Further studies on the development of gamonts and oocysts of Eimeria magna in cultured cells

Monolayer, cell-line cultures of embryonic bovine trachea, Madin-Darby bovine kidney (MDBK), and monolayers (RK-1) or aggregates of primary rabbit kidney cells were inoculated with merozoites obtained from rabbits that had been inoculated 3 to 5 1/2 days earlier with Eimeria magna. Merozoites obtained from from rabbits 3 days entered cells and underwent only merogony, whereas 3 1/2-5 1/2-day-old merozoites formed gamonts as well as meronts. Merozoites arising from the first or second meront generation in culture formed another meront generation or gamonts. Third-generation merozoites formed only gamonts. Most merozoites remained within the parasitophorous vacuole of the original host cell and transformed into macro- or microgamonts or meronts. Some such macro- and microgamonts then fused with each other to form larger multinucleated bodies. Such microgamonts formed microgametes, but multinucleate macrogamonts did not form oocysts. Mature microgamonts were 34 microns in diameter, and contained several hundred biflagellate microgametes. Mature macrogamonts measured 29.1 x 21.5 microns, unsporulated oocysts were 31.2 x 22 microns, and sporulated oocysts were 32 x 23.1 microns. Oocysts obtained from cell cultures were sporulated and then inoculated by gavage into rabbits, which passed E. magna oocysts 6--10 days later. Sporozoites, obtained from oocysts produced in culture or from rabbits that had been inoculated with the vitro-produced oocysts, developed to first- and second-generation meronts in MDBK or RK-1 cultures.     

The endogenous stages of Eimeria utahensis (Protozoa: Eimeriidae) in the kangaroo rat, Dipodomys ordii

The endogenous life cycle of Eimeria utahensis is described from experimentally infected kangaroo rats, Dipodomys ordii. The endogenous asexual cycle consisted of 4 generations of meronts. First-generation meronts were concentrated in the anterior third of the small intestine. The succeeding generations of meronts and the sexual stages were concentrated in the middle third of the small intestine. First-generation meronts had a mean diameter of 9.7 micrometer and contained 12 to 16 merozoites. Second-generation meronts had a mean diameter of 8.0 micrometer and contained 12 to 16 merozoites and a residual body. Third-generation meronts had a mean diameter of 12.4 micrometer and contained 4 to 8 merozoites. Fourth-generation meronts had a mean diameter of 8.6 micrometer and contained 16 to 24 merozoites. Young gamonts were located in epithelial cells of the crypts of the small intestine. Shortly after the parasites entered the epithelial cells, the infected cells became displaced into the lamina propria, and most of the mature gamonts were in this location. The nuclei of host cells containing young sexual stages became greatly elongated and flattened. A few young gamonts were seen in cells in which the host cell nuclei were dividing. During development, nuclei of microgamonts became arranged on the periphery of numerous compartments. Only one type of wall-forming body could be distinguished in the macrogamonts.     

Establishment of Besnoitia darlingi from opossums (Didelphis virginiana) in experimental intermediate and definitive hosts,propagation in cell culture,and description of ultrastructural and genetic characteristics

Besnoitia darlingi from naturally infected opossums (Didelphis virginiana) from Mississippi, USA, was propagated experimentally in mice, cats, and cell culture and was characterised according to ultrastructural, genetic, and life-history characteristics. Cats fed tissue cysts from opossums shed oocysts with a prepatent period of nine or 11 days. Oocysts, bradyzoites, or tachyzoites were infective to outbred and interferon-gamma gene knockout mice. Tachyzoites were successfully cultivated and maintained in vitro in bovine monocytes and African green monkey cells and revived after an 18-month storage in liquid nitrogen. Schizonts were seen in the small intestinal lamina propria of cats fed experimentally-infected mouse tissues. These schizonts measured up to 45 x 25 microm and contained many merozoites. A few schizonts were present in mesenteric lymph nodes and livers of cats fed tissue cysts. Ultrastructurally, tachyzoites and bradyzoites of B. darlingi were similar to other species of Besnoitia. A close relationship to B. besnoiti and an even closer relationship to B. jellisoni was indicated for B. darlingi on the basis of the small subunit and ITS-1 portions of nuclear ribosomal DNA.     

