RAG expression is restricted to the first year of life in the Mexican axolotl

The developmental expression of the RAG1 gene in the Mexican axolotl hemato poietic organs was studied. RAG1 mRNAs were first detected in trunk extract s from 6-week-old larvae, and in head and trunk extracts of 8- and 9-week-o ld larvae. RAG1 is expressed in the thymus at all stages of development, un til its natural involution after 12 months of age. In contrast, although RA G1 transcripts were present in the spleen and liver of the young larvae, th ey were not detected in the liver after 4.5 months and in the spleen after 8 months. No RAG1mRNA expression was observed in the spleens or livers of 2 4-month-old hyperimmunized axolotls. The developmental expression of the RA G2 protein was also analyzed in axolotl thymus, spleen, and liver extracts using specific anti-RAG2 antibodies. RAG2 was readily detected at 7 months, but not in hematopoietic organs of 12- and 24-month-old axolotls. The pres ence of RAG1 transcripts was limited to the sub-capsular area of the thymus lobes, as detected by in situ hybridization. Discrete clusters of labeled cells were observed in the spleen sections, and a relatively large number o f labeled cells were located in the hepatic peripheral hematopoietic layer of 3-month-old axolotls. The first appearance of RAG1 gene products in the axolotl hematopoietic organs is thus well correlated with the first product ion of rearranged T-cell and B-cell receptor mRNAs, 40-60 days after fertil ization.