Meiotic spindle location and identification and its effect on embryonic cleavage plane and early development

To examine the relationship between the meiotic spindle, the first cleavage plane and any resulting influence on embryonic development parameters.

Sibling oocytes (n = 246) were allocated to either a control [polar body (PB)-aligned] or a treatment (spindle-aligned) microinjection group by use of a random numbers table. Spindles were identified by PolScope((R)) and the early embryo development parameters, and angle of first cleavage plane in relation to a defined animal-vegetal pole were analysed.

Most oocytes (92.7%) had a visible spindle at the time of microinjection; however, 62.6% of first PBs (1PBs) were not located above the spindle (average deviation 37.3 +/- 33.2 degrees; range 0-176.6), with 6.9% of 1PBs in the opposite hemisphere to the spindle. The second PBs (2PBs) can also have an unpredictable deviation from the position of the meiotic spindle (12.5 +/- 16.7 degrees; range 0-91.8). This increased when the 1PB was above the spindle, forming a physical barrier to extrusion (average 24.7 +/- 16.1 degrees; range 7.9-91.8). Embryos developing from the spindle-aligned microinjection group had significantly more blastomeres per embryo (P = 0.044), a higher morphology score per embryo (P = 0.008) and a significantly higher average embryo score parameter (P = 0.003), with more embryos developing without any detectable fragmentation (P < 0.05) than the PB-aligned control group. Non-fragmented embryos undergo meridional cleavage, with a small angle between the spindle location and first cleavage plane (16.4 +/- 14.0 degrees ) compared with embryos with some degree of fragmentation (P = 0.002). This angle increased with the degree of fragmentation, with worst quality embryos having a spindle:cleavage angle of 45.1 +/- 17.7 degrees.

The 1PB and, to a lesser degree, the 2PB can be unreliable predictors of the exact meiotic spindle location in human oocytes. Embryos from spindle-aligned oocytes have an increase in all measured development parameters over control siblings. When the animal pole is defined as the meiotic spindle location, non-fragmented embryos tend to develop from a meridional cleavage; with the most fragmented embryos developing from a more equatorial initial cleavage plane. This study proposes that the spindle accurately marks the animal pole in human oocytes, and provides evidence linking the meiotic spindle location to the first cleavage plane and resulting early embryo development parameters in human embryos.