, Torquentia and
Penetrantia, appear to have ovoid and usually unbranched pseudocoelomocytes. In general,
pseudocoelomocytes of nematodes tend to be variable in shapes from rounded, ovoid,
cuboidal, spindle-shaped to stellate or branched cells (Figures 1 and 2) with relatively fixed positions [34]. In Caenorhabditis elegans, they are
small rounded cells with small yet discernible nuclei (Figures 3(a), 3(b), and 3(g)). Some coelomocytes may have a
reticulate network (Figure 3(l)) while others may have several small vacuoles
(Figures 3(a), 3(d), and 3(h)) contained within. In ascarids, there are two or four large branched cells with a
central nucleated body and numerous radiating branches. The amoeboid extensions
of the coelomocytes
of Ascaris suum and Parascaris equorum show terminal swellings [10].  In free-living rhabditid nematodes, pseudocoelomocytes are usually
small spherical (Figures 3(a)–3(g), and 3(p)) or ovoid cells (Figures 3(i) and 3(o)) with rather
obscure nuclei. However, in an insect-associate rhabditid, Distolabrellus veechi  the coelomocytes tend
to be larger and prominent (Figure 3(o)). Earlier B.-G. Chitwood and M. B. Chitwood [27] reported
two round binucleated coelomocytes-like bodies from Cephalobellus papilliger and four
rounded bodies in Blatticola blattae. Ancylostoma duodenale possesses strand-like organs with refractive particles, in the body cavity
[20]. Unlike Secernenteans (Figures 4(c) and 4(g)), the coelomocytes are
not conspicuous in Adenophoreans, however, the monhysterids (Figure 4(d)),
araeolaimids (Figure 4(i)), chromadorids (Figure 4(h)) and few enoplids (Figures 4(a) and 4(b)) show prominent ovoid coelomocytes with relatively fixed positions.
Coelomocytes could also be detected in few species of dorylaims (Figure 4(f))
and mononchs (Figure 4(e)).Figure 1
Coelomocytes in free-living and parasitic nematodes. (a)
Stellate cell inStrongylus sp. (b) Spindle-shaped coelomocytes in Blatticola sp. (c), (f), (l) Round coelomocytes
in rhabditids. (d) Coelomocyte close to developing genital primordium. (e) Paired
coelomocytes in Panagrolaimus sp. (g) Branched cell in Ascaris sp. (h) Cuboidal coelomocyte in Monhystera sp. (i), (j) Angular coelomocytes in rhabditids. (k) Bilobed coelomocyte in Curviditis sp. (m), (o) Coelomocyte in strongylid. (n) Reniform coelomocyte in Plectus sp. (p) Bipolar
coelomocyte in Oscheius sp.(a)–(h) Light micrograph (DIC) of coelomocytes. (a) Round
granular type. (b) Reticulate type. (c) Ovoid type with distinct nucleus. (d)
Round vacuolated type. (e), (g) Bipolar type. (f) Bilobed type. (h) Ovoid type.
(i) Transmission electron micrograph of coelomocyte: g = granule; n = nucleus;
nl = nucleolus; m = mitochondria; v = vesicle (scale bar A–H = 5μm;  I=0.5μm).
(a)(b)(c)(d)(e)(f)(g)(h)(i)Figure 3
Position of coelomocytes with respect to developing
gonad indicated by arrows. (a) Coelomocytes in second stage juvenile. (b) Coelomocyte in third
stage female juvenile. (c) Coelomocyte in fourth stage female juvenile. (d), (e) Coelomocyte
in adult female. (f), (g) Coelomocytes adjacent to testis in adult male. (h)–(j), (o), (p)
Coelomocytes between cardia and anterior reflexed ovary. (l)–(n), (q) Coelomocytes
with varied morphologies close to genital system in posterior body region
(scale bar = 5μm).Figure 4
Coelomocytes in different nematode groups indicated by
arrows. (a), (b) Ovoid coelomocyte in an enoplid. (c) Bipolar coelomocyte in a rhabditid. (d)
Stellate coelomocyte in a monhysterid. (e) Rounded coelomocyte in a mononchid.
(f) Inconspicuous coelomocyte in a dorylaim. (g) Paired spindle-shaped
coelomocytes in a panagrolaimid. (h) Elongate coelomocyte in a chromadorid. (i)
Binucleate coelomocyte in an araeolaimid (scale bar = 5μm).