Clinical coccidiosis in raccoons (Procyon lotor)

Clinical coccidiosis was diagnosed in wild-caught and captive raccoons. Eimeria procyonis-like oocysts were seen in 15 of 15 captive raccoons. In 6 of 6 juvenile wild-caught raccoons examined at necropsy, endogenous coccidian stages were seen in the small intestine. Two types of schizonts (large and small) were identified. Large schizonts were up to 110 microm long, contained 10-microm-long merozoites, and were in crypt glandular epithelial cells. Smaller schizonts were 10 microm long, contained 5-microm-long merozoites, and were at the tips of the villi. Only a few gamonts and no oocysts were seen in sections. These stages were thought to be of E. procyonis.     

Discovery of three novel coccidian parasites infecting California sea lions (Zalophus californianus), with evidence of sexual replication and interspecies pathogenicity

Enteric protozoal infection was identified in 5 stranded California sea lions (Zalophus californianus). Microscopically, the apical cytoplasm of distal jejunal enterocytes contained multiple stages of coccidian parasites, including schizonts with merozoites and spherical gametocytes, which were morphologically similar to coccidians. By histopathology, organisms appeared to be confined to the intestine and accompanied by only mild enteritis. Using electron microscopy, both sexual (microgametocytes, macrogamonts) and asexual (schizonts, merozoites) coccidian stages were identified in enterocytes within parasitophorous vacuoles, consistent with apicomplexan development in a definitive host. Serology was negative for tissue cyst-forming coccidians, and immunohistochemistry for Toxoplasma gondii was inconclusive and negative for Neospora caninum and Sarcocystis neurona. Analysis of ITS-1 gene sequences amplified from frozen or formalin-fixed paraffin-embedded intestinal sections identified DNA sequences with closest homology to Neospora sp. (80%); these novel sequences were referred to as belonging to coccidian parasites "A," "B," and "C." Subsequent molecular analyses completed on a neonatal harbor seal (Phoca vitulina) with protozoal lymphadenitis, hepatitis, myocarditis, and encephalitis showed that it was infected with a coccidian parasite bearing the "C" sequence type. Our results indicate that sea lions likely serve as definitive hosts for 3 newly described coccidian parasites, at least 1 of which is pathogenic in a marine mammal intermediate host species.     

In vivo action of the anticoccidial diclazuril (Clinacox) on the developmental stages of Eimeria tenella: an ultrastructural evaluation

A single 5-mg/kg oral dose of diclazuril affected both the asexual and sexual development of Eimeria tenella in experimentally inoculated chickens. In second-generation schizonts, early growth and nuclear divisions progressed normally, but a marked inhibition of merozoite formation was observed. Exogenesis of merozoites was largely prevented, whereas production of micronemes, amylopectin granules, and dense bodies and the formation of rhoptries, conoid, and pellicle continued. All these subcellular organelles accumulated, together with differentiated nuclei, within the main cytoplasmic mass. In the end, complete necrosis of the schizonts occurred. In macrogamonts, dilation of the rough endoplasmic reticulum around type II wall-forming bodies, fusion of type II wall-forming body contents, disturbance of the normal parallel arrangement of rough endoplasmic reticulum, and disruption of row formation of amylopectin granules became evident. In the microgamonts, normal evagination of microgametes was prevented; the flagellar complex formed within the main cytoplasmic mass and the differentiated nuclei remained present within the parasite body. The macro- and microgamonts also ended up in a stage of complete necrosis. These data indicate that diclazuril treatment primarily affects the normal differentiation of the respective endogenous stages during parasite development. This leads to complete degeneration of schizonts and gamonts indicating the lethal effect of this new anticoccidial compound.     

