70
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
tion occurs, with the capsule comprised of leucocytes and connective
tissue fibers (Fig. 11). Concurrent with the completion of encapsulation,
the larvae commence to atrophy and are eventually resorbed (Fig. 13).
Thus it would appear that in the unnatural host, encapsulation,
although delayed, does occur and the capsules, including the cells and
cell products involved, contribute in some yet unknown way to the
death and resorption of the parasites.
The origin and chemical nature of the encapsulating fibers surrounding Tylocephalum metacestodes in C. virginica have been studied by
Rifkin and Cheng (1967). We have found that the fibers, except for
those comprising the innermost layer, are not transformed from
leucocytes or fibroblasts but are concentrically deposited thickened
intercellular material originating from between the Leydig cells located
on the periphery of the parasite. These fibers are reticular rather than
collagenous and include glycoproteins and/or mucoproteins and neutral
mucopolysaccharides. The innermost layer of each capsule is comprised of fibroblast-like cells which possess a y-metachromatic and acid
mucopolysaccharide-rich matrix. The positive acid mucopolysaccharide
histochemical reaction is due to the occurrence of chondroitin sulfate B.
The origin of these cells is uncertain. It is also uncertain as to what
stimulates the intracellular material between surrounding Leydig cells
to thicken and become fibrous or what attracts leucocytes to migrate
into the area of the fibrous capsule. It has been noted that extremely
thin periodic acid-Schiff-positive strands are commonly found connecting the surface of each metacestode with the surrounding capsule.
These strands may represent some parasite-secreted material which
serves as the stimulatory agent for the host tissue reactions. Direct
proof of this, however, is currently unavailable.
Another interesting aspect of this study is that the capsules surrounding metacestodes situated in the zone surrounding the oyster’s
alimentary tract are consistently thicker than those surrounding
metacestodes located in the interdiverticular spaces of the digestive
gland. Since it has been shown that most of the fibers forming each
capsule originate from intercellular reticular fibers, one possible
explanation for this difference in capsule thickness is that there is less
Leydig tissue in the digestive gland than in the area surrounding the
alimentary tract, and hence less intercellular material. This interpretation lends further support to our earlier contention (Cheng and
Cooperman, 1964; Cheng et al., 1966a) that the presence or absence of
fibers in molluscan encapsulating cysts is dependent on the availability
and nature of the surrounding host tissues.
For
Other instances of encapsulation in molluscs are known.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
tion occurs, with the capsule comprised of leucocytes and connective
tissue fibers (Fig. 11). Concurrent with the completion of encapsulation,
the larvae commence to atrophy and are eventually resorbed (Fig. 13).
Thus it would appear that in the unnatural host, encapsulation,
although delayed, does occur and the capsules, including the cells and
cell products involved, contribute in some yet unknown way to the
death and resorption of the parasites.
The origin and chemical nature of the encapsulating fibers surrounding Tylocephalum metacestodes in C. virginica have been studied by
Rifkin and Cheng (1967). We have found that the fibers, except for
those comprising the innermost layer, are not transformed from
leucocytes or fibroblasts but are concentrically deposited thickened
intercellular material originating from between the Leydig cells located
on the periphery of the parasite. These fibers are reticular rather than
collagenous and include glycoproteins and/or mucoproteins and neutral
mucopolysaccharides. The innermost layer of each capsule is comprised of fibroblast-like cells which possess a y-metachromatic and acid
mucopolysaccharide-rich matrix. The positive acid mucopolysaccharide
histochemical reaction is due to the occurrence of chondroitin sulfate B.
The origin of these cells is uncertain. It is also uncertain as to what
stimulates the intracellular material between surrounding Leydig cells
to thicken and become fibrous or what attracts leucocytes to migrate
into the area of the fibrous capsule. It has been noted that extremely
thin periodic acid-Schiff-positive strands are commonly found connecting the surface of each metacestode with the surrounding capsule.
These strands may represent some parasite-secreted material which
serves as the stimulatory agent for the host tissue reactions. Direct
proof of this, however, is currently unavailable.
Another interesting aspect of this study is that the capsules surrounding metacestodes situated in the zone surrounding the oyster’s
alimentary tract are consistently thicker than those surrounding
metacestodes located in the interdiverticular spaces of the digestive
gland. Since it has been shown that most of the fibers forming each
capsule originate from intercellular reticular fibers, one possible
explanation for this difference in capsule thickness is that there is less
Leydig tissue in the digestive gland than in the area surrounding the
alimentary tract, and hence less intercellular material. This interpretation lends further support to our earlier contention (Cheng and
Cooperman, 1964; Cheng et al., 1966a) that the presence or absence of
fibers in molluscan encapsulating cysts is dependent on the availability
and nature of the surrounding host tissues.
For
Other instances of encapsulation in molluscs are known.