## 2.2. Size

The coelomocytes in earthworms tend to be of two sizes: small
(cytotoxic) and large (phagocytic) [35]. In sea urchins,
coelomocytes are free-wandering cells that populate the coelomic cavity of which two-thirds are phagocytic, and the rest
are vibratile cells, or colorless and red spherule cells [36].Most descriptive studies [9, 37] have indicated
that nematode coelomocytes frequently become giant cells during the parasitic phase of
development, whereas in free-living species the cells remain relatively small. In C. elegans a coelomocyte is about 10–20 μm in diameter whereas the large stellate pseudocoelomocyte of adult parasite, P. equorum, measures approximately 5mmlong×3mmwide and 250 μm thick.


## 2.3. Number

In the purple sea urchin (Strongylocentrotus
purpuratus), there are on average 7.5×106  putative coelomocytes per mL of coelomic fluid [38]. However, in
nematodes their number is considerably smaller and varies from 2 to 6. There
are five coelomocytes in males and six in adult hermaphrodites of C. elegans [39]. In Thelastomatidae,
the voluminous pseudocoel contains 3 large pseudocoelomocytes. In Sphaerolaimus gracilis, two pairs
of coelomocytes occurred laterally posterior to the pharyngeointestinal junction
[40]. The anterior pair of the coelomocytes between the renette
cell and gonad lies on the left or the right lateral side of the body. The
posterior pair of coelomocytes lies on the opposite side [41].


## 2.4. Structural Details

The sipunculids were investigated to possess coelomic cell
complexes in the coelomic fluid, using Transmission Electron Microscopy [42]. Each complex was made of a
central glandular cell and the outer layer of podocytes. Peculiar cell
complexes (“urns”), comprising ciliary and granular cells, were described in Thysanocardia nigra [42, 43].In
pseudocoelomate phylum Rotifera, video-enhanced differential interference
contrast microscopy has
revealed a network of filopodia in the pseudocoelomocytes that show motility
[44].
The filopodial junctions are regularly displaced. Thereafter, the free-ending
filopodia form and
extend, and further show retraction. There occur  enlargements, diminutions,
and extinctions of filopodial
polygons, and the formation of new polygons in these rotifers.In
the group Priapulida, the mesenteries that suspend organs are reported to be
formed from extracellular material produced by coelomocytes or amoebocytes that
move freely in the body cavity. These amoebocytes produce fibers that suspend
the digestive system and other organs in pseudocoelom.
Two types of coelomocytes have been observed in the body cavity ofPriapulus caudatus by Mattisson and Fänge [45]. The “erythrocytes” are
nucleated and contain marginal bands,
vacuoles and occasionally crystals. The cytoplasm has few organelles. The
“leucocytes” are amoeboid,
motile cells, the cytoplasm of which contains numerous organelles.In
nematodes, the coelomocyte is a highly specialized cell containing a swollen,
ramifying rough endoplasmic
reticulum, which is filled with an amorphous material, and is associated with
many large
complex Golgi bodies. A heterogeneous population of electron-dense bodies
constitutes the major
cellular inclusion. These structures may represent either a product of the cell
and/or material taken
up by the coelomocytes [46].In
animal parasitic nematode,P. equorum, a large stellate
cell lies in the pseudocoelom, firmly attached
to the surrounding tissue [10] with a large nucleus and cytoplasm
containing numerous granules and vesicles. A. suum like P. equorum contains a large irregular nucleus lying in a central
cytoplasmic mass which branches into many amoeboid extensions [10, 20, 47–49]. These extensions terminate in
spherical bodies of cytoplasm that have been referred to as “cytoplasmic
pearls”  (see Hurlaux [10]). Fukuda [49] performed detailed
study on the branched cell of A. suum. The transmission
and scanning electron microscopic studies have elucidated finer details of
coelomocytes of adult A. suum. The extraordinarily large cell with a
main body of cytoplasm, contains a centrally located nucleus including many
large nucleoli. The central cytoplasm branches extensively, and each branch
ends in small bulbs composed of a fibrous core surrounded by a region
containing most of the cell organelles. The branches of the coelomocytes
increase the cell surface area and extend over the gut, lateral line, and body
muscles. The free-floating spherical coelomocytes lying in the pseudocoelomic
cavity of larvae and adult
C. elegans
have been found to
endocytose many compounds, possibly for immune surveillance. The coelomocytes
in adult hermaphrodites, often lying pairwise together, display prominent
cytoplasmic inclusions and vacuoles. These coelomocytes become more granulated
and vacuolated during postembryonic development of C. elegans [41].


