A morphological, enzyme-cytochemical, and physiological study of the blood-gonad barrier in the hermaphroditic snail Lymnaea stagnalis.

In the hermaphroditic pulmonate snail Lymnaea stagnalis a blood-gonad (blood-testis) barrier appears to exist. Septate junctions between Sertoli cells and epithelial cells of the neck areas of the gonadal acini constitute this barrier; they separate the male from the female compartment. Experiments with tracer substances (colloidal gold particles, lanthanum nitrate, tannic acid) showed that the basal lamina around the acini hardly forms a barrier; only the larger colloidal gold particles do not pass this lamina. Physiological, the blood-gonad barrier is apparent in studies on the composition of gonadal fluid, which differs considerably from that of haemolymph. The osmolarity and the concentration of protein and amino acids in gonadal fluid exceed those of haemolymph. As to the major ions, in the gonadal fluid Na+ is partly replaced by K+, and HCO-3 is almost totally replaced by Cl-. Such a distribution of HCO-3 and Cl- is indicative of metabolic acidosis. The cytochemical localization of carbonic anhydrase activity in cells lining the acinar lumen (Sertoli cells, epithelial cells) suggests that these cells are involved in the process of ion exchange. The metabolic acidosis in the gonad might result from the anaerobic production of lactate and succinate by Sertoli cells; these cells lack the enzymes cytochrome oxidase, lactate dehydrogenase, and succinate dehydrogenase. Spermatogenic cells, on the other hand, do possess these enzymes. This probably indicates that these cells metabolize lactate and succinate secreted by Sertoli cells.