Hypotheses for the functions of intercellular bridges in male germ cell development and its cellular mechanisms.

In oogamous reproduction of multicellular organisms, a striking phenomenon is the prevailing synchronous development of male germ cells connected by wide intercellular bridges (IBs, 0.1-2 microm), which is well conserved in both animal and plant species ranging from algae to human. In the literature, IBs are believed either to allow genetically segregated haploid spermatids to share diploid gene products after meiosis, or to mediate rapid transfer of some vital signals or nutrients. Although intercellular sharing of gene transcripts has experimental evidences, these hypotheses are still not satisfactory. To explore the unknown roles of IB, we assume that developing male germ cells may be especially sensitive to stochastic gene expression to become heterogeneous. To achieve best gamete quality, such heterogeneity must be eliminated so that relatively uniform gametes with normal functions can be produced. Development within a common syncytium may be the only way for this purpose. The process may require not only the intercellular exchange of a few molecular signals but also the mixing of protoplasm between the connected cells so that they have similar levels/states of mRNAs, proteins and organelles, which can be achieved only through wide IBs. This hypothesis can explain some quite intriguing aspects of male gametogenesis and provide unique predictions that can be tested experimentally. Copyright 2004 Elsevier Ltd.