## 3. Origin and Location of Coelomocytes

Nematode
development provides an excellent model system for studying the relationships
and connections
between cell growth, cell division, and cellular differentiation [41, 50]. The differentiation is not evident during the early
phases of cleavage program,
however, at later stages, cellular populations begin to show morphological and
molecular differences
that reflect their final fate. Though the coelomocytes, strands, and membranes
in the pseudocoelom
were all considered to be a type of mesenchyme, quite early by Hyman [51], nevertheless,
Maggenti [52] questioned the reliability of the fact. It has been proved now
that these
coelomocytes are mesodermal in origin generating from blast cells that also
give rise to mesoderm.
The M-lineage includes cells that form body wall muscles, coelomocytes (similar
to macrophages), and sex myoblasts—precursors to
vulval and uterine muscles. Coelomocyte precursors fail to differentiate in
some mutants but instead undergo an extra division to generate cells that can
enlarge like sex myoblasts.Unlike annelids and echinoderms, the nematode coelomocytes are
fixed in position and firmly attached to the inner surface of the body wall by filopodia (Figures2(d), 2(e), and 2(g)). However, aberrantly positioned coelomocytes suggest the
possibility that detachment and migration may occur in some instances. The
coelomocytes are usually located ventral, lateral, or even dorsal in position,
in close vicinity of the genital primordium (Figure 3) and keep on changing
their position with successive development and moulting. In adults, one or two
coelomocytes definitely exist between the base of pharynx and the anterior end
of gonad (Figures 3 and 4). Stefanski [26] described four round cells in Rhabditella axei, two of which were located
between the base of pharynx and the anterior end of ovary/testis, one at the
blind end of anterior gonad and another at (3/4)th of body length from anterior extremity.A progenitor blast cell in the posterior part of the newly hatched
larva ofNippostrongylusbrasiliensis produced two coelomocytes C5 and C6 as reported by Weinstein
[53]. Most sex-specific tissues of the adult are formed from the descendents
of blast cells.InC. elegans, the coelomocytes arise from two separate lineages. The six oblong celomocytes residing as 3 pairs (ventral anterior, ventral posterior, and
dorsal) in the pseudocoelomic cavity adjacent to the somatic musculature of the hermaphrodite, are
derived from two parts of the mesodermal lineage. Four of these coelomocytes are present at hatching and two are
generated in the first larval stage. Four are born during embryogenesis from MS
lineage symmetrically [50]; two are
postembryonically derived from divisions of the M blast cell [41]. The anterior four embryonic coelomocytes are
derived in pairs from progeny of two different MS grand daughters, MSap and
MSpp [50], and lie on the ventral side between
the pharynx and vulva. The postembryonic M-derived coelomocytes arise from divisions
of M blast cell and do not share a common parentage [41]
and reside dorsally
in the tail. In males, one of the proximal germ line  coelomocytes migrates
posteriorly, and one M-derived coelomocyte is formed. In first-stage juvenile
(J1), destined to develop into male, one of the ventral left-side coelomocytes
is located posterior to the gonad primordium rather than anterior as in J1
developing into hermaphrodite. The ventral anterior pair is located on the
right side and the ventral posterior pair on the left as observed by Wood et
al.
[54] in C. elegans grown at 20°C but the positions were
reversed when cultivated at 10°C  [55]. Yanowitz and Fire
[56] identified and characterized a set of mutations that affect coelomocyte number,
specification, and morphology. The mutants were reported with an increased
number of coelomocytes, random decrease in the number, lineage-specific
decrease, and premature degradation. The lineage-specific decrease was further
classified into three types: defective in both pairs of embryonic coelomocytes;
defective in the MSap-derived coelomocytes; defective in the postembryonically
derived coelomocytes [56]. Although many genes determining
muscle cell fates have been characterized, the genes for the distinction
between muscle and nonmuscle cell fates are still to be identified.The number and position of the coelomocytes show some
variations in different groups
of nematodes (Figure5) as also reported between free-living and parasitic
species [9, 37]. In a careful study, the rhabditids such as Oscheius sp.
(Figure 5(a)) and Rhabditis sp. were observed to possess six small
coelomocytes as found in C. elegans hermaphrodites. In
the monhysterid Geomonhystera pervaga
two pairs of coelomocytes were observed: one pair close
to pharyngeo-intestinal junction while the other lying closely anterior to
vulval opening (Figure 5(b)). The araeolaimid Plectus
parvus possesed four relatively larger coelomocytes with the two pairs located between cardia and
anterior ovary (Figure 5(c)). In Achromadora
ruricola,  the chromadorid was observed to have only one pair of coelomocytes in the
pseudocoelom lying slightly posterior to pharyngeo-intestinal junction (Figure 5(d)).
One pair of coelomocytes was observed to exist at the level of vulva in Prismatolaimus
matoni. The two pairs of pericardial cells
at the junction of pharynx and intestine showed striking similarity to these
coelomocytes in morphology and staining properties (Figure 5(e)).Number and relative positions of coelomocytes in nematodes
(a)Oscheius sp. (b) Geomonhystera pervaga. (c) Plectus parvus. (d) Achromadora ruricola. (e) Prismatolaimus matoni (scale bar = 100 μm).
(a)(b)(c)(d)(e)


