Somatic tissue-male germ cell barrier in Hydra viridis (hydrozoa, coelenterata)

In Hydra viridis, cordons of male germ cells lie in gonadal compartments, which are enlarged spaces between the elongated and "spongy" epidermal cells. The germ cells are surrounded by these cells, except for small areas where the interstitial cells and spermatogonia are in direct contact with the mesoglea. Cells from both epidermis and gastrodermis project cytoplasm into the mesoglea, where they contact each other and form trans-mesogleal bridges. The latter exhibit gap junctions, which are particularly abundant at the spermary region. Here, the mesoglea is thinner then elsewhere in the body. Both epithelia are joined by septate junctions toward their apical ends, which are totally impermeable to horseradish peroxidase (HRP). HRP gained entry to the cells of both epithelia by pinocytosis. Incorporation into the cells was high at the basal disk, in the tentacles, and in the mesoglea in the lower part of the body stalk. The tracer was never found within the gonadal space of the testis during spermatogenesis. In mature spermaries during spermiation, tracer-filled intracellular vacuoles fused with the gonadal spaces as the thin cytoplasmic columns of the epidermal cells ruptured; HRP thus gained access to the germ cells. During spermatogenesis, germ cells of Hydra viridis are in a closed compartment. The barrier that controls the access of metabolites to the germ cells is formed by epidermal cells, thinned-out mesoglea, and numerous transmesogleal interepithelial bridges. The presumed role of the barrier is the control of the environment (1) where interstitial cells are differentiating into spermatogonia and meiosis occurs and (2) in which ripe spermatozoa are kept immotile until spermiation.