ULTRASTRUCTURAL STUDIES ON THE FORM AND FUNCTION OF THE GASTRODERMIS OF PROTOPOLYSTOMA XENOPI (MONOGENOIDEA: POLYOPISTHOCOTYLEA)

¹ Department of Pure and Applied Zoology, University of Leeds, England, U. K.

1. Ultrastructural studies reveal that the cecal epithelium of the monogenean Protopolystoma xenopi is composed of two cell types. Hematin-containing cells are discontinuously arranged and alternate with a thin cytoplasmic layer, the connecting syncytium.

2. The connecting syncytium forms a flat sheet overlying the cecal wall, it surrounds the individual hematin cells and is perforated only by their distal tips. The ultrastructure of the syncytium suggests that an active physiological role is unlikely.

3. The hematin cells are involved in the visible processes of digestion. After initial extracellular hemolysis the blood meal is absorbed by pinocytosis; intracellular breakdown continues within large membrane-bound vacuoles and leads to the accumulation of hematin; this insoluble product is eventually eliminated by the discharge of intact vacuoles, apparently by simple extrusion.

4. There is no evidence for the detachment or partial disintegration of the hematin-laden cells, and the cecal epithelium remains at all times a continuous structure.

5. Since the hematin cells are partially overlapped by the connecting syncytium, active absorption is limited to a relatively small area of hematin cell surface. A system of microtubules communicating between the free cell border and the hematin vacuole may participate in hemoglobin uptake and compensate for the reduced absorptive area. The exposed hematin cell border bears numerous cytoplasmic processes and these may facilitate contact digestion.

6. The structural and physiological organization of the polyopisthocotylean gastrodermis is characterised by the formation of dense iron-containing pigment within the relatively vulnerable digestive cells. It is considered that the connecting syncytium performs a skeletal role, giving support and protection to the hematin cells and the underlying tissues.