Cytoplasmic Bridges as Cell-Cell Channels of Germ Cells
Transient intercellular bridges are seen between a wide variety of cells before the completion of cytokinesis.1 However, these are distinct from stable intercellular bridges that remain persistent after incomplete cytokinesis.2 The diameter of the cytoplasmic bridges is rather big, 1-10 mm, compared with the very tiny gap junctions which allow passage of only small molecules or peptides (< 1-2 kDa). Among somatic cells there are a number of examples of intercellular bridges, for instance in muscle cells and neurons. The best studied entity at both functional and molecular level is cytoplasmic bridges connecting germ cells. Many conserved features exist in cytoplasmic bridge formation and function during germ cell development: the diameters of the bridges increase during gametogenesis and is 1-10 mm in Drosophila oogenesis and 1-3 mm in mammalian spermatogenesis depending on the developmental stage of the gametes. The transportation mechanisms, e.g., the importance of cytoskeleton during transportation, are quite similar in both sexes from insects to mammals. Obviously the function of cytoplasmic bridges is to facilitate the sharing of cytoplasmic constituents between neighbouring cells.3 This is probably most energy-efficient way and allows germ cell differentiation to be directed by the products of both parental chromosomes. In this article special features and recent investigations of cytoplasmic bridges as cell-cell channels during gametogenesis are reviewed.