The life cycle of Eimeria falciformis var. pragensis (Sporozoa: Coccidia) in the mouse, Mus musculus

The life cycle of Eimeria falciformis var. pragensis, established from a single oocyst, is described in experimentally infected mice (Mus musculus). The coccidium had a prepatent period of 7 days and a patent period of 10--16 days. Oocysts were spherical to ellipsoidal in shape and measured 21.2 x 18.3 micron. Sporulation time was 3 to 3.5 days. Sporocysts measured 12.2 x 7.2 micron and contained a circular to avoid granular sporocyst residuum measuring 5.5 X 5.0 micron. One, 2 or 3 circular to rectangular polar granules were observed within each sporulated oocyst. The endogenous stages developed primarily in the cecum and colon and only occasionally in the lower ileum. Four generations of schizonts were found. Mature 1st-generation schizonts, first observed 48 hr postinfection (PI), measured 17.8 x 12.3 micron and had 12 merozoites that measured 13.3 x 2.0 micron. Mature 2nd-generation schizonts appeared 78 hr PI. They measured 10.2 x 9.3 micron and had 8 merozoites measuring 5.0 x 1.6 micron. Mature 3rd-generation schizonts appeared first at 114 hr PI and measured 17.5 x 10.2 micron and had 10 merozoites that measured 12.4 x 1.8 micron. Mature 4th-generation schizonts appeared first at 144 hr PI. They measured 18.2 x 15.3 micron and had 18 merozoites. The merozoites of the 4th-generation schizont were 4.5 x 1.2 micron. Mature macrogamonts and microgamonts developed simultaneously appearing at 156 hr PI. Macrogamonts measured 16 x 14.5 micron and microgamonts were 18.2 x 15.3 micron. In experimentally infected rats (Rattus norvegicus), development of E. falciformis var. pragensis progressed only as far as mature 1st-generation schizonts.     

Life Cycle of Eimeria ferrisi Levine & Ivens, 1965 in the Mouse, Mus musculus

SYNOPSIS. The life cycle of Eimeria ferrisi is described from experimentally infected Mus musculus. The prepatent period was 3 days and the patent period was 3–4 days. The endogenous stages were found only in the cecum and colon. Three generations of schizonts were found. Mature 1st-generation schizonts first seen 24 hr postinoculation (PI) measured 10.9 (7–14) × 10.2 (6–13) μm and had 9.6 (7–14) merozoites. Some 2nd-generation schizonts had uninucleate merozoites and others had multinucleate merozoites. The former were first seen in small numbers 36 hr PI and were most abundant 48 hr PI. They measured 9.6 (5–13) × 7.9 (6–12) μm and had 18 (6–25) merozoites. Schizonts with multinucleate merozoites were seen 72 hr PI. Mature 3rd-generation schizonts were seen 72 hr PI. They measured 14.0 (12–18) × 11-0 (9–13) μm and had 12.5 (5–16) merozoites. Macrogamonts were first seen in 72 hr sections. Each young macrogamont had a large nucleus with a prominent nucleolus. Only one type of cytoplasmic granule appeared to be involved in the formation of the oocyst wall. Mature macrogamonts were 11.0 (5–14) × 10.0 (6–13) μm. Crescent-shaped bodies were observed in the parasitophorous vacuole of trophozoites and young macrogamonts. Early microgamonts were first recognized at 96 hr by the presence of darkly stained and irregularly shaped nuclei. Usually, mature microgametes were arranged in long, narrow whorls at the periphery of the microgamont or in whorls at the surface of 2–5 compartments.     

