Pronuclear morphology evaluation for fresh in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) cycles: a systematic review

Abstract

The current systematic review was aimed to assess the effectiveness of the zygote morphology evaluation in fresh in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) cycles. All available studies reporting on zygote morphology and clinical and/or biological outcomes were analyzed. Forty studies were included in the final analysis. Fourteen different zygote scoring systems were employed. Zygote morphology correlated significantly with embryo quality and cleavage, blastocyst stage, embryonic chromosome status, in a high proportion of the studies which assessed the specific outcome [15/25 (60%), 15/20 (75%), 7/8 (87.5%), 6/6 (100%), respectively]. On the other hand, only a reduced proportion of papers showed a statistically significant relationship between implantation, pregnancy and delivery/live-birth rates and zygote morphology score [12/23 (52.2%), 12/25 (48%), 1/4 (25%), respectively]. In conclusion, our findings demonstrate the lack of conclusive data on the clinical efficacy of the zygote morphology evaluation in fresh IVF/ICSI cycles, even if biological results showing a good relationship with embryo viability suggest a role in cycles in which the transfer/freezing is performed at day 1.

Figure 1

Study flow-chart.

Figure 2

Zygote scoring system of Scott and Smith [50]. In this classification, if PN are close or aligned they are assigned a score 5 (A). If they are well separated or unequal in size they are classified as score 1 (B). Nucleoli aligned in a row at the PN junction are scored as 5 (C), beginning to align as 4 (D), scattered as 3 (E). The cytoplasm is scored as follow: heterogeneous in appearance with a clear halo around the edges, occasionally with a clear area in the centre around the PN and darkened ring/halo in the middle, scored 5 (F), whereas zygotes with a clear homogeneous cytoplasm or a pitted and/or darkened cytoplasm scored 6 (G).

Figure 3

Zygote scoring system of Tesarik and Greco [51]. pattern 1 includes zygotes with big difference (>3) in the number of NPBs in both PN (A), pattern 2 includes zygotes showing a small number (<7) of NPBs without polarization in at least one PN (B), pattern 3 includes zygotes with a large number (>7) of NPBs with polarization in at least one PN (C), pattern 4 includes zygotes characterized by a very small number (<3) of NPBs in at least one PN (D) and pattern 5 includes zygotes showing polarized distribution of NPBs in one PN and non-polarized in the other. These 5 patterns are considered as “abnormal”; zygotes not included in pattern 1–5 are classified as pattern 0, and are considered “normal” (F and G).

Figure 4

Zygote scoring system of Tesarik et al. [12]. Zygotes showing a normal morphology are classified as “pattern 0”, and zygotes showing an abnormal morphology are classified as “non-pattern 0”. Specifically, pattern 0 include zygotes with a difference in a NPBs number between the two PN < 3, the same distribution (random or polarized) of NPBs in both PN, and at least one NPB in each PN (A and B). All the other NPBs configurations lead to classified zygotes as “non-pattern 0”.

Figure 5

Zygote scoring system of Scott et al. [10]. Z1 includes zygotes with equal number of nucleoli aligned at PN junction (A), Z2 includes zygotes with equal number and size of nucleoli (between 3 and 7) which are equally scattered in the two PN (B), Z3 includes zygotes with either very small/large nucleoli (C and D), and Z4 includes zygotes showing PN separated or different in size and small nucleoli, partially aligned or scattered (E and F).

Figure 6

Zygote scoring system of Gianaroli et al. [19]. They identified 5 different patterns according to PN morphology: (i) juxtaposed and centralized (A), (ii) juxtaposed and peripheral (B), (iii) centralized and separated (C), (iv) unequal size (D), and (v) fragmented (E). At regard to nucleolar morphology, 4 different patterns were proposed: (i) large size, aligned (F), (ii) large sized scattered in both PN (G), (iii) large size, aligned in one PN and scattered in the other (H), (iv) small size in at least one PN, scattered (I). Finally, the orientation of polar bodies was described in relation to the longitudinal axis of PN: (i) the longitudinal axis (L); (ii) perpendicular to the longitudinal axis (M); (iii) in different position (N).

Figure 7

Zygote scoring system of Wittemer et al. [11]. They defined zygotes with “normal” pattern 0 as zygotes with the four following characteristics: (i) the number of NPBs never differed by more than 3 (A); (ii) NBPs always polarized when fewer than 7 and never polarized if more than 7 in at least one PN (B and C); (iii) the number of NBPs in PN never fewer than 3 (D); (iv) the distribution of NPBs either polarized or not in both PN (E and F). Whereas zygotes that did not conform to this morphological pattern were considered as “abnormal”.

