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Possible defensive mechanism of Hymenolepis diminuta cysticercoids to hemocytes of the beetle Tribolium confusum
JOURNAL
OF
INVERTEBRATE
Possible
PATHOLOGY
16, 310-312
(1976)
Defensive Mechanism of Hymenolepis diminuta Cysticercoids to Hemocytes of the Beetle
Tribolium Defensive mechanisms of insects to larval cestodes have been little studied. Generally the reaction of insects against foreign bodies involves phagocytosis by amoebocytes, nodule formation, encapsulation, and segregation (G. Salt, Parasitology 53, 527-642, 1963). The factors involved in successful transit of Hymenolepis diminuta oncospheres across the intestinal wall of Tribolium confusum have been discussed by M. Voge and M. Graiwer (J. Parasitol. 50, 267-270, 1964). After entry into the hemocoel of the insect the oncosphere encounters hemocytes possibly stimulated by metabolic products of the parasite (J. E. Larsh, G. J. Race, and G. IV. Esch, 1. Parasitol. 51, 4% 52, 1965). Little information exists concerning defensive mechanisms on the part of larval cestodes against these reactions by the host. An ultrastructural study of the cystic wall of H. diminuta larva developing in beetles, Tribolium confusum, has revealed a possible defensive mechanism on the part of larva for destroying the invasive hemocytes of the host. Eggs of H. diminuta were exposed to 8to lo-week-old Tribolium confusum. The beetles were then maintained for 8 days at 30” C after which they were dissected in cold fixative by separating the abdomen and head from the thorax. The cysticercoids were fixed in cold 4% paraformaldehyde and prepared for electron microscopy. The outer wall of the cysticercoid is a syncytium and contains numerous branched microvilli which are unlike surface projections described for other larval cestodes (Fig. 1). The microvilli of the cystic wall of H. diminuta are covered by a single unit membrane with an external filamentous 310
confwm coating (Inset A). The matrix within the villus is generally more dense than the distal cytoplasm of the syncytium and contains scattered filaments. The microvilli are branched, usually near the base although occasionally the branching occurs more terminally. The microvilli are distended at various levels along the main axis, and the distensions appear to pinch off the apical end. The resulting vesicle, probably containing secretory products, is membrane bound (Inset B ) and covered externally by a filamentous coat similar to that of the branched microvilli. Secretory vesicles are most numerous in regions of the body wall that are in close contact with the hemocytes of the host. The numerous vesicles associated with deteriorating amoebocytes suggest to us that they are probably responsible for lysis of the hemocytes and thus provide an active protective or defensive function for the microvillar surface of the cysticercoid. Lysis of the hemocyte results in complete disruption of all cellular organelles except mitochondria which appear to be resistant. Membranes and cell fragments concentrate at the opening of the anterior canal and various cell fragments and intact mitochondria can be observed throughout the canal and inner cavity of the cyst. Secretions from the tegument of several adult Platyhelminthes are suggested to afford resistance to host digestive enzymes. A type of secretion, occasionally via branched microvilli, has been described in adhesive organs of trematodes where the secretion was believed to involve lysis of host tissue greatly facilitating the breakdown and uptake of nutrients by the para-
FIG. 1. Electron micrograph from anterior canal of outer capsule wall (H!/menoZepk diminnta). The distal cytoplasm (C), which supports the branched microvilli, rests upon a basal lamina ( BL). Three hemocytes of insect origin are in various states of decay, presumably from material (S) released by the branched microvilli. Inset A shows the structure of the microv;illi under higher magnification. Note thrsir repeated branching and the filamentous coat. Inset B shows the origin of a filamentous coated vesicle ( V ) from the tip of a microvillous and a larger vesicle in close proximity to the limiting membrane of the hemocyte. Note the various breaks (F) in the membrane of the hemocyte. Fragments of the hemocytes are found throughout the anterior canal and larval chamber. X9,450, Inset ,4 X57,400, Inset B x 20.000.
