Association of SPARC (Osteonectin, BM-40) with extracellular and intracellular components of the ciliated surface ectoderm of Xenopus embryos

SPARC (Secreted Protein, Acidic, Rich in Cysteine) was detected by immunohi stochemistry in the sensorial layer of the bilayered embryonic epidermis of Xenopus laevis during neurulation, when a subset of the sensorial cells ar e selected to differentiate into ciliated cell precursors. After the ciliat ed cells had intercalated into the outer layer and had undergone ciliogenes is, intense SPARC immunostaining was associated with the cilia and remained associated with the cilia throughout their persistence on the epidermis. C ircumferential SPARC immunostaining was also detected at the interface betw een surface epithelial cells. Animal cap explants indicated that the embryo nic activation of SPARC expression in the dorsal ectoderm does not require signaling from factors secreted by the underlying mesoderm. Immunoelectron microscopy revealed that SPARC is intimately associated with the 9 + 2 micr otubule arrays of cilia. Our data indicate that SPARC plays a role in the d evelopment and function of the surface ciliated epidermis of Xenopus embryo s. We propose that the counter-adhesive activity of SPARC facilitates the i ntercalation of ciliary cell precursors to the surface epithelial layer, wh ere its Ca2+-binding abilities promote cell-cell adhesion. Based on its ass ociation with ciliary microtubule arrays, we also propose that intracellula r SPARC may play a role in regulating ciliary beat frequency and polarity. (C) 2000 Wiley-Liss, Inc.