Morphological Changes of Mammalian Nucleoli during Spermatogenesis and Their Possible Role in the Chromatoid Body Assembling

Figure 6

Hypothetical schematic view of the nucleolar role on the CB assembly in male germ cells. Spermatogonium is an undifferentiated male germ cell, originating in a seminal tubule and dividing by mitosis into a new generation of spermatogonia or into two first spermatocytes. In this type of cell the nucleolus is organized and active. Germ granules and granular localization of PIWI and Tudor proteins are not well characterized in spermatogonia, but MILI, MAEL, TDRD1, and TDRD9, which are molecular components of CBs, have been demonstrated in the cytoplasm of spermatogonia and/or early spermatocytes [50]. In late first spermatocytes there is a peak of nucleolar activity followed by nucleolar disorganization, and some of the nucleolar components become disperses through the cytoplasm. In this stage there is an intense transcription of mRNA that will be silent for later translation during germ-cell differentiation, and also many other nuclear products, which will play a role in the CB molecular composition, are in transit nucleus cytoplasm. There is a drastic inhibition of transcriptional activity during the meiotic divisions, and mRNA storage and translational regulation by RNA-binding proteins have an important role in the control of the synthesis of many spermatids and spermatozoa proteins [45]. In postmeiotic round spermatids, nucleolar reorganization is observed, and the transcription is active until the silencing of the haploid genome owing to compaction of the genome by the replacement of histones with protamines. In this type of cell the CB is condensed to its final shape, and it contains (a) components that play a role in the RNA processing (Miwi, KIF17b, Dicer, Ago, MVH DCP1a, miRNA–purple shapes); (b) components that act as proteasome folding and degradation sites (AcPs, HSP70, DNase/RNase–blue shapes). These components possibly play a role also in the acrosomal system formation; (c) components related to CB movement (actin-red shape). Due to the CB high mobility probably related to the presence of actin, there is a hypothesis that CB would link to mitochondria aggregates playing a role in the migration of mitochondria to the nuclear posterior region, where the formation of mitochondrial sheath and spermatozoon tail will take place. (d) The probable function of nucleolar proteins (light green shape) is still unclear. They could play some physiological role in the RNA metabolism, or even they could be targeted for degradation. After the spermiogenesis process mature male germ cell does not possess any residue of nucleolar nor even CB components, indicating that these materials were possibly digested at the end of the spermatogenesis process. RNPs: ribonucleoproteins; P:  pachytene; D:  diakinesis; A: acrosome.