Tegument (helminth)

Tegument ˈtɛɡjʊmənt is a term in helminthology for the outer body covering of members of the phylum Platyhelminthes. The name is derived from a Latin word tegumentum or tegere, meaning "to cover". It is characteristic of flatworms including the broad groups of tapeworms and flukes. Once considered to be a non-living component, it is now known to be a dynamic cellular structure. In fact it is a living structure consisting of proteins, lipids, carbohydrates and RNA. It forms the protective layer and the host-parasite interface of the worms, serving both secretory and absorptive functions. The fine structure of tegument is essentially the same in both the cestodes and trematodes. A typical tegument is 7-16 μm thick, with distinct layers. It is a syncytium consisting of multinucleated tissues with no distinct cell boundaries. The outer zone of the syncytium, called the "distal cytoplasm," is lined with a plasma membrane. This plasma membrane is in turn associated with a layer of carbohydrate-containing macromolecules known as the glycocalyx, that varies in thickness from one species to another. The distal cytoplasm is connected to the inner layer called the "proximal cytoplasm", which is the "cellular region or cyton or perikarya" through cytoplasmic tubes that are composed of microtubules. The proximal cytoplasm contains nuclei, endoplasmic reticulum, Golgi complex, mitochondria, ribosomes, glycogen deposits, and numerous vesicles. The internal most layer is bounded by a layer of connective tissue known as the "basal lamina". The basal lamina is followed by a thick layer of muscle. A large number of important enzymes has been detected in the tegument. Glutathione S-transferase, ATP diphosphorylase, alkaline and acid phosphatases, β-glucuronidase, amino peptidase, acetylcholine esterase, phosphofructokinase, glucose transporters, serine hydrolases and several glycolytic enzymes have been detected with their biological roles. The external surface of tegument is associated with unique defining structures in cestodes and trematodes.