Figure 8

Zygote scoring system of Montag et al. [16]. This classification subdivided the zygotes with a “normal” pattern 0 in two further different patterns according to NPBs number and distribution: pattern 0A (A) (>7 equally distributed NPBs) and pattern 0B (≤ polarized NPBs) (B).

Figure 9

Zygote scoring system of Balaban et al. [27]. The authors identified two group: group 1, zygotes corresponding to “pattern 0” of the original classification (A and B); group 2, zygotes showing a single PN; group 3, zygotes with 2PN originally classified as pattern 1–5 (C-G).

Figure 10

Zygote scoring system of Guerif et al. [35]. This classification grouped the patterns 1–5 as defined by Tesarik and Greco [51] into a single class called “non-pattern 0” (A-E). Zygotes with “non-pattern 0” were considered abnormal zygotes in opposition to “pattern 0” zygotes (normal zygotes) (F and G).

Figure 11

Zygote scoring system of Brezinova et al. [42]. Zygotes exhibiting some number of NPBs evenly distributed in the PN or large NBPs with polarized distribution between the two PN were grouped in pattern 0 (and B), whereas all the other non symmetrical alignments of NPBs were classified as pattern “other” (C-G).

Figure 12

Zygote scoring system of Aydin et al. [48]. In the zygote scoring system another pattern was adding to those originally described [51](A-G). Zygotes presenting disconnected PN with unequal size and difference in the number of NPBs less than 3 in both PN were included in this new pattern (H).

Figure 13

Zygote scoring system of Lan et al. [23]. Zygote Z1 had equal number of nucleoli (between 3 and 7) aligned at the PN junction (A), Z2 had nucleoli equally in number and size equally scattered in both PN (B), Z3 had equal number of nucleoli of equal size in the same PN but with one PN having alignment at the PN junction and the other with scattered nucleoli (C), and Z4 had PN not aligned, grossly different in size or not located in the central part of the zygote (D). Zygotes with unequal number (a difference of more than one nucleolus), and/or size of nucleoli were considered as Z3.

Figure 14

Zygote scoring system of De Placido et al. [17]. The authors combined five previous scoring systems [10,11,50-52] and considered 3 main parameters: (i) the position of PN in relation to the cytoplasm (A-E); (ii) the morphology and orientation of nucleoli (F-L); (iii) the presence of a dense area of the cytoplasm aggregate around the PN (cytoplasmic flare) (M-Q). Zygotes showing two opposed PN with equal size in centre of the cytoplasm, equal number of juxtaposed nucleoli and the cytoplasmic flare were considered as top quality zygotes.

Figure 15

Zygote scoring system of James et al. [30]. Zygotes were grounded in scores from 1 to 4: score 1, zygotes with equal numbers of nucleoli that are aligned at the furrow between the PN (A); score 2, zygotes with equal numbers of nucleoli that are not aligned at the furrow (B); score 3, zygotes with marked differences in size and/or number of nucleoli with nucleoli not aligned (C and D); and score 4, zygotes with different size PN, non central PN or PN that were not in contact with each other (E and F).

Figure 16

Zygote scoring system of Nicoli et al. [38]. Z1 group included zygotes with PN juxtaposed and centralized, nucleoli of large size, aligned and with the polar bodies aligned and oriented in the longitudinal axis (A); Z2 group includied zygotes showing PN juxtaposed and peripheral, nucleoli of large size scattered in both PN, and polar bodies orientated in the longitudinal axis of the PN (B); and Z3 group included zygotes show a different PN morphologies (centralized and separated, of unequal size and fragmented), different position of nucleoli (large size and scattered in both PN, large size and aligned in one PN and scattered in the other, and small sized in at least one PN and scattered) and different polar bodies orientation (perpendicular to the longitudinal axis; in different positions) (C-E).

Figure 17

ESHRE zygote scoring system [1]. symmetrical zygotes show two PBs, two centrally located and juxtaposed PNs with distinct membranes, equal size and equivalent numbers and size of NPBs equatorially aligned at the region of membrane juxtaposition (A), non-symmetrical zygotes (differing from the ideal configuration A) (B), and abnormal zygotes with no NPBs and those with a single NPB (C).