The Principal Forces of Oocyte Polarity Are Evolutionary Conserved but May Not Affect the Contribution of the First Two Blastomeres to the Blastocyst Development in Mammals

Abstract

Oocyte polarity and embryonic patterning are well-established features of development in lower species. Whether a similar form of pre-patterning exists in mammals is currently under hot debate in mice. This study investigated this issue for the first time in ovine as a large mammal model. Microsurgical trisection of unfertilized MII-oocytes revealed that cortical cytoplasm around spindle (S) contained significant amounts of total maternal mRNAs and proteins compared to matched cytoplast hemispheres that were located either near (NS) or far (FS) -to-spindle. RT-qPCR provided striking examples of maternal mRNA localized to subcellular substructures S (NPM2, GMNN, H19, PCAF, DNMT3A, DNMT1, and STELLA), NS (SOX2, NANOG, POU5F1, and TET1), and FS (GCN) of MII oocyte. Immunoblotting revealed that specific maternal proteins DNMT3A and NANOG were asymmetrically enriched in MII-spindle-half of the oocytes. Topological analysis of sperm entry point (SEP) revealed that sperm preferentially entered via the MII-spindle-half of the oocytes. Even though, the topological position of first cleavage plane with regard to SEP was quite stochastic. Spatial comparison of lipid content revealed symmetrical distribution of lipids between 2-cell blastomeres. Lineage tracing using Dil, a fluorescent dye, revealed that while the progeny of leading blastomere of 2-cell embryos contributed to more cells in the developed blastocysts compared to lagging counterpart, the contributions of leading and lagging blastomeres to the embryonic-abembryonic parts of the developed blastocysts were almost unbiased. And finally, separated sister blastomeres of 2-cell embryos had an overall similar probability to arrest at any stage before the blastocyst (2-cell, 4-cell, 8-cell, and morula) or to achieve the blastocyst stage. It was concluded that the localization of maternal mRNAs and proteins at the spindle are evolutionarily conserved between mammals unfertilized ovine oocyte could be considered polar with respect to the spatial regionalization of maternal transcripts and proteins. Even though, the principal forces of this definitive oocyte polarity may not persist during embryonic cleavages.

Fig 1. Schematic illustration of the procedure used for microsurgical trisection of ovine unfertilized MII-oocytes using a manual method of oocyte trisection.

A, B) In vitro matured oocytes were denuded of cumulus cells and oocytes with evident first polar body (1Pb) were selected for zona removal. C) Ovine oocytes partially extrude the MII-spindle and associated chromosomes (S) as a cytoplasmic protrusion which is particularly evident following zona removal. D) Using this cytoplasmic protrusion as a reference point, oocytes were first bisected to oocyte hemispheres that were either near-to (NS) or far-from (FS) the MII-spindle (S). E) The obtained NS oocyte hemispheres were then used for preparation of cortical materials containing MII-chromosomes (S) and the enucleated NS oocyte hemispheres.

Fig 2. Investigation of transcript assymetry within oocyte and early embryo.

A) Unfertilized ovine MII-oocytes were micro surgically trisected to generate cortical materials containing MII-spindle material (S) and oocyte hemispheres that were either near to (NS) or far from (FS) the MII-spindle. The pools of S, NS, and FS cytoplasmic fragments were prepared and used for comparative RT-qPCR using 21 developmentally important genes. Obtained profiles revealed four patterns of transcript abundances within the three fragments of the oocyte. B) To understand whether ovine early embryos inherit differential transcripts localization observed within MII-oocyte, the transcriptional distribution of transcripts between early embryonic sister blastomeres was investigated using RT-qPCR. For this purpose, one blastomere of ovine early embryo was labeled with Dil and the embryos were followed during the first and second embryonic divisions to generate pools of leading and lagging blastomeres. The leading and lagging pool of blastomeres were used for quantitative analysis of those transcripts that were found to be differentially located with unfertilized MII oocytes.

Fig 3. Lineage tracing of the leading and lagging blastomeres.

