Prepubertal heifers versus cows-The differences in the follicular environment

The oocyte quality is to a large extent influenced by the sexual maturity of the donor female. Although this phenomenon has already been broadly described in domestic animals, the underlying mechanisms are poorly understood. Published data focus on oocyte ultrastructure, fertilization abnormalities, and blastocyst developmental rate. The goal of the present experiment was to characterize the follicular environment (oocyte, cumulus [CC] and granulosa (GC) cells as well as follicular fluid [FF]) in ovarian follicles of prepubertal heifers and cows. Each experimental replicate included the following set of traits within individual follicles: lipid droplets (LDs) number in oocytes, expression of seven genes involved in energy metabolism (fatty acids [FAs] metabolism-ELOVL2, ELOVL5, SCD, FADS2, glucose transport-GLUT1, GLUT3, GLUT8) in CC and GC as well as FA composition and glucose concentration in FF. According to our results, cow oocytes were larger in diameter and contained more LD than those from prepubertal heifers, both before and after IVM. The LD number was also higher in cow oocytes after IVM, when compared to immature oocytes. The FF from cow follicles had elevated glucose content similarly to the majority of the analyzed FA. Transcript analysis revealed differences for five out of seven analyzed genes (ELOVL, FADS2, SCD, GLUT3, GLUT8) in CC and GC cells. However after considering the female category, the only difference was noticed for the mRNA of SCD gene, which was more abundant in cow GC. This finding may indicate distinct roles of CC and GC in follicular energy metabolism. In conclusions, we suggest that distinct properties of follicular environment in prepubertal heifers and cows may be responsible for differences in the quality of oocytes from the two categories of donors. We hypothesize that suboptimal environment in heifer follicles (glucose and FA lower content in FF) determines reduced quality of their oocytes (lower diameter and LD number) and limited maturation potential. Besides, energy demands of heifer oocytes may be restricted due to a low LD number, exerting a negative effect on the development of the future embryo. The advantages of cow gametes (e.g., higher LD number and diameter) attributed to oocytes of superior quality may support the statement that cows donate oocytes of better quality than heifers.

Three-dimensional positioning of B chromosomes in fibroblast nuclei of the red fox and the chinese raccoon dog

Great progress has been achieved over the last years in studies on chromosome arrangement in mammalian cell nuclei. Growing evidence indicates that the genome's spatial organization is of functional relevance. So far, no attention has been paid to the nuclear organization of B chromosomes (Bs). In this study we have examined nuclear positioning of Bs in 2 species from the Canidae family--the red fox and the Chinese raccoon dog. Using 2D and 3D fluorescence in situ hybridization and 2 gene-specific probes (C-KIT and PDGFRA), we analyzed the location of Bs in fibroblast nuclei. We found that small Bs of the red fox occupied mostly the interior of the nucleus, while medium-sized Bs of the Chinese raccoon dog were observed in the peripheral area of the nucleus as well as in intermediate and interior locations. The more uniform distribution of B chromosomes in the Chinese raccoon dog may be the result of differences in their size, since 3 morphological types of Bs are distinguished in this species. Our results indicate that 3D positioning of B chromosomes in fibroblast nuclei of the 2 canid species is in agreement with the chromosome size-dependent theory.

Early cleaved bovine embryos show reduced incidence of chromosomal aberrations and higher developmental potential on day 4.5 post-insemination

The present study aimed to investigate whether the timing of the first zygotic cleavage (FZC) influences the speed of embryo development expressed by the total cell count and the rate of chromosomally aberrant embryos. Bovine embryos were produced in vitro and divided into two categories according to the timing of FZC: early cleavers (at 30 hpi; EC) and non-early cleavers (at 48 hpi; NEC). On day 4.5 pi, embryos were grouped into three classes depending on the number of blastomeres: delayed (<8 BL), normal (8-16 BL) and advanced (>16 BL). We applied fluorescence in situ hybridization (FISH) with probes for bovine chromosomes 6 and X. The only form of chromosomal imbalance observed was mixoploidy [(2n/3n; 2n/4n); 19.9%, 54/271]. Early cleavers were less often chromosomally unbalanced (13.9%, 20/144) than their NEC counterparts (26.7%, 34/127). Among embryos developing at a normal speed, the NEC embryos were more often abnormal (NEC 20/80; EC 10/79; p < 0.05). The advanced embryos were not observed among the NEC category, whereas such embryos from EC category displayed no chromosomal aberrations. The majority of embryos arrested at the 8 BL stage were of NEC category and were carriers of chromosomally abnormal blastomeres. With regard to embryonic sex, we demonstrated that although males dominate among bovine embryos developing in vitro, the incidence of mixoploidy was equal for both sexes. It can be suggested that a good-quality bovine embryo is usually an early cleaver that develops at higher speed and contains less aberrant cells. The present study also confirmed the usefulness of the FZC as a marker of embryo quality by demonstrating a significantly lower incidence of aberrations in early embryos.

