Soma-germ contact across basement membrane in ovary

Epithelial cells generally interact with other cells and environments at their apical side. At the basal surface, the basement membrane impedes such an interaction. One notable instance is communication between soma and germ cells within the ovary across numerous bilaterian taxa. This contact underlies proper oogenesis and subsequent embryogenesis through facilitating nutrient supply to gametes, exchange of molecules and ions, and participation in signal transduction for primary axis formation. Throughout layers of the history of morphology and cellular biology, there has been emphasis on this heterocellular interaction primarily occurring at the apical side of epithelial cells. Contrary to this long-standing understanding, we uncover that ovarian follicle cells in three myriapod species belonging to phylogenetically basal myriapod clades extend their cytoplasmic processes, penetrating the basement membrane to establish direct contact with oocytes. These discoveries demonstrate the ovarian soma-germ interaction transverse the basement membrane, suggesting that the basal matrix does not always a physical barrier to soma-germ communication. Furthermore, we find that the ovarian somatic cells in a myriapod directly connect with the oogonia or young oocyte before the formation of their basement membrane. This result suggests an overlooked construction manner of heterocellular communication at the basal domain of epithelial cells.