site (D. A. Erasmus and C. Ohman, Ann. N.Y. Acad. Sci. 113, 7-35, 1963). Large terminal vesicles at the tips of microvilli have been reported for Tylocephalum metacestodes by E. R&in, T. C. Cheng, and H. R. Hohl (J. Morphol. 130, 11-24, 1970). Since these vesicles also were pinched off, a secretory mechanism was suggested which functioned to prevent intimate body contact and keep the host fluids in a state of flux permitting more uniform uptake of nutrient materials. External coats of the cell membrane h ave been regarded as a defensive mech-
anism against host enzymes by acting as a barrier to large particles (L. Monne, A&iv. Zool. 12, 343358, 1959). The greatly increased area of coated membrane exposed to the environment due to the branching of the microvilli suggests an enhancement of this type of defense mechanism. As has been suggested for Tylocephalum metacestodes, the bands of circular muscles which underlie the surface of the cystic wall of H. diminuta probably cause the formation of surface undulations by contracting. In response, the microvillar border, which is of definite shape but probably Aes-
312
NOTES
ible, facilitates the sweeping of materials down the anterior canal. Amoebocytes in various states of breakdown as well as fragments of cytoplasm and other substances from which nutrients may be derived are observed along the anterior canal which opens into the larval chamber. A rhythmic contraction could explain the movement of nutrients through the canal toward the developing metacestode. Although the presence of branched microvilli with secretory ability suggests a defensive mechanism on the part of the
cysticercoid, the origin and ultimate disposition of the branched microvilli as well as the exact nature and ultimate function of the secretion remain to be determined. JOHN E. UBELAKER NANCY B. COOPER v. F. ALLISON
Department of Biology Southern Methodist University Dallas, Texas 75222 Received May 28, 1970
OF
INVERTEBRATE
Possible
PATHOLOGY
16, 310-312
(1976)
Defensive Mechanism of Hymenolepis diminuta Cysticercoids to Hemocytes of the Beetle
Tribolium Defensive mechanisms of insects to larval cestodes have been little studied. Generally the reaction of insects against foreign bodies involves phagocytosis by amoebocytes, nodule formation, encapsulation, and segregation (G. Salt, Parasitology 53, 527-642, 1963). The factors involved in successful transit of Hymenolepis diminuta oncospheres across the intestinal wall of Tribolium confusum have been discussed by M. Voge and M. Graiwer (J. Parasitol. 50, 267-270, 1964). After entry into the hemocoel of the insect the oncosphere encounters hemocytes possibly stimulated by metabolic products of the parasite (J. E. Larsh, G. J. Race, and G. IV. Esch, 1. Parasitol. 51, 4% 52, 1965). Little information exists concerning defensive mechanisms on the part of larval cestodes against these reactions by the host. An ultrastructural study of the cystic wall of H. diminuta larva developing in beetles, Tribolium confusum, has revealed a possible defensive mechanism on the part of larva for destroying the invasive hemocytes of the host. Eggs of H. diminuta were exposed to 8to lo-week-old Tribolium confusum. The beetles were then maintained for 8 days at 30” C after which they were dissected in cold fixative by separating the abdomen and head from the thorax. The cysticercoids were fixed in cold 4% paraformaldehyde and prepared for electron microscopy. The outer wall of the cysticercoid is a syncytium and contains numerous branched microvilli which are unlike surface projections described for other larval cestodes (Fig. 1). The microvilli of the cystic wall of H. diminuta are covered by a single unit membrane with an external filamentous 310
confwm coating (Inset A). The matrix within the villus is generally more dense than the distal cytoplasm of the syncytium and contains scattered filaments. The microvilli are branched, usually near the base although occasionally the branching occurs more terminally. The microvilli are distended at various levels along the main axis, and the distensions appear to pinch off the apical end. The resulting vesicle, probably containing secretory products, is membrane bound (Inset B ) and covered externally by a filamentous coat similar to that of the branched microvilli. Secretory vesicles are most numerous in regions of the body wall that are in close contact with the hemocytes of the host. The numerous vesicles associated with deteriorating amoebocytes suggest to us that they are probably responsible for lysis of the hemocytes and thus provide an active protective or defensive function for the microvillar surface of the cysticercoid. Lysis of the hemocyte results in complete disruption of all cellular organelles except mitochondria which appear to be resistant. Membranes and cell fragments concentrate at the opening of the anterior canal and various cell fragments and intact mitochondria can be observed throughout the canal and inner cavity of the cyst. Secretions from the tegument of several adult Platyhelminthes are suggested to afford resistance to host digestive enzymes. A type of secretion, occasionally via branched microvilli, has been described in adhesive organs of trematodes where the secretion was believed to involve lysis of host tissue greatly facilitating the breakdown and uptake of nutrients by the para-
FIG. 1. Electron micrograph from anterior canal of outer capsule wall (H!/menoZepk diminnta). The distal cytoplasm (C), which supports the branched microvilli, rests upon a basal lamina ( BL). Three hemocytes of insect origin are in various states of decay, presumably from material (S) released by the branched microvilli. Inset A shows the structure of the microv;illi under higher magnification. Note thrsir repeated branching and the filamentous coat. Inset B shows the origin of a filamentous coated vesicle ( V ) from the tip of a microvillous and a larger vesicle in close proximity to the limiting membrane of the hemocyte. Note the various breaks (F) in the membrane of the hemocyte. Fragments of the hemocytes are found throughout the anterior canal and larval chamber. X9,450, Inset ,4 X57,400, Inset B x 20.000.
site (D. A. Erasmus and C. Ohman, Ann. N.Y. Acad. Sci. 113, 7-35, 1963). Large terminal vesicles at the tips of microvilli have been reported for Tylocephalum metacestodes by E. R&in, T. C. Cheng, and H. R. Hohl (J. Morphol. 130, 11-24, 1970). Since these vesicles also were pinched off, a secretory mechanism was suggested which functioned to prevent intimate body contact and keep the host fluids in a state of flux permitting more uniform uptake of nutrient materials. External coats of the cell membrane h ave been regarded as a defensive mech-
anism against host enzymes by acting as a barrier to large particles (L. Monne, A&iv. Zool. 12, 343358, 1959). The greatly increased area of coated membrane exposed to the environment due to the branching of the microvilli suggests an enhancement of this type of defense mechanism. As has been suggested for Tylocephalum metacestodes, the bands of circular muscles which underlie the surface of the cystic wall of H. diminuta probably cause the formation of surface undulations by contracting. In response, the microvillar border, which is of definite shape but probably Aes-
312
NOTES
ible, facilitates the sweeping of materials down the anterior canal. Amoebocytes in various states of breakdown as well as fragments of cytoplasm and other substances from which nutrients may be derived are observed along the anterior canal which opens into the larval chamber. A rhythmic contraction could explain the movement of nutrients through the canal toward the developing metacestode. Although the presence of branched microvilli with secretory ability suggests a defensive mechanism on the part of the
cysticercoid, the origin and ultimate disposition of the branched microvilli as well as the exact nature and ultimate function of the secretion remain to be determined. JOHN E. UBELAKER NANCY B. COOPER v. F. ALLISON
Department of Biology Southern Methodist University Dallas, Texas 75222 Received May 28, 1970