Trophectoderm Biopsy

In 1990, Handyside et al. reported a successful pregnancy utilizing preimplantation genetic diagnosis (PGD) technology for embryo sex selection in IVF for the first time [1]. Nowadays, the technology has been used in IVF centers worldwide. The term PGD has been replaced by preimplantation genetic testing (PGT) in 2017 [2]. PGT refers to a test performed to analyze the DNA from oocytes (polar bodies) or embryos (cleavage stage or blastocyst) for HLA-typing or for determining genetic abnormalities. These include PGT for aneuploidies (PGT-A); PGT for monogenic/single gene defects (PGT-M); and PGT for chromosomal structural rearrangements (PGT-SR). With the continuous development of molecular biology techniques, some tests can be performed using non-invasive or minimally invasive methods to obtain genetic material [3, 4]. However, the accuracy of the test results based on such sampling methods is still controversial [5]. For this reason, genetic material is still predominantly retrieved by invasive means, i.e., biopsy. The biopsy consists of two micro-manipulation steps: zona pellucida (ZP) opening and cell removal. PGT biopsies are usually performed in three different ways: (a) polar body biopsy: the first and second polar bodies are biopsied at the oocyte or zygote stage; (b) cleavage stage biopsy: one or two blastomeres are taken for biopsy at the cleavage stage; and (c) blastocyst biopsy: 5 to 10 trophectoderm (TE) cells are taken for biopsy at the blastocyst stage. Biopsy of polar bodies is mainly used for diagnosing maternal genetic mutations and specific laws and regulations prohibit embryo biopsy in some regions. Before 2010, approximately 90% of PGD biopsies were performed on cleavage stage embryos [6]. However, a prospective cohort study found that the number of blastomeres biopsied at the cleavage stage would affect blastocyst formation and pregnancy outcomes [7]. Another randomized paired study also showed that biopsy of even only one blastomere of a cleavage embryo would significantly reduce its implantation potential. In contrast, biopsies of blastocysts had no measurable impact on implantation rate [8]. Therefore, following the improvement of controlled ovarian stimulation and blastocyst vitrification technology, blastomere biopsy has been gradually replaced by trophectoderm biopsy in the last decade [9, 10].