Ultrastructural studies of microgametogenesis and macrogametogenesis of Eimeria truncata of the lesser snow goose

Microgamonts and macrogamonts of Eimeria truncata were observed in renal epithelial cells of collecting tubules and ducts and occasionally in macrophages of experimentally infected lesser snow geese (Anser c. caerulescens) beginning 8.5 days post inoculation. Intraparasitophorous vesicles in parasitophorous vacuoles of both types of gamonts appeared to originate in host cell cytoplasm and enter gamonts through micropores by budding of plasmalemma or by pinocytosis. Within the parasite's cytoplasm, the vesicles were broken down in Golgi-associated vacuoles. The surfaces of microgamonts were highly invaginated to facilitate extrusion of numerous microgametes. Formation and maturation of microgametes were similar to those of other eimerian species. Each microgamete had two flagella, a mitochondrion, and a peculiarly shaped electron-dense nucleus that was oval anteriorly in cross section and somewhat dumbbell-shaped posteriorly. A longitudinally arranged inner membrane complex lay between a portion of the mitochondrion and the plasmalemma. About five subpellicular microtubules extended the length of the microgamete body. Macrogametogony differed little from that described in other eimerian species. Type 1 wall-forming bodies (WFB) formed in Golgi complexes early in macrogametogony, and type 2 WFB formed in cisternae of endoplasmic reticulum in intermediate stages of macrogamont development.     

Clinical Toxoplasma gondii, Hammondia heydorni, and Sarcocystis spp. infections in dogs

Concurrent infections with coccidians Toxoplasma gondii, Sarcocystis spp., and a Hammondia heydorni-like parasite were identified in tissues of three littermate pups on a Kelpie dog breeding farm in Australia. In total, 20 pups in four litters had died following vaccination with an attenuated distemper virus vaccine. Toxoplasma gondii tachyzoites were identified immunohistochemically in tissues of two dogs. Sarcocystis sp. sporocysts were seen in the intestinal lamina propria of two dogs. Asexual and sexual stages of H. heydorni-like parasite were found in enterocytes of the small intestine of two dogs. Ultrastructural development of schizonts and gamonts of this parasite is described. None of the protozoa in these dogs reacted with antibodies to Neospora caninum. Feeding of uncooked tissue of sheep was considered to be the likely source of infection for these coccidians in dogs.     

Endogenous development of Eimeria minasensis in experimentally infected goats

The endogenous development of Eimeria minasensis was studied in 9 coccidia-free goat kids inoculated with 10(5) sporulated oocysts/kg body weight. Kids were killed 4, 7 (2 animals), 10, 13, 16, 18, 19, and 22 days after inoculation (DAI). In tissue sections of the intestines stained with hematoxylin and eosin and examined by light microscopy, 2 generations of meronts, gamonts, gametes, and oocysts were found. The first generation of meronts developed in cells deep in the lamina propria of the jejunum and ileum. Mature giant meronts (299.4x243.8 microm) found 16 DAI were visible to the naked eye and contained a large number of crescent-shaped merozoites. The second generation of meronts developed in the epithelial cells of crypts of the ileum and above the host cell nuclei. Mature meronts (11.5x10.1 microm) with 18-28 comma-shaped merozoites were first seen 16 DAI. Gametogenesis took place in epithelial cells of the crypts and villi of the terminal part of the ileum, cecum, and colon. Macrogametes (27.8x17.6 microm), mature microgamonts (21.3x17.0 microm), microgametes, and oocysts (30.5x19.4 microm) were found 19 DAI. Sexual stages were below the host cell nucleus.     

Ultrastructural Studies of Microgametogenesis and Macrogametogenesis of Eimeria truncata of the Lesser Snow Goose1

