Trematodes nutrient absorption

The external surfaces of parasitic helminths, termed the tegument in cestodes and trematodes, and the cuticle in nematodes, are adapted to serve a wide range of biological functions. Though most research has focused on structural aspects or their roles in immune evasion (Chapter 16) and nutrient absorption, the external surfaces of helminths also serve important roles in locomotion, excretion and regulation of electrochemical and osmotic gradients. 

Its numerous pits and channels provide an enormous surface area for absorption, and the tegument is constantly exposed to a nutrient-rich environment. Many enzymes that function in amino acid absorption are located in the tegument, and some of these, such as leucine aminopeptidase, are not present in the gut. Trematodes possess an incomplete digestive tract, and both S. mansoni and F. hepatica can survive extended in vitro incubations in the absence of detectable nutrient absorption across the intestine (63,64). Glucose absorption in trematodes can be detected during immature stages of the life cycle, which lack an intestine (65). 

The trematode gastrodermis epithelium is probably the principal site of absorption for large-molecular-weight nutrients. These nutrients seem to be critical for growth and long-term survival and for egg production by females (88a). The mechanisms of absorption are uncharacterized. Endocytosis occurs to a very limited extent in F. hepatica, but there is no evidence for this process in schistosomes. There is no information available on the role of the trematode gastrodermis in the transport of inorganic ions or water. 

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