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Blastocyst Scoring System

Figure 8 Scoring system for human blastocysts. Initially blastocysts are given a numerical score from 1 to 6 based upon their degree of expansion and hatching status: (1) early blastocyst; the blastocoel being less than half the volume of the embryo; (2) blastocyst; the blastocoel being greater than or equal to half of the volume of the embryo; (3) full blastocyst; the blastocoel completely fills the embryo; (4) expanded blastocyst; the blastocoel volume is now larger than that of the early embryo and the zona is thinning; (5) hatching blastocyst; the trophectoderm has started to herniate through the zona; (6) hatched blastocyst; the blastocyst has completely escaped from the zona. The initial phase of the assessment can be performed on a dissection microscope. The second step in scoring the blastocysts should be performed on an inverted microscope. For blastocysts graded as 3 to 6 (i.e., full blastocysts onwards) the development of the inner cell mass (ICM) and trophectoderm can then be assessed. ICM grading: A—tightly packed, many cells; B—loosely grouped, several cells; C—very few cells. Trophectoderm grading: A—many cells forming a tightly knit epithelium; B—few cells; C—very few cells forming a loose epithelium. Source: From Ref. 307.

Figure 8 Scoring system for human blastocysts. Initially blastocysts are given a numerical score from 1 to 6 based upon their degree of expansion and hatching status: (1) early blastocyst; the blastocoel being less than half the volume of the embryo; (2) blastocyst; the blastocoel being greater than or equal to half of the volume of the embryo; (3) full blastocyst; the blastocoel completely fills the embryo; (4) expanded blastocyst; the blastocoel volume is now larger than that of the early embryo and the zona is thinning; (5) hatching blastocyst; the trophectoderm has started to herniate through the zona; (6) hatched blastocyst; the blastocyst has completely escaped from the zona. The initial phase of the assessment can be performed on a dissection microscope. The second step in scoring the blastocysts should be performed on an inverted microscope. For blastocysts graded as 3 to 6 (i.e., full blastocysts onwards) the development of the inner cell mass (ICM) and trophectoderm can then be assessed. ICM grading: A—tightly packed, many cells; B—loosely grouped, several cells; C—very few cells. Trophectoderm grading: A—many cells forming a tightly knit epithelium; B—few cells; C—very few cells forming a loose epithelium. Source: From Ref. 307.

reports are now leaning to blastocyst transfer, especially with regard to single embryo transfer (303,305,306). Furthermore, it appears that cryopreser-vation of human embryos is most effective at the blastocyst stage (300,302). Consequently, extended culture, facilitated by the culture systems outlined in this chapter, may represent a way to increase the overall efficiency of human IVF.

Figure 7 shows the morphology of human blastocysts following culture in sequential media, together with an alpha-numeric scoring system (Fig. 8). It is evident that blastocyst score is correlated with subsequent implantation potential (307), with implantation rates of > 60% being attainable.

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