## 4. Changes in Coelomocytes during Developmental Cycle

The coelomocytes in the early developmental stages are
usually minute, hence cryptic and hard to recognize (Figure3(a)). Some of the detailed researches on
nematodes have indicated changes and differentiation in the pseudocoelomocytes during
postembryonic development. B. G. Chitwood and M. B. Chitwood [57] stated four coelomocytes to be present in
the J1 of most rhabditids including C.elegans though their number reaches 5 in males
and 6 in hermaphrodites. These glandular cells are located in the pseudocoelom adjacent to the somatic
musculature. In J1, their nuclei are granulated and do not contain visible
nucleoli. During larval development, the cytoplasm of the coelomocytes acquires both granules (of high refractive index) and
vacuoles (of low refractive index), giving these cells a very characteristic
appearance. In the J1 destined to develop into hermaphrodite, four coelomocytes are located subventrally between the pharynx
and the genital primordium; the two on the right are anterior to the two on the
left. In the male, one of the left coelomocytes is located posterior to the gonad primordium. Weinstein [9, 37]
studied the morphology and differentiation of the pseudocoelomocytes present in
the body cavity of the second rhabditiform stage and the exsheathed third stage
of N. brasiliensis. He reported four linearly arranged
pseudocoelomocytes to be present in the newly hatched larva from the base of the
pharynx to the genital primordium. They were located subventrally: the anterior
two (C1 and C2) on the right side of the ventral nerve cord, the posterior two
(C3 and C4) on its left side. Another marked difference reported between the
free-living rhabditiform and infective third-stage larva was the coloration of
pseudocoelomocytes that changed from colorless to pink-rose hue. During the
parasitic cycle the pseudocoelomocytes soon lost their pigmentation and became
colorless.Weinstein [53] further analyzed the formation and
differentiation of pseudocoelomocytes in the same nematode, N. brasiliensis. In
the first-stage larva, 7 single seam cells appeared in tandem in the hypodermis
on both the right and left sides. The seam cells 1–5 and 7 were reported to
undergo two divisions and the resultant quartets maintained equal spaces.
However, seam cell 6 underwent an unusual series of divisions resulting in the
formation of a huge amoeboid nurse cell enclosing a quartet of small cells in a vacuole. Finally, all the seam
cells along with nurse cells regressed and disappeared, except for the quartet cells that were released
from their vacuole. The latter remained inactive during the life of the free-living stages. By the
time, the 5th and 6th coelomocytes aligned themselves closely to seam cells 6 and their progeny; some were
attached with or penetrated the nurse cells at the level of the vacuole. The coelomocytes usually
were located subdorsal and posterior to the genital primordium, C5 on the right and C6 on the left.At the time of the second molt, tiny vesicles appeared in
the cytoplasm of coelomocytes 5th and
6th in the early third
stage infective larva ofN. brasiliensis. An increasing
number of vesicles aggregated into a mass at either the anterior or posterior
pole of the cells. Of the six coelomocytes, 1–4 situated anterior to the
genital primordium contained larger numbers of vesicles that concentrated extraordinary
amounts of vitamin B12,
which was recognized as a red pigment. However, the number of vesicles remained low and the 5th and 6th coelomocytes looked colorless with absence of
red pigment.


