Fasciola hepatica: basal infolds and associated vacuoles of the tegument.

The tegument of Fasciola hepatica has been shown to contain long invaginations of its basal plasma membrane, hereafter called basal infolds. Associated with the membranes of the infolds, and with the apical and basal plasma membranes, is a Na⁺ K⁺ ATPase (EC 3.6.1.3). Furthermore, polymorphic masses of acid mucopolysaccharide lie close to or against the sides of the infolds and the basal plasma membrane and also fill cytoplasmic tubules which connect the tegument with the tegumental cells. Fixation and incubation of flukes in hypertonic and hypotonic media have shown that the infolds respond to changes in external osmolarity by collapsing in the former and swelling in the latter. This is not simply a passive response, however, since the infolds return to near normal configuration and morphology within 1 hr in either hypertonic or hypotonic media, even though the whole fluke may be shrunken or turgid depending on medium osmolarity. The tegument, therefore, has many of the characteristics of a transporting epithelium. A theory outlining the possible mode of operation of the tegument as a transporting epithelium is proposed on the basis of the present structural, chemical, and physiological findings combined with current ideas of the role and functioning of standing gradients, and forward and backward channel systems in other transporting epithelia.