Histochemical and ultrastructural observations on feeding and digestion in Reighardia sternae (Pentastomida: Cephalobaenida)

Abstract

Histological, histochemical and ultrastructural methods have been used to study gut structure and function in the cephalobaenid pentastomid Reighardia sternae (Diesing, 1864). R. sternae feeds exclusively on blood. Haemolysis of ingested erythrocytes is thought to be initiated by non-specific esterase secreted by cells lining the posterior oesophagus. Only one other enzyme, acid phosphatase, was demonstrated; its activity is confined to the microvillar layer of the gastrodermis. Digestion appears to be largely extracellular, as large volumes of haematin form in the gut lumen after haemolysis. However, this is supplemented by a significant amount of intracellular digestion: accumulations of iron-positive granules occur in approximately 20% of gastrodermal cells. All gastrodermal cells are alike, and all can undertake the same intracellular digestive process. This is characterized by a number of morphologically distinct stages which have been studied using the electron microscope. The iron-positive particles distinguished under the light microscope were easily identifiable ultrastructurally. Spherical, weakly iron-positive bodies form within the cisternae of the endoplasmic reticulum. Phagosomes containing haemoglobin, or some breakdown product of haemoglobin, apparently bud off from the termini of a labyrinth system of tubules under the microvilli and fuse with the ER to form these phago-lysosomes. Intensely iron-positive heterolysosomes, also present in the cytoplasm of these cells, accumulate haemosiderin particles which are thought to be released into the cytoplasm during phago-lysosomal digestion. The ensuing complex changes in cell morphology are, apparently, related to the elimination of spent phago-lysosomes. It has been suggested that the purpose of intracellular digestion is to provide a specific metabolite which the extracellular degradation of haemoglobin cannot supply.