## 5. Functions of Coelomocytes

The function of coelomocytes in different groups
of invertebrates has been investigated through various experiments. The immune
system of the deuterostome, purple sea urchin (Strongylocentrotus purpuratus), is the most studied among invertebrates where numerous
coelomocytes wander in the coelomic cavity [38]. About two-thirds of
them are phagocytic, and the rest are vibratile cells, and colorless or red
spherule cells. They tend to accumulate at sites of injury and form cellular
clots to clear bacteria and other foreign substances from the coelomic cavity,
and participate in allograft rejection [38, 58, 59].The population of echiuran wormUrechis caupo was compared by Arp et al. [60] at two sites in California,
US, namely, Elkhorn Slough (a high-density site) and Bodega Bay (a high-sulfide
site). The population at the latter site had greater concentrations of hematin,
a nonglobin heme compound, in the coelomocytes, and exhibited a greater
tolerance to sulfide in the laboratory. Therefore, it was concluded that
hematin in the coelomocytes worked as a sulfide-detoxifying agent thus
enhancing the survival rate of the worm.Haug et al. [61] found that coelomocytes from the European common sea star Asterias rubens responded to trauma stress. The repair phase was observed after
the first 24 hours postamputation of the arm tip, by modulating over the time
the total number of circulating coelomocytes. Mangiaterra and Silva [62], through in vitro and in vivo studies, confirmed the
endocytic activity of free phagocytic amoebocytes (coelomocytes) in the sea
urchin that showed an ability to respond to an inflammatory stimulus.The coelomocytes of the annelidE. fetida were
classified into four major categories based on cytomorphology and cytochemistry-acidophils, basophils,
chloragocytes cells, and neutrophils [63]. The enzyme acid phosphatase
was present in all coelomocytes, but was especially abundant in basophils and neutrophils. Alkaline
phosphatase was detected in basophils, while basophils and neutrophils actively killed the tumor
target, with K562 reflecting their role in the earthworm's immune system.Engelmann et al. [35] further demonstrated that
earthworm innate immunity depended upon small and large leukocytes
(coelomocytes) that synthesized and secreted humoral antimicrobial molecules (e.g., lysenin, fetidin,
eiseniapore, coelomic cytolytic factor (CCF), and Lumbricin I).
Coelomocytes contain several lysosomal enzymes involved in phagocytosis and a
pattern recognition molecule (CCF) that may trigger the prophenoloxidase
cascade, a crucial innate immune response. Earlier, Field et al.
[64] studied the correlation between number of PO (Prophenoloxidase)-active coelomocytes and infection level of the parasitic
protozoan Monocystis sp. in the earthworm Lumbricus
terrestris that reflected a positive relationship. Such system has also
been found to operate in crustaceans [65].Even the coelomocytes ofLumbricus terrestris responded
to an increase in environmental osmotic pressure from isotonic conditions (170 mOsm) to hypertonic
conditions (715 mOsm) by changing from a round/petalloid morphology to a filopodial
morphology. Earthworms also show a change in number of coelomocytes in response
to pollutant exposure and the normal 1:1 ratio of coelomocytes to eleocytes is
disturbed with a rise in coelomocytes to 62%, 65%, and 69%, respectively after
Cd, Cu, and Zn treatments [1]. In the earthworm Lumbricus
rubellus, the polluted conditions affected the behavior of
coelomocytes where copper and several other contaminants were found to impair
the retention of a dye (Neutral Red) by lysosomes from coelomocytes [66]. Membrane permeability and stability are crucial factors
when such soil invertebrates are exposed
