SMN, the affected protein in spinal muscular atrophy (SMA), is a cytoplasmic 
protein that also occurs in nuclear structures called "gems" and is involved in 
snRNP maturation. Coilin-p80 is a marker protein for nuclear Cajal bodies 
(coiled bodies; CBs) which are also involved in snRNP maturation, storage or 
transport. We now show that gems and CBs are present in all fetal tissues, even 
those that lack gems/CBs in the adult. Most gems and CBs occur as separate 
nuclear structures in fetal tissues, but their colocalization increases with 
fetal age and is almost complete in the adult. In adult tissues, up to half of 
all gems/CBs are inside the nucleolus, whereas in cultured cells they are almost 
exclusively nucleoplasmic. The nucleolar SMN is often more diffusely 
distributed, compared with nucleoplasmic gems. Up to 30% of cells in fetal 
tissues have SMN distributed throughout the nucleolus, instead of forming gems 
in the nucleoplasm. The results suggest a function for gems distinct from Cajal 
bodies in fetal nuclei and a nucleolar function for SMN. Spinal cord, the 
affected tissue in SMA, behaves differently in several respects. In both fetal 
and adult motor neurons, many gems/CBs occur as larger bodies closely associated 
with the nucleolar perimeter. Uniquely in motor neurons, gems/CBs are more 
numerous in adult than in fetal stages and colocalization of gems and CBs occurs 
earlier in development. These unusual features of motor neurons may relate to 
their special sensitivity to reduced SMN levels in SMA patients.