Sexing of human embryos and fetuses by fluorescent in situ hybridization (FISH) to paraffin-embedded tissues with sex chromosome-specific DNA probes.

A fluorescent in situ hybridization (FISH) study was carried out on paraffin sections of human embryonic and fetal tissues with two DNA probes, DXZ1 and DYZ1 (Oncor), for X and Y chromosome-specific DNA sequences, respectively. The specificity of the DNA probes was confirmed on metaphase and interphase lymphocytes of healthy normal adult males. The paraffin blocks of the human embryonic and fetal tissues examined in the present study had been stored at room temperature for up to 5 years after fixation in 4% paraformaldehyde. All the seven embryos and five fetuses examined were successfully sexed by FISH. The cells from three embryos and four fetuses were positive for a hybridization signal with each of the DXZ1 and DYZ1 probes and they were classified as male. The cells from the remaining four embryos and one fetus were positive for two identical hybridization signals with the DXZ1 probe in a nucleus instead of the absence of the signal hybridized with DYZ1, indicating that their cells have two X chromosomes but no Y chromosomes. The FISH results for the five fetuses examined were consistent with their genital sex and/or gonadal histology. The FISH results were highly specific and no false positive or false negative results were obtained. Thus, the FISH technique has been shown to visualize specific DNAs in situ on paraffin sections and to be useful to determine the sex of fixed embryos and fetuses retrospectively.