ULTRASTRUCTURE OF THE CELLS OF THE BOUNDARY LAYER OF SMALL VESSELS IN THE DIGESTIVE GLAND OF THE GASTROPOD PULMONATE EOBANIA VERMICULATA (MÜLLER)

SUMMARY We have described the vascular network of the digestive gland of Eobania vermiculata (Muller), with special reference to the ultrastructure of endothelial-like cells forming the capillary wall. These cells functionally correspond to elements of the normal endothelium but possess some exclusive ultrastructural features. The particular position of the cell nuclei and the relationship to the surrounding connective tissue (the loose perivascular connective tissue and interstitial vesicular cells) are shown. Irregular microvilli and pinocytosis vesicles are found on both inner and outer surfaces and oriented bundles of filaments occupy the central portion of the cytoplasm, anchoring to the plasmalemma. The hypothesis is advanced that these filaments are contractile in nature and, therefore, endothelial-like cells are retained capable of autoregulative processes, in order to adjust the calibre of the capillary lumen according to the rate of blood-tissue exchanges.