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. In cestodes the tegument is further covered with highly specialized
microvilli, called "
microtriches", projecting from the outer limiting membrane of the tegument. These microtriches are fine hair-like filaments distributed throughout the surface of the body, and give the body surface a smooth and silky appearance. Since cestodes are devoid of any
digestive and
excretory systems, the tegument with its microtriches constitute the principal site of absorption of
nutrients and elimination of waste materials. In fact the tegument highly resembles the
gut of
animals turned inside out. In trematodes the tegument contains a number of invaginations or surface pits, and is externally lined with minute
tubercles among which are dispersed bristle-like projections called "
spines". Spines are embedded in the basal lamina and the tip is finely pointed to the external surface. They are made up of
paracrystalline arrays of
actin filaments. Tubercles are numerous, fairly regularly arranged, rounded protuberances. ==Functions==