The Cestode Tegument
In this page the details of the tegument found in the Eucestodes is detailed. The related cestodarians that also belong within the cestodes, have a tegument that appears to be intermediate with that of the eucestodes and monogeneans. This is another piece of evolutionary evidence that indicates a monogenean origin for the tapeworms. In this case the surface of the cestodarian tegument is covered with numerous microvilli, similar in form to the eucestode microtriche (see below), but lacking the electron dense cap seen in these parasites.
The cesode tegument is a syncytial layer, showing many features typical of that found in other parasitic platyhelminthes.
The are however a number of distinguishing features present in these parasites. On the very outer surface of the tegument a surface glycocalyx is seen to cover the outer plasma membrane. Below this glycocalyx, a characteristic feature of the eucestode tegument are the presence of numerous microtriches (Mt), long spine like processes that are in fact a highly modified form of micovilli. Each microthrix has a hard, pointed, electron dense cap which is seperated from the rest of the microthrix by a crescent shaped membranous cap. The mirotriches are thought to serve two functions. Firstly, the tapeworms do not possess a gut and must absorb all of their nutrients across the surface tegument. The microtiches greatly increase the surface area of the parasite, and can be seen as an adaptation to maximise the amounts of nutrients available to the parasite. This is supported by the finding of microtubles in the shaft of the microtriches. Secondly the spine like character of the microtriches probably help the parasite maintain its position in the gut. This can more clearly seen by comparing the microtriches found in different regions of the parasites body. It has been noted in many species that the microtriches found covering the scolex, the attachment organ of the parasite, were much longer than those covering the strobila, and in some species show special adaptations. For example the microtriches covering the strobila of E. granulosus have been found to shown to be curved with hooks or sometimes even barbs.
Below the layer of microtriches the main syncytial layer of the tegument is found. This has been seen to contain numerous vesicles and membrane bound, electron dense rod-like structures, refered to as disc-shaped bodies (Db). Finally numerous mitochondria, mainly in the distal region of the tegument, may be seen. These are unusual in that they do not have many cristae, reflecting the anaerobic metabolism of the organism.
The tegumental nuclei are however not located in this outer layer, but are found within subtegumental cell bodies (StC), located beneath the circular (Cm) and longitudinal muscle (Lm) layers, embedded within the parenchymal tissues (P) and mesenchymal musculature (M). These subtegumental cell bodies also contain other cellular elements such as golgi apparatus and lipid inclusion bodies (L) of are connected to the outer syncytium and areas of glycogen storage (Gs)by long protoplasmic extensions (Pe). The location of these important cellular elements away from the outer surface of the parasite, exposed to immunological attack by the parasites host, is an important adaptation to a parasitic lifecycle adopted by all of the parasitic platyhelminthes. The parenchymal tissues are similar to those of the trematodes and fill the spaces between the parasites internal organs (all cestodes and other platyhelminthes being acoelomate organisms). These tissues are a syncytial network formed by anastomosis of mesenchymal cells, with spaces filled with carbohydrate rich parenchymal fluid.