The use of different fixatives and hydrophilic embedding media (Historesin and Unicryl) for the study of embryonic tissues.

The effects of different fixatives, dehydration procedures, and embedding media on the structural and ultrastructural preservation of young chick embryos (Hamburger and Hamilton stages 18-24) have been studied by means of light and electron microscopy techniques. Under the light microscope, the results obtained with the use of Bouin, glutaraldehyde, or glutaraldehyde-paraformaldehyde mixtures, followed by partial dehydration of the samples and the embedding with two different polar resins (Historesin and Unicryl), were compared with the results obtained using conventional paraffin-embedding methods. Cell and tissue shrinkage was determined by comparing blood cells from those embryos embedded in either of resins with those embedded in paraffin. Samples were also compared with blood smears, either methanol-fixed or unfixed, obtained from embryos at the same Hamburger and Hamilton stages. The results obtained when Unicryl and Araldite were used for electron microscopy have also been compared. When ultrastructural images from glutaraldehyde-tannic acid/osmium tetroxide fixed, Unicryl embedded samples were compared with those from araldite embedded samples, the same good results were observed with either of the resins. Araldite embedding requires a complete dehydration of the samples, while Unicryl allows the embedding of partially dehydrated embryos with optimal ultrastructural results. We suggest that these polar resins can be considered as complementary tools for embedding delicate embryonic tissues, allowing partial dehydration of the specimens with an excellent cell and tissue preservation.