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Fig. 5. The chromatoid body in post-meiotic male germ cells. Hypothetical model of how the chromatoid body might function. After transcription, haploid gene products are assembled in the ribonucleoprotein particles containing RNA-binding proteins. A kinesin KIF17b transports mRNAs through nuclear pore complexes into the cytoplasm. The chromatoid body in the cytoplasm of haploid spermatids moves actively, makes frequent contacts with the nuclear envelope and collects mRNAs, KIF17b and other material directly from nuclear pores. In the chromatoid body, KIF17b interacts with the testis-specific PIWI/Argonaute family member, MIWI. Chromatoid bodies also contain other RNA-binding and RNA-processing proteins, such as the ATP-dependent DEAD-box RNA helicase MVH (mouse VASA homolog), and components of the RNA decay pathway and the miRNA pathway such as miRNAs, Dicer and Argonaute proteins Ago2 and Ago3 (Kotaja et al., 2006). MIWI is proposed to function as a testis-specific component of the miRNA pathway. RNA-processing enzymes act on their target mRNAs, which might be either degraded by the RNA decay enzymes or translationally repressed and stored by order of miRNAs. The presence of several separate processing pathways suggests that the chromatoid body functions as a sorting center that determines the destiny of mRNAs. KIF17b could also be involved in the regulation of the active movements of the chromatoid body. Txn, transcription.

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