A cohort group of blastocysts derived from leading-labeled two-cell embryos that are observed using appropriate Hoechst-33342 (A) and Rhodamine (B) filters. C) Criteria for sorting of Dil-labeled embryos. Scoring the orientation of the Em-Ab axis in ovine blastocysts relative to the first cleavage plane of the 2-cell embryo. After drawing imaginary lines of the Em-Ab axis and equatorial plane, the blastocysts were classified to three groups depending on the angular departure of the boundary line of the fluorescent/nonfluorescent cells from the equatorial plane: i) specified: when the angular departure was almost 90°, ii) semi-specified: when the angular departure was ≥45°, iii) un-specified: when the angular departure was <45°. D1-F4) Representative images of specified (D1-D4), semi-specified (E1-E4) and un-specified (F1-F4) observed under bright light (D1, E1, & F1), and H33342 (D2, E2, & F2) and Rhodamine (D3, E3, & F3) filters. D4, E4, & F4 images show merged images of H33342 and Rhodamine. Bar represents 100 μm.

Fig 4. Topological assessment of sperm entry point (SEP) in ovine eggs.

Fertilzied oocytes were rotated under constant UV-light until MII-spindle was positioned to 3 O’clock. Then, the spatial relationship between SEP and MII-spindle was measured in 4 imaginary oocyte zones. In this scheme, zones I and IV are the closest and the furthest from the MII-spindle, respectively. Table represents the results of topological assessment of SEP.

Fig 5. Comparative analysis of total mRNA contents between subcellular structures of MII-oocyte.

^a-c: values with different superscripts are significantly different at p<0.05.

Fig 6. Expression of selected genes in subcellular structures of MII-oocyte.

^a-c: values with different superscripts are significantly different at p<0.05.

Fig 7. The importance of orientation of the embryonic divisions.

Cleavage can occur either equatorially or meridionally, along the animal-vegetal axis, with reference to MII-spindle as the hypothetical animal pole. A result of equatorial first cleavage division is the transcriptome asymmetry between the balstomeres of 2-cell embryo that could persist even after the second cleavage division. By contrast, a result of meridional first cleavage division is the transcriptome symmetry between the balstomeres of 2-cell and 3-cells embryos, despite oocyte transcript polarity. Moreover, indiscriminate separation of 2-cell embryos yields constraints that exclude the possibility that transcriptome asymmetry within MII-oocyte leads to programmed asymmetric transcriptome inheritance between blastomeres of 2-cell embryos.

Fig 8

A) Comparative analysis of total protein contents within subcellular structures of MII-oocyte. ^a-c: values with different superscripts are significantly different at p<0.05. B) Protein patterns of oocyte substructures revealed by SDS-PAGE electrophoresis. C) Western blot analysis of NANOG and DNMT3A between NS+S and FS oocyte fragments.

Fig 9. Representative figures and Nile-red staining for investigation of lipid droplet distribution with MII-oocyte (A-A”) and between sister blastomeres of 2-cell embryo (B-C”).

Lipid-droplets are evenly distributed within MII-oocyte (A-A”), and in majority of 2-cell embryos (A-A” & D). However, in some 2-cell embryos, one balstomere had significantly higher lipid droplets (C-C” and E). Bar represents 100 μm. *: significantly different at P<0.05.

Fig 10. Contributions of leading and lagging blastomeres to the total cell count of the developed blastocysts.

In majority (75%) of blastocysts, leading blastomeres contributed to significantly higher cells of the blastocyst compared to the lagging counterpart. In the remaining 25%, the contributions of leading and lagging blastomeres to total cell count of the blastocysts were comparable. *: significantly different at P<0.05.

Fig 11. Developmental competence of corresponding sister blastomeres derived from ovine 2-cell embryos.

The proportion of sister blastomeres that developed to certain developmental stages (y-axis: 2-cell block (2CB), two further cleavage (4-cell block: 4CB), three further cleavage (8-cell block: 8CB), four further cleavage (morula block: MB), and those blastomeres that reached the blastocyst stage (BLS)). The size of circles correspond their relative proportions (numbers within the circles).