K. Sato et al., Localization of mitochondrial ribosomal RNA on the chromatoid bodies of marine planarian polyclad embryos, DEVELOP GR, 43(2), 2001, pp. 107-114
Electron-dense cytoplasmic structures, referred to as chromatoid bodies, ar
e observed in the somatic stem cells, called neoblasts, and germline cells
in adult planarians. Although it has been revealed that the chromatoid bodi
es morphologically resemble germline granules in Drosophila and Xenopus emb
ryos, what essential role it plays in the planarian has remained unclear. i
n the present study, to examine whether chromatoid bodies in planarian embr
yos are responsible for germline formation, the presence and behavior of ch
romatoid bodies during embryogenesis were examined. Mitochondrial large rib
osomal RNA and mitochondrial small ribosomal RNA were used as candidate mar
kers for components of the chromatoid body. Starting from the fertilized eg
g, extramitochondrial signals of both RNA (mtrRNA) were observed. At the ul
trastructural level, mtrRNA were localized on the surface of the chromatoid
bodies. At subsequent stages, the signals of mtrRNA were observed in certa
in restricted blastomeres that contribute to the formation of larval struct
ures. The signals gradually decreased from the gastrula stage. These result
s suggest that the chromatoid bodies associated with mtrRNA in embryogenesi
s are not germline granules. The chromatoid bodies of blastomeres may be co
ncerned with the toti- or pluripotency and cell differentiation as proposed
in adult planarian neoblasts.