Cajal bodies, formerly referred to as "coiled bodies", appear in the electron microscope as a ball of tangled threads 0.15 to 1.5 um in diameter. They are highly dynamic structures that may play a role in snRNP transport, maturation or both. Cajal bodies contain spliceosomal snRNPs as well as several nucleolar antigens, including fibrillarin, NOPP140 and U3 snoRNP and a human autoantigen called p80 coilin which is highly enriched in these structures and widely used as a marker for the Cajal body.

The function of this subnuclear body is still unknown. It might be involved in regulating snRNA gene expression, but is probably not a site of coordinated snRNA transcription, processing and assembly because it lacks nascent RNA and because snRNP particle assembly from snRNA and Sm proteins occurs in the cytoplasm.

A key unexplained feature of Cajal bodies is their intimate relationship with the nucleolus. They have occasionally been observed within nucleoli of mammalian cells, and the interaction between the two may be regulated by phosphorylation. This morphological association of Cajal bodies and nucleoli appears to be of genuine functional importance, and our laboratory continues to study it.

For a review of Cajal body structure and function, see:
Cajal bodies: a long history of discovery" (2005) Cioce, M. and A.I. Lamond. Annu. Rev. Cell Dev. Biol. 21:105-31.

HeLa cells stained with 5P10*, a monoclonal antibody to p80 coilin. *Almeida, F., Saffrich, R., Ansorge, W., Carmo-Fonseca, M. (1998) J. Cell Biol. 142:899-912.	HeLa cells expressing EYFP-tagged p80 coilin. images provide by Dr. Judith Sleeman