ABSTRACT. Microgamonts and macrogamonts of Eimeria truncata were observed in renal epithelial cells of collecting tubules and ducts and occasionally in macrophages of experimentally infected lesser snow geese (Anser c. caerulescens) beginning 8.5 days post inoculation. Intraparasitophorous vesicles in parasitophorous vacuoles of both types of gamonts appeared to originate in host cell cytoplasm and enter gamonts through micropores by budding of plasmalemma or by pinocytosis. Within the parasite's cytoplasm, the vesicles were broken down in Golgi-associated vacuoles. The surfaces of microgamonts were highly invaginated to facilitate extrusion of numerous microgametes. Formation and maturation of microgametes were similar to those of other eimerian species. Each microgamete had two flagella, a mitochondrion, and a peculiarly shaped electron-dense nucleus that was oval anteriorly in cross section and somewhat dumbbell-shaped posteriorly. A longitudinally arranged inner membrane complex lay between a portion of the mitochondrion and the plasmalemma. About five subpellicular microtubules extended the length of the microgamete body. Macrogametogony differed little from that described in other eimerian species. Type 1 wall-forming bodies (WFB) formed in Golgi complexes early in macrogametogony, and type 2 WFB formed in cisternae of endoplasmic reticulum in intermediate stages of macrogamont development.     

Ultrastructural Observations on the Gametocytic Stages of the Coccidium Tyzzeria chalcides Probert, Roberts & Wilson, 1988 from the Ocellated Skink Chalcides ocellatus

Gametogenesis of Tyzzeria chalcides Probert, Roberts & Wilson, 1988, from the ocellated skink, Chalcides ocellatus , occurs within the epithelium of the gali bladder. Transmission electron microscopy reveals that macrogamonts contain 2 types of wall-forming bodies. Type I bodies are large densely stained structures associated with rough endoplasmic reticulum and the Golgi apparatus. They appear to be formed within the Golgi itself. Type II bodies are less densely stained, smaller and appear to form directly from the rough endoplasmic reticulum. Canaliculi are associated with Type I wall-forming bodies and probably function to transport the wall-forming bodies to the pellicle. Micropores occur in the pellicie and large amylopectin granules, lipid globules and dense bodies are found within the cytoplasm of the macrogamont. Mature microgamonts contain in excess of 20 microgametes, each of which has 2 flagella and an associated mitochondrion. Both types of gamont are found within a parasitophorous vacuole, in the host cell, which is filled with vesicular material on which the gamonts probably feed.     

A Comparison of Endogenous Development of Three Isolates of Cryptosporidium in Suckling Mice1

ABSTRACT. Suckling mice were used as a model host to compare the endogenous development of three different isolates of Cryptosporidium: one from a naturally infected calf, one from an immunocompetent human with a short-term diarrheal illness, and one from a patient with acquired immune deficiency syndrome (AIDS) and persistent, life-threatening, gastrointestinal cryptosporidiosis. After oral inoculation of mice with oocysts, no differences were noted among developmental stages of the three isolates in their sites of infection, times of appearance, and duration, morphology, and fine structure. Sporozoites excysted within the lumen of the duodenum and ileum, penetrated into the microvillous region of villous enterocytes, and developed into type I meronts with six or eight merozoites. Type I merozoites penetrated enterocytes and underwent cyclic development as type I meronts or they became type II meronts with four merozoites. Type II merozoites did not exhibit cyclic development but developed directly into sexual forms. Microgamonts produced £16 small, bullet-shaped microgametes, which were observed attaching to and penetrating macrogametes. Approximately 80% of the oocysts observed in enterocytes had a thick, two-layered wall. After sporulating within the parasitophorous vacuole, these thick-walled oocysts passed through the gut unaltered and were the resistant forms that transmitted the infection to a new host. Approximately 20% of the oocysts in enterocytes consisted of four sporozoites and a residuum surrounded only by a single oocyst membrane that ruptured soon after the parasite was released from the host cell. The presence of thin-walled, autoinfective oocysts and recycling of type I meronts may explain why a small oral inoculum can produce an overwhelming infection in a suitable host and why immune deficient persons can have persistent, life-threatening cryptosporidiosis in the absence of repeated oral exposure to thick-walled oocysts.     

