Fasciola gigantica: Ultrastructure of the Adult Tegument

Abstract

The tegument of adult Fasciola gigantica can be divided into four layers based on ultrastructural characteristics. The first layer includes ridges and pits which are covered by a trilaminate membrane about 8 nm thick, underlined by a dense lamina about 15 nm thick. The membrane is coated externally by the glycocalyx which consists of two layers: the inner dense homogeneous layer about 10-15 nm, and the outer fibrillar layer about 100-300 nm thick which is intensely stained with ruthenium red. The cytoplasm is composed of densely-packed microtrabecular network, and contains many ovoid granules (G1) whose size is about 90 x 180 nm, and numerous discoid granules (G2) whose size is about 40 x 250 nm. G1 contain dense ruthenium red-positive matrix while G2 contain translucent matrix, and both are surrounded by a trilaminate membrane. G1 close to the surface invariably exocytose their content into bottoms of the pits, while some G2 are fused and have their membrane joined up with the surface membrane. It is, therefore, suggested that G1 contribute to the formation of glycocalyx while G2 are the main contributor to the surface membrane. The second layer of the tegument is a narrow zone of cytoplasm that contains high concentrations of G1, G2 granules and lysosomes. The third layer is the widest middle portion of the tegument which contains numerous and evenly distributed mitochondria. Both G1 and G2 granules are present but in much fewer number than in the first and second layers. The fourth layer is the innermost zone that rests on and couples with the 120-140 nm thick basal lamina. Its cytoplasm is loosely packed and contains numerous infoldings of the basal plasma membrane which have mitochondria in close association. It contains fairly large numbers of G1 and G2 granules which are produced and transported to the tegument by one type of tegumental cells lying in rows underneath the muscular layers. Spines in the tegument are numerous, each is a wedge-shaped crystalline structure with the lattice spacing about 4 nm, and its rootlets are firmly implanted in the basal lamina.