to freezing temperatures or desiccation thus causing substantial dehydration of
the cells.Another
role attributed [63] to the coelomocytes in
annelids is the homeostasis. The brown bodies in the coelomic cavity are formed
from an aggregation of coelomocytes around offending foreign cells such as
bacteria, gregarines, incompatible graft fragments, and altered self-structures
or necrotic muscle cells.The
function of pseudocoelomocytes in the nematodes has been debatable. Nassonov
[47, 48] suggested
that these cells were part of the excretory system; however, Hurlaux [10]
showed a definite
discontinuity between the excretory system and the coelomocytes. Nevertheless,
Maggenti [67]
suggested an excretory function in Plectus spp.
These cells have been proposed to play a scavenger
role, based on their ability to continuously endocytose low molecular weight
dyes and proteins
(e.g., GFP) and to extra cellularly accumulate larger materials from the body
cavity [28, 68]. Because of their ability to
take up a variety
of molecules from the body cavity fluid, these cells have been suggested to act
as scavenger cells
[69]. The plasma membrane of coelomocytes shows active
endocytosis, with
multiple “omega-figures” lying in close proximity to one another [55]. In C.elegans, coelomocytes have been shown to uptake various substances
from pseudocoelom such as india  ink, rhodamine-dextran, fluorescein isothiocyanate (FITC)-BSA, and FITC- lipopolysaccharide from S. typhimurium [68]. Under
exceptional cases, coelomocytes were induced
to take up GFP-tagged yolk particles, apparently due to the presence of the GFP
moiety [70]. This scavenging behavior may
form a part of a primitive immune surveillance function. However, this activity
does not seem to be essential for the animal's survival or fertility as nematodes with toxin-ablated coelomocytes continue to grow
and bear progeny [68]. Peregrine [12], Poinar and Jansson [33],
and A. F. Bird and J. Bird [34] have suggested their role in secretory and endocrine
functions or processes and in phagocytic, immunologic, and denitrifying activities.
In some instances, they have been thought to have a secretory function or a
respiratory role in the animal parasitic species [11] although
there is little direct evidence for either function. The phagocytic function
demonstrated for coelomocytes is their capacity to accumulate extraordinary
amounts of vitamin B12 (an intensely red pigment) in animal parasitic strongylids
and trichostrongylids [29, 71]. Oya and Weinstein
[72] demonstrated that vitamin B12 is converted into metabolically active
coenzyme from adenosylcobalamin in A. suum.
These cells were also suggested to function in the synthesis and secretion of a protein product. In A. suum the
accumulated products may represent either a product of the cell and/or material taken up by the coelomocytes.
Coelomocytes in larger nematodes may take up dyes (methylene blue, crystal
violet) and even injected bacilli or invading organisms [28, 46].Turpeenniemi
[40], after studying the ultrastructure of the coelomocytes in S.
gracilis, identified them to be absorptive cells. They were found to contain cell
organelles, with the characteristics of lysosomes and peroxisomes and also contained crystalloids.
These cell organelles were investigated using enzyme histochemical analyses and
the results indicated that they contained both catalase and acid phosphatase
and, therefore, were neither peroxisomes nor lysosomes [73]. These organelles were described as coelomocyte organelles
(CC-organelle). Both CC-organelles and peroxisomes indicated presence of urate
oxidase [73–76], an enzyme that catalyzes the oxidation
of uric acid to allantoin [77]. The studies further concluded
CC-organelles to be associated with formation of vacuoles that contained
