Carbonic anhydrase III, an early mesodermal marker, is expressed in embryonic mouse skeletal muscle and notochord

Carbonic anhydrase III (CAIII) is an abundant soluble protein in adult mammalian slow twitch skeletal muscle fibers. It is thought to be an early marker for myogenesis based upon its high level of expression in myoblasts in vitro prior to fusion. Using in situ hybridization, we have studied the in vivo distribution of CAIII gene transcripts in mouse embryos and fetuses from 7.25 days to 17.5 days post coitum (p.c.). CAIII mRNAs are first detected in the myotomes of somites between 9.5 and 10.5 days p.c. (20-30 somites). At 15.5 days p.c., CAIII begins to be restricted to developing slow muscle fibers. By two weeks post partum (p.p.), CAIII mRNAs are detected mainly in slow muscle fibers. CAIII transcripts are detected at an earlier stage (7.25 days p.c.) in the developing notochord. CAIII is expressed at a much higher level in the notochord than it is in developing skeletal muscle. As the notochord forms the nucleus pulposus in fetal mice, CAIII mRNA levels remain very high. Expression of CAIII in the notochord is of interest in the context of skeletal myogenesis because the notochord is thought to play an important role in somite formation. In addition to the notochord, CAIII transcripts are detected prenatally in several other non-muscle tissues: in cells of the choroid plexus, endocardial cushion and ureter, and in adipocytes.