A comparison of endogenous development of three isolates of Cryptosporidium in suckling mice

Suckling mice were used as a model host to compare the endogenous development of three different isolates of Cryptosporidium: one from a naturally infected calf, one from an immunocompetent human with a short-term diarrheal illness, and one from a patient with acquired immune deficiency syndrome (AIDS) and persistent, life-threatening, gastrointestinal cryptosporidiosis. After oral inoculation of mice with oocysts, no differences were noted among developmental stages of the three isolates in their sites of infection, times of appearance, and duration, morphology, and fine structure. Sporozoites excysted within the lumen of the duodenum and ileum, penetrated into the microvillous region of villous enterocytes, and developed into type I meronts with six or eight merozoites. Type I merozoites penetrated enterocytes and underwent cyclic development as type I meronts or they became type II meronts with four merozoites. Type II merozoites did not exhibit cyclic development but developed directly into sexual forms. Microgamonts produced approximately 16 small, bullet-shaped microgametes, which were observed attaching to and penetrating macrogametes. Approximately 80% of the oocysts observed in enterocytes had a thick, two-layered wall. After sporulating within the parasitophorous vacuole, these thick-walled oocysts passed through the gut unaltered and were the resistant forms that transmitted the infection to a new host. Approximately 20% of the oocysts in enterocytes consisted of four sporozoites and a residuum surrounded only by a single oocyst membrane that ruptured soon after the parasite was released from the host cell. The presence of thin-walled, autoinfective oocysts and recycling of type I meronts may explain why a small oral inoculum can produce an overwhelming infection in a suitable host and why immune deficient persons can have persistent, life-threatening cryptosporidiosis in the absence of repeated oral exposure to thick-walled oocysts.     

Coccidiosis in wild grey kangaroos

Deaths occurred among wild grey kangaroos (Macropus giganteus) whose habitat was subject to flooding. A sample of the involved juvenile animals was hypoproteinaemic, and extensive haemorrhagic enteritis, associated with the presence of coccidial life cycle stages was found in the small intestine at necropsy. Masses of small schizonts infected cells of the lamina propria; large schizonts occupied clusters of hypertrophied cells; macrogametes and microgametocytes were observed in cells of the lamina propria. The species of coccidia involved could not be determined, although oocysts of Eimeria cunnamullensis and E. kogoni were found in faeces. Overcrowding, food shortage, damp conditions and possibly feed supplementation with hay on the ground were considered probable epizootiological factors contributing to an outbreak of coccidiosis among juvenile animals.     

Development of Eimeria meleagrimitis Tyzzer from sporozoites and merozoites in turkey kidney cell cultures

Sporozoites and 1st-, 2nd-, and 3rd-generation merozoites of Eimeria meleagrimitis were inoculated into primary cultures of turkey kidney cells. In vitro-excysted sporozoites developed into mature macrogamonts in 8 days; in vivo-excysted sporozoites developed into 2nd- or 3rd-generation schizonts within 5 to 7 days. First-generation merozoites obtained from infected turkeys produced mature 2nd-generation schizonts within 24 h. Second-generation merozoites from turkeys produced mature macrogamonts and oocysts within 72 h, whereas 3rd-generation merozoites produced these stages within 48 h. The oocysts that developed from 3rd-generation merozoites sporulated at 25 C and were infective for turkeys. The timing of the early stages and the intervals between schizogonic generations in cultures were comparable with those in turkeys. Morphologic parameters, however, indicated that some differences existed between in vitro and in vivo development. Second- and 3rd-generation schizonts and gamonts that developed after inoculation of cultures with merozoites were similar to stages in turkeys. Oocysts, however, were significantly smaller (P less than 0.05) in cultures. All stages that developed after inoculation of cultures with sporozoites were smaller (P less than 0.05) than their in vivo counter parts.     

The ultrastructure of gametogenesis of cryptosporidium baileyi (eimeriorina; cryptosporidiidae) in the respiratory tract of broiler chickens (Gallus domesticus).