catalase enzyme. Turpeenniemi
[40] also observed the body-cavity fluid to be transported in
pinocytotic vesicles to the vacuoles and suggested the role of these cells in purine degradation and
as important centers of intermediary metabolism in nematodes.Earthworms and other invertebrates possess natural,
nonspecific, nonclonal, and nonanticipatory immune response influenced by germ line genes [35]. Coelomocytes of individual sea urchins express multiple scavenger receptor
cysteine-rich (SRCR) genes from among a multigene family in diverse
transcription profiles. Various immunological assays revealed presence of
special proteins in different vertebrates. Nevertheless, the invertebrates are
also reported to possess two coelomocyte proteins homologous to factor B and
C3/C4/C5 complement proteins in vertebrates [59, 78, 79]. Coelomocytes displayed
transcriptional response to injury [80]. TOLL and
TOLL-like receptor signaling is essential for phagocytosis and antimicrobial
peptide synthesis and secretion in insects and vertebrates. Dewilde et al. [81]
stated that Mb1-like tissue globins occur intracellularly in circulating
“coelomocytes” and extracellularly dissolved in the haemolymph in molluscs and
annelids.The nematodeC. elegans has
recently been used as an attractive model system [82, 83] to gain insight
into mechanisms of immunity in invertebrate organisms. However, there appears a
lineage-specific loss of innate immune mechanisms in C. elegans [84]. The hypothesis is supported by the fact
that although its genome encodes a number of homologs of Toll pathway components, these genes appear not to contribute directly to immunity
[85, 86].
The nematode is found to be
equipped with the antimicrobial and digestive peptides, a transforming growth
factor b-like pathway, an insulin receptor-like pathway, a programmed cell death (PCD)
pathway, and finally three MAP kinase pathways (p38 MAP kinase). These pathways play
important roles in various developmental processes and interact with each other and most of them also
respond to stress conditions, suggesting that the nematode uses elements of stress response as part
of its inducible immune defense against pathogens. Interestingly, nematodes appear to be free from
diseases caused by viruses possibly due to the reason that the worms may use RNAi (RNA interference) as an antiviral
defense [87].As many parts of the more complex innate systems of insects
and vertebrates show enormous similarities (e.g., components of cellular defenses or the
Toll pathways), these are likely to have a single origin. InC. elegans the
coelomocytes may serve certain immune or hepatic functions.However, unlike immune cells in other organisms, the
coelomocytes are not migratory; rather, they are attached to the hypodermis, and would rely on the
movement of the animal to bring foreign agents into their proximity. These cells, however, show high
endocytotic activity, and may still contribute to immunity by supporting detoxification
processes [68].A combination of forward and reverse genetics has identified
a number of new membrane trafficking factors. Most of them have mammalian homologues which
function in the same transport events.SAND genes have been found to be of special importance in
this connection. They are present in major eukaryotic taxa, with two SAND genes in vertebrate
species. SAND proteins are ancient and belong to an absolutely distinct protein family. Poteryaev
and Spang [88] described a gene sand-1, the malfunction of which causes profound endocytic defects
in many tissues of C. elegans. The most conspicuous feature of sand-1 (or 552)
mutants is the presence of large intracellular membrane-bounded granules in the
early embryo. In coelomocytes, this led to accumulation of extremely large vacuoles
of endocytic origin.


