Structural damage to the foot-sole epithelium of Bulinus africanus following exposure to a plant molluscicide

Adult Bulinus africanus were exposed to sublethal and lethal concentrations of the crude aqueous extract of the plant Agave attenuata for a 24-hour pe riod. Sublethal toxic effects included retardation of mobility, swelling of the cephalopedal mass, and haemorrhagic blistering in the subepithelium of the foot sole, while lethal concentrations resulted in a cessation of mobi lity, severe swelling of the cephalopedal mass, increased mucous secretion, and haemorrhage. Light microscopy showed that the molluscicide had induced gross structural damage to the epithelium of the foot sole, especially at lethal concentrations. TEM revealed such cellular injuries as the reduction and degradation of cilia, and the breakdown of the connective tissue and b lood sinuses, resulting in the accumulation of haemolymph below the epithel ium. This caused the partial basal detachment and distortion of adjacent ep ithelial cells. Other molluscicide-induced effects included the accumulatio n of electron-dense vesicles in the apical region of epithelial cells, the discharge of glycogen, lateral compression of the nuclei, contraction of th e nuclear envelope, swelling of the mitochondria and disruption of their cr istae. Exposure to lethal concentrations caused the complete disintegration of the epithelium and accentuated the cellular damage observed at subletha l levels. These data imply that the active ingredients of A. attenuata alte r the physiology of the epidermal tissue particularly that of osmoregulatio n.