269 RELATIVE TRANSCRIPT ABUNDANCE OF EIGHT MARKER GENES IN OOCYTES OF PREPUBERTAL AND CYCLIC GILTS

Oocytes from prepubertal (P) females usually show impaired quality when compared with those of adult animals. Recently, we have shown that although P oocytes mature in vitro at similar rates as oocytes from cyclic (C) gilts, they are more often chromosomally unbalanced. We have also reported some impaired processes in cytoplasmic maturation of the P oocytes. This was especially evident in cases of a disturbed mitochondria redistribution and more frequent apoptosis. It is therefore hypothesised that incomplete cytoplasmic maturation is responsible for the decreased developmental competence of porcine oocytes. A factor reflecting the status of cytoplasmic maturation is the relative transcript abundance (RA) of developmentally important genes. We investigated the RA of 8 genes known as markers of oocyte quality. Ovaries of unknown origin were collected from P and C gilts in a local slaughterhouse. Cumulus–oocyte complexes (COC) were aspirated from 2- to 6-mm follicles and evaluated morphologically. Pools of 25 denuded oocytes with a proper morphology were frozen in liquid nitrogen (pre- and post-IVM). Altogether, 4 groups of oocytes were analysed: P pre-in vitro maturation (IVM), P post-IVM, C pre-IVM, and C post-IVM. A two-step IVM protocol was performed in NCSU23 medium for 44 h. Transcript analysis included total RNA isolation followed by reverse transcription and real-time PCR using fluorescence resonance energy transfer probes for 8 genes (BMP15, GDF9, ZAR1, ATP5A1, EEF1A1, GSTA2, TOP2B, SMARCA4). Each of the four groups was represented by 6 independent oocyte pools (6 × 25 oocytes × 4 groups = 600 oocytes), which were analysed in duplicate. Among the pre-IVM oocytes, no significant differences in RA were observed. However, the post-IVM oocytes of C gilts were characterised by higher RA of 3 marker genes [BMP15 (P < 0.05), GDF9 (P < 0.01), ZAR1 (P < 0.05)]. The RA of the GSTA2 and ATP5A1 genes showed a tendency to be higher in C oocytes, whereas the RA of the EEF1A1, TOP2B, and SMARCA4 genes did not differ between P and C gilts. Although the reduced quality of P oocytes has been well documented, only 10% of genes were differentially expressed in oocytes of P and C gilts (Paczkowski et al. 2011). In this study, we demonstrated that the sexual maturity of the donor gilt affected transcript abundance of 3 out of 8 developmentally important genes. Our results corroborate the previous findings and extend them to new transcripts (GDF9, BMP15, ZAR1). We did not show differences in the RA of genes regulating energy production and transcription (ATP5A1, EEF1A1). Some genes, such as TOP2B (apoptosis control) and SMARCA4 (a chromatin remodelling factor), were expressed at similar levels. We suggest that the RA of the ZAR1, BMP15, and GDF9 genes may serve as markers of the reduced quality of oocytes from P gilts and, because of their function, might alter cytoplasmic maturation. Funding was from the Polish Ministry of Science and Higher Education, COST GEMINI FA0702-DWM/N190/COST/2008.

264 MITOCHONDRIAL DNA COPY NUMBER IN OOCYTES OF PREPUBERTAL AND CYCLIC GILTS

In many domestic species (pig, cow, sheep), oocytes from prepubertal females show impaired quality when compared with those from adult animals. Incomplete cytoplasmic maturation is thought to be the main factor responsible for reduced developmental competence of embryos derived from prepubertal oocytes. The status of ooplasm maturation is also reflected by the copy number of mitochondrial DNA (mtDNA). Because replication of mtDNA ceases when oocytes reach their final size and occurs again at the blastocyst stage, the mtDNA copy number is a proved marker of oocyte quality in the pig (El Shourbagy et al. 2006 Reproduction 131, 233–245). The number of mtDNA copies in the grown oocyte is crucial to sustain the first embryonic divisions. To increase the rate of good-quality blastocysts, oocytes of domestic animals have been evaluated by the brilliant cresyl blue test (BCB). According to El Shourbagy et al. (2006), more competent BCB+ oocytes possess higher copy number of mtDNA (on average 222 446) than do their BCB– counterparts (115 352). However, there are no published data on the variation in mtDNA copy number in oocytes derived from ovaries of prepubertal (NCL) and cyclic (CL) gilts. Ovaries of NCL and CL gilts were collected in a local slaughterhouse. Cumulus–oocyte complexes (COC) were aspirated from nonatretic follicles 2 to 6 mm in diameter and evaluated morphologically. Only COC with a proper morphology were subjected to the BCB test. A group of non-BCB-treated COC served as control. Four groups of COC were collected: BCB+ (CL, NCL) and control (CL, NCL). Follicular cells attached to oocytes were removed by pipetting, and completely denuded gametes were individually frozen in liquid nitrogen. Analysis of the mtDNA copy number included isolation of the total DNA followed by amplification of the Cytochrome b (CYTB) gene by real-time PCR (one copy per one mitochondrial genome). Differences in mtDNA copy number among experimental groups were evaluated by Student’s t-test. To date, 30 BCB+ oocytes have been analysed individually (15 CL and 15 NCL). The analysed parameter varied in a wide range from 79 852 to 522 712 copies in CL oocytes and from 52 270 to 287 852 copies in NCL oocytes. Oocytes from cyclic gilts contained significantly more mtDNA copies (on average 267 524) than did gametes of prepubertal females (179 339; P < 0.05). The data on the mtDNA copy number in the control oocytes are currently under investigation. The preliminary results indicate that impaired oocytes quality of prepubertal gilts may be also attributed to the reduced copy number of mtDNA. This project was sponsored by MSHE Poland (grant no. 451/N-COST/2009/0).