## 6. Conclusions

The invertebrate immune mechanisms can be varied and diverse
with the different key players being the antimicrobial peptides, RNA interference,
phagocytic cells/cytolytic cells, lysozymes, production of toxic oxygen, and nitrogen
metabolites or the most advanced defense mechanisms utilizing the C3 and Toll receptors. The
interaction between invertebrate model hosts and pathogens provides insights into the
mechanisms of pathogen virulence and host immunity, and complements the use of mammalian models
by enabling whole-animal high throughput infection assays [89].
Each alternative model system has advantages and disadvantages, which emphasize the need to
use many models to understand the mechanisms by which pathogens manipulate the innate
immune system to cause diseases.D. melanogaster and C. elegans have been used successfully to study microbial pathogenesis and defense responses. However, there are some
limitations as both cannot survive at 37°C and the inoculation dose of antimicrobial
substances is technically challenging in these systems [90]. In addition, there are many differences in the innate immune systems of invertebrates
and vertebrates, for example, the recognition mechanism by Toll receptors seems to be lacking
in C. elegans. Despite the differences, C. elegans is
being used as a model system  [91] due to extensive genetic and molecular information available facilitating an in-depth
analysis of host defense factors and pathogen virulence factors. Many of these factors are
conserved of insects and mammals, indicating the relevance of the nematode model for
vertebrate innate immune responses. The nematode has been found to exhibit protective responses to a
variety of fungal and bacterial pathogens; however, it lacks the cellular arm of innate
immunity in its classic form. Although coelomocytes are present (five in males and six in
hermaphrodites), but are not involved in phagocytosis or encapsulation of bacteria as seen in diverse
invertebrate species.Coelomocytes also regarded as nonmobile scavenger cells
continuously and nonspecifically endocytose fluid from the pseudocoelom (body cavity)
although toxin-mediated ablation of coelomocytes indicated that endocytosis is not essential for
growth or survival ofC. elegans under normal laboratory conditions. However, there exists
rather a complex link of intracellular signaling cascades contributing to the
antibacterial defenses of C. elegans [92]. Another indication is that
intercellular communication between different cell types plays a role in worm innate immunity.
Four signaling pathways involved in immune responses have been identified so far: the p38 MAP
kinase pathway, the programed cell death pathway, the TGF-β pathway, and the
DAF-2 insulin/IGF-I like signaling pathway. The signaling pathways and the effectors
produced depend on the type of infection, indicating the nematode’s ability to detect
and distinguish between infecting micro-organisms. ILR (insulin-like receptors) pathway plays
an important role into both resistance and avoidance behavior against a potentially
natural pathogen. ILR signaling also contributes to physiological resistance against diverse
stressors including heavy metals, hypoxia [93], and so forth. However, the
molecules involved in recognition of pathogens or stresses are yet to be identified. The various
proteins encoded in genome, such as those with C-type lectin domains, may have this
recognition function.Recently, coelomocytes have been used in pollution studies
for detection of stress [94]. Coelomocytes in earthworms exposed to heavy metals
exhibited significant impairment of pinocytosis and plastic adherence [95]. Nematodes of the
ideal bioindicators, in view of their ubiquity and hardy nature, can be tested for such
studies to detect stress at cellular as well as molecular levels and its impacts on coelomocytes, if any.
Although the understanding of C. elegans defenses is developing rapidly, future work is needed to
analyze how infection is perceived by the nematode, what are the types of responses
triggered by different pathogens, or by stress, despite the fact that they involve linked signaling
cascades. The tractability and relative simplicity of C. elegans are important to understand the complexity of
conserved innate immune defenses, and to explain the role of cellular stress in response to
infection.



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*Source: 218197-2009-02-10.xml*