Historical survey on chromatoid body research

Abstract

The chromatoid body (CB) is a male reproductive cell-specific organelle that appears in spermatocytes and spermatids. The cytoplasmic granule corresponding to the CB was first discovered some 130 years ago by von Brunn in 1876. Thirty years later the German term "chromatoide Körper" (chromatoid body) was introduced to describe this granule and is still used today. In this review, first, the results obtained by light microscopic studies on the CB for the first 60 years are examined. Next, many findings revealed by electron microscopic studies are reviewed. Finally, recent molecular cell biological studies concerning the CB are discussed. The conclusion obtained by exploring the papers on CB published during the past 130 years is that many of the modern molecular cell biological studies are undoubtedly based on information accumulated by vast amounts of early studies.

Keywords: P-body function; chromatoid body; lysosomal function; research history.

Fig. 1

Chromatoid body (CB) depicted in early papers. a. The first description of a CB-like granule of rat spermatids (arrows) by von Brunn in 1876. The granule is absent in late spermatids. b. CBs of rat spermatids illustrated by Niessing in 1897 (arrows). Movement of CB during spermiogenesis is shown. Finally, CB enters the residual body at the base of flagellum. c. CBs depicted by Wilson in 1913. CBs (c) of an insect, cabbage bug, are surrounded by clear hollow, which seems to correspond to CB vesicles revealed by electron microscopy.

Fig. 2

Formation of CB. The CB and intermitochondrial cement (“nuage”) appear in pachytene spermatocytes. Immediately after the second meiotic division, the CB is dispersed in the cytoplasm as small dense vesicles. At the same time, the intermitochondrial cement also disappears. During steps 1 and 2, the vesicles assemble again to form the CB near the nucleus. At this stage the CB receives nuclear materials such as mRNA and is surrounded by small clear vesicles and tubules, which are positive for lysosomal membrane proteins, LAMP1 and LAMP2. Communication of the CB with the intermitochondrial cement is not clear but ATP-dependent DEAD-box RNA helicase MVH (mouse VASA homolog) is detected in both structures. During steps 3 and 6 the CB moves slowly toward the cytoplasm at the caudal pole of nucleus. At steps 7 and 8, the CB reaches the base of flagellum, where it is enclosed compactly by small vesicles and its matrix becomes denser. Gradually the CB decreases in size and finally disappears.

Fig. 3

Typical CB of step 3 spermatid observed by electron microscope. The CB consists of electron dense matrix and is surrounded by small clear vesicles and tubules. The CB has no limiting membrane. Frequently, multivesicular bodies are located near the CB (arrows). Bar=0.5 µm.

Fig. 4

Immunoelectron microscopic localization of lysosomal membrane proteins, LAMP1 and LAMP2 in small vesicles surrounding the CB. a. CB (CB) of step 1 spermatid. Gold particles are associated with the vesicles. N: nucleus. Bar=0.5 µm. b. CBs (CB) of step 10 spermatid. LAMP1 signals are present in small vesicles surrounding CB. Bar=0.5 µm. c. CB (CB) of step 2 spermatid. Weak gold label is seen in vesicles surrounding CB. N: nucleus. d. CBs (CB) of step 8 spermatid. Gold labeling is noted in the vesicles and multivesicular body (MVB). Arrow indicates developing flagellum. Bar=0.5 µm.

Fig. 5

Immunoelectron microscopic localization of proteins relating to ubiquitin-proteasome proteolytic system. a. Ubiquitin signals in the CB of step 3 spermatid. Bar=0.5 µm. b. Localization of proteasome subunit p52 in the CB of step 1 spermatid. Note that gold particles are seen on the surface of dense matrix of the CB. Bar=0.5 µm. c. Gold labeling for proteasome activator PA700 in the CB. Most of the gold particles are located on the surface of dense material. Bar=0.5 µm. d. Gold labeling for ubiquitin conjugating enzyme, E2 in the CB. Gold particles are present on the CB matrix. Bar=0.5 µm.

Fig. 6

Hypothetical model for the function of CB. Haploid gene products are assembled into the ribonucleoprotein particles containing RNA-binding proteins. The mRNA-protein complex is then transported through the nuclear pore into the cytoplasm with the help of kinesin KIF17b. In the cytoplasm, moving CBs make frequent contact with the nuclear pores and collect mRNAs. KIF17b interacts with the testis-specific PIWI/Argonaute family member, MIWI. CB contains RNA-binding and RNA-processing proteins such as MVH (mouse VASA homolog), and components of the RNA decay pathway and the miRNA pathway such as miRNA, Dicer, decapping enzyme Dcp1a and Argonaute proteins. Stored mRNAs are released to the cytoplasm by appropriate stimulus and translated. The complex of RNA-binding protein and KIF17b may be reused. The CB also contains various proteins derived from different subcellular compartments such as the cytosol, mitochondria and nucleus, and enzymes for ubiquitination such as E2, that are not depicted here. Some of them are likely polyubiquitinated and degraded by proteasomes located on the surface of the CB (area shaded by dots). Other proteins are degraded by lysosomes (small vesicles) closely attached to the CB. Thus, in post-meiotic male germ cells, the CBs seem to have a P-body-like role on the one hand and an aggresome-like role on the other hand.