**5. Incomplete cytokinesis providing a cellular communication as a feature of male generative cells**

Syncytial development during the formation of male generative cells is conservative and is characteristic for different animals [59, 60]. It is assumed that the link between the cells in the syncytium is important primarily for synchronizing differentiation processes [59]. The cytoskeleton and the molecules providing a cellular communication are the main components supporting of equality of each cell in the syncytium.

The cytokinesis of spermatogenic cells is characterized by the formation of ring canals linking the cells that have undergone meiosis or mitosis [61]. During cell division, a contractile ring, made of actin and myosin II filaments, assembles as a result of the interaction between the plus ends of the microtubules and the cell cortex [62]. The contractile ring is double-sided. The microtubules whose minus ends directed toward centrosome on one pole of the dividing cell bind to one side of the contractile ring by their plus ends. The microtubules that have their minus ends turned toward centrosome on the other pole of the dividing cell bind to the opposite side of the contractile ring. Thus, the ring attached equatorially to the membrane of the dividing cell forms an actin-myosin cleavage furrow [60]. The ingression of the cleavage furrow is accompanied by the growth of the daughter cell membranes.

+TIPs—complexes linked to the plus-ends of the microtubules—play a special role in the interaction of the plus-ends of the microtubules with the cell cortex [63]. +TIPs also play an important role in finding and capturing microtubule targets—the cortex and the chromosomes [64, 65]. Localized mRNAs are anchored at the plus ends of microtubules especially in polarized cells [66]. SBR also may be one of the factors that interact with the plus-ends of microtubules.

It is worth noting that the Hs NXF1 (TAP) factor in humans was initially identified *not* as a protein enabling transport of mRNA from the nucleus into the cytoplasm, but as a factor important for cellular adhesion and involved in cell signaling [67]. For this reason, the protein Hs NXF1 was called TAP—Tip-associated protein, where Tip stands for tyrosine kinase-interacting protein.

Tyrosine phosphorylation is important for regulating the assembly and disassembly of the actin cytoskeleton at cell-cell junctions. In many cases, tyrosine phosphorylation in proteins of the junction complex—plakoglobin and β-catenin in adherens junctions—disrupts interactions between cytoskeleton and membrane [68]. It is believed [61] that tyrosine phosphorylation in targets leads to the disassociation of phosphorylated actin from the contractile ring that forms during the cytokinesis of the cells comprising the syncytium during spermatogenesis. This stabilizes the interaction between the walls of the ring and the cell membrane, ends cell division, and forms the ring canal [61]. Building a new cell wall at the cleavage site during cytokinesis requires the involvement of components of the cell membrane and the signaling molecules [69]. A system of transport molecules ensures the delivery of the necessary elements to the region where the cell membrane is being formed [70].

Interactions of NXF1 with cytoskeleton [27] and the cellular membrane [67] may help finding partners of NXF1 in the cytoplasm. Not surprisingly, the closest evolutionary relative of NXF1 is dynein. Dynein plays a significant role in cytokinesis, chromosome segregation, and in enabling the movement of the flagellum of the sperm [70, 71].
