Regassa et al. BMC Genomics 2011, 12:57 http://www.biomedcentral.com/1471-2164/12/57 maturation processes, the underlying transcriptional mechanism must be robust. Interestingly, some of the genes that are under expressed due to removal of CCs before in vitro maturation have vital roles in gene expression. The most important one is RNA polymerase II, an enzyme that plays a significant role in gene tran- scription. Reduced expression of this gene due to removal of CCs before in vitro maturation means the expression of other genes is greatly affected and hence the developmental competence of such oocytes is compromised. In vitro studies have shown that follicle stimulating hor- mone (FSH) dependent cyclic adenosine monophosphate (cAMP), the activator of MAPK signaling, is produced by CCs and diffuses to the oocyte via the gap junction [75,76]. The activated MAPK in turn activates the compo- nents of MPF to initiate meiotic resumption [77] and simulate mos mRNA cytoplasmic polyadenylation during Xenopus [78,79] and mouse [80] oocyte maturation. Low expression of molecules that play roles in biochemistry of oocyte maturation (MAP3K2, MAP3K3 and MAP4K14) in OO-CCs samples may imply defects in the maturation process due to removal of CCs before maturation. The capacity of the oocyte to metabolize glucose is positively correlated with its developmental potential and this depends on the presence of companion somatic cells [81,82]. Glucose is a pivotal metabolite for COCs and is metabolized via various pathways. During oocyte maturation, a large proportion of total glucose is meta- bolized in the CCs via the glycolytic pathway to provide substrates such as pyruvate for energy production [83]. Consistent with this, some genes that are involved in carbohydrate metabolism are under expressed due to removal of CCs indicating defective energy metabolism in the groups cultured without CCs and hence poor developmental competence. In general, removal of companion CCs at GV stage appeared to affect the gene expression of MII oocytes as a number of genes are over expressed in oocytes cul- tured with CCs relative to those cultured without. As explained above, some of these genes have been impli- cated to be involved in various biological processes that are pertinent to oocyte meiotic resumption and matura- tion supporting the notion that the presence of CCs during in vitro maturation is crucial for oocyte develop- mental competence. However, the majority of these over expressed genes are uncharacterized and/or their func- tions, particularly with regard to oocyte development and maturation, are poorly understood. Paradoxically, several previously identified and biologically important OSFs (GDF9, BMP6, 15, TGFBs), zona pellucida pro- teins (