Distribution of type II collagen mRNA in Xenopus embryos visualized by whole-mount in situ hybridization.

We have developed a whole-mount histochemical method to monitor the distribution of expressed genes within the intact, developing vertebrate embryo. Background problems that result from alkaline phosphatase- or horseradish peroxidase-based stains have been minimized, enabling both early and late stages of Xenopus embryogenesis to be monitored. The feasibility and utility of this non-isotopic method has been demonstrated by using a specific DNA probe to localize Xenopus laevis Type II collagen mRNA expression to areas surrounding the vacuoles of the notochord in Stage 30 embryos. Expression expands by Stage 41/42 to form a visually striking distribution pattern that includes a variety of chondrogenic tissues such as the vertebrae, otocysts, mandible, and periocular region. Although these experiments focused on expression of a structural gene, the high resolution and sensitivity of the method should allow it also to monitor expression of less abundant mRNA products of non-structural genes such as transcription factors, cytoplasmic regulators, and growth factors. In addition, this approach should be a successful tool to probe expression in normal and perturbed embryos not only of amphibians but also of other vertebrates, including avians and mammals.