Demonstrating the Osseous Skeleton of Human Embryos and Fetuses

Transparent human embryos and fetuses whose osseous skeletons are stained in toto by alizarin red S are successfully prepared when the KOH clearing of the soft tissues and the alizarin staining of the bones are performed simultaneously instead of independently. This modification minimizes the possibility of macerating and staining the soft tissues. Fetuses over 50 mm. CR length are skinned, eviscerated, decerebrated, defatted by dissection, fixed in 95% alcohol, bleached in H₂O₂, cleared and stained simultaneously in an aqueous solution of KOH (from 2% to 10% depending upon the size of the specimen) and .0001 to .00005% alizarin red S (solution has a pale lavender color). This solution is changed periodically to maintain the concentration of the KOH until the clearing of the tissues is complete and of the alizarin until the bones are properly stained. Tissues are dehydrated in increasing concentrations of glycerin and stored in white glycerin plus thymol.