Expression and distribution of SPARC in early Xenopus laevis embryos.

SPARC (Secreted Protein, Acidic, Rich in Cysteine) is a highly conserved, calcium-binding, extracellular matrix protein. To investigate its role in early embryogenesis, we examined its tissue distribution in early Xenopus embryos. SPARC mRNA transcripts were detectable by Northern blot analysis beginning at the early neurula stage. SPARC transcripts then rapidly accumulated, reaching their highest embryonic level at the tailbud stage. The levels of SPARC mRNA were similar in dorsalized (LiCl-treated) and ventralized (UV-treated) embryos, indicating embryonic expression of SPARC mRNA was not obviously altered by changes in pattern formation. Immunofluorescence staining showed SPARC was an abundant protein in tissues of mesodermal and ectodermal origin in tailbud embryos, with the exception of the notochord. Of particular interest was the distribution of SPARC within the intersomitic clefts during somitogenesis. The widespread distribution of SPARC in early embryogenesis suggests it may play a role in the formation of a variety of tissues.