Somatic tissue–male germ cell barrier in three hermaphrodite invertebrates: Dugesia biblica (Platyhelminthes), Placobdella costata (Annelida), and Levantina hierosolyma (Mollusca)

The gonads from three hermaphrodite species of different invertebrate phyla were studied at the ultastructure level. In the flatworm Dugesia biblica , male germ cells in different stages of development lay in the lumen of testicular follicles surrounded by overlapping parietal cells. The intercellular space formed by cytoplasmic extensions running parallel to the testis wall is occluded by septate junctions and by an electron‐dense material. In the leech Placobdella costata , the testis is lined by a unicellular layer of parietal cells surrounded by densely packed connective tissue fibers. No specialized occluding junctions were found between the parietal cells; however, plasmalemma thickenings and electron‐dense material in the intercellular space near to the testis lumen were observed. In the lumen, germ cells develop connected to cytoplasmic masses, the cytophore. In the land snail Levantina hierosolyma , male and female germ cells are found together in the same acini; each acinus is surrounded by a thick basement membrane. At the periphery of the acinus is the ovarian layer; centrally to it is the testicular layer. Intercalated between them is a double cellular layer of follicular cells and of Sertoli cells. The inter‐Sertoli space is characterized by elaborate septate junctions. In the three species studied male germ cells develop within the lumina of compartments isolated from the somatic tissue. This separation is brought about by specialized septate junctions, and/or by electron‐dense material between the cells that form the testis walls, and also by densely packed connective tissue fibers. Our observations strengthen the view that a male germ cell‐somatic tissue barrier as described in the literature of the testes of vertebrates and of invertebrates from various phyla is of general occurrence in the animal kingdom.