The ultrastructural features of sexual development of Cryptosporidium baileyi in the respiratory tract of experimentally infected broiler chickens were studied using transmission electron microscopy. Sexual stages of C. baileyi were seen attached to the tracheal epithelium and free in the tracheal lumen. These stages included intracellular type III merozoite-like stages, microgamonts, microgametes, macrogamonts, thin-walled oocysts, and thick-walled oocysts. These stages were developmentally similar to those observed for other Cryptosporidium species. All of the above stages were observed during each study day. Thin-walled oocysts, microgamonts, and microgametes were seen less frequently than other sexual stages. Microgamonts, macrogamonts, and oocysts attached to the epithelium were all contained in a host cell membrane or within a parasitophorous vacuole. Thin-walled oocysts of C. baileyi were observed for the first time on an ultrastructural level in the respiratory tract of chickens.     

In vitro development from sporozoites to first-generation merozoites in Eimeria contorta Haberkorn, 1971. A fine structural study.

The asexual development of Eimeria contorta from sporozoites to first-generation merozoites in tissue culture was investigated with the electron microscope. Sporozoites with a three-layered pellicle, 26 subpellicular microtubules, a conoid, 4-7 rhoptries, and an abundance of micronemes actively entered host cells and showed direct contact to the host cell's cytoplasm. Shortly after penetration, small vacuoles surrounding the parasite merged into a parasitophorous vacuole. Inside this vacuole, sporozoites assumed a definite U-shape before transformation into schizonts took place. This process was characterised by the occurrence of subpellicular microtubules exclusively in the anterior half of the sporozoite, by a degeneration of the 2 inner pellicular membranes, by an outpocketing of the parasite's surface, and by the arrangement of microtubules in clusters. About 25 merozoites were formed at the surface of mature schizonts, to which they remained attached at their posterior pole. A polar ring was present at that area. Anterior and posterior refractile bodies were conspicuous in merozoites and showed close association with mitochondria. The significance of a fibrillar substructure in rhoptries and micronemes is discussed, and special attention is drawn to the pathway of nutrient transport from host cell mitochondria and dictyosomes through intravacuolar folds, parasitophorous vacuole and crescent body into the parasite's food vacuoles.     

Histochemical distribution of four types of enzymes and mucous cells in the gastrointestinal tract of reared half‐smooth tongue sole Cynoglossus semilaevis

The histochemical distribution of acid phosphatase (ACP), alkaline phosphatase (ALP), non‐specific esterase (NSE), peroxidase (POD) and mucous‐cell types was evaluated in the gastrointestinal tract of the half‐smooth tongue sole Cynoglossus semilaevis. The enzymes were detected in the entire stretch of the gastrointestinal tract. ACP activity was found in the supranuclear region of enterocytes and the lamina propria of the intestine, as well as the cytoplasm of epithelial cells of the stomach. The staining intensity of ACP in the anterior and posterior intestines was stronger than in the stomach. ALP activity was detected in the striated border of enterocytes and muscularis of the whole intestine, lamina propria and supranuclear cytoplasm of the enterocytes in the anterior intestine, as well as in the blood vessels of the stomach. The staining intensity for ALP in the anterior intestine was stronger than in the posterior segment and the latter was stronger than in the stomach. NSE activity was detected in the cytoplasm of the epithelial cells in the entire gastrointestinal tract, with the anterior intestine showing stronger intensity than the stomach. POD activity was located in the blood cells of the lamina propria of the gastrointestinal tract and the levels in the stomach were similar to the anterior and posterior intestines. Alcian blue (pH 2·5) periodic acid Schiff (AB‐PAS) histochemical results revealed three types of mucous cells in the gastrointestinal tract. Type I cells (PAS+AB‐) were observed among the gastric mucosa columnar cells in the stomach and enterocytes in the basal region of the villi and in the middle and top regions of the intestinal villi. Type II cells (PAS‐AB+) and type III cells (PAS+AB+) were not detected in the stomach but were distributed ubiquitously among enterocytes in the middle and top regions of the intestinal villi.