Model proposing regulated delivery of the paracrine factors to the oocyte-granulosa cell interface [6, 7]. Four communication pathways are described. (1) Localized uptake of growth factors, like GDF-9, from the oocyte (yellow ball) by endocytosis at attachment sites of transzonal projections (TZPs) at the oolema. The vectorial transport of endocytic vesicles (VEs) to the cytoplasm of the granulosa cells takes place through the microtubules (MT). (2) Granulosa-zona pellucida interactions are necessary for the orientation of the TZPs. Contact sites may play an important role in the signaling role of oocytes and granulosa cells. (3) The gap junctions that allow direct intercellular communications between the microvilli of the oocyte and the granulosa cell TZPs. (4) Pathway for delivery of granulosa cell factors (red ball) packaged in secretory vesicles (VSs), which undergo endocytosis through the specific receptor on the oocyte surface. N: nucleus, MT: microtubules, ZP: zona pellucida, red ball: factors of the granulosa cells, yellow ball: factors released by the oocyte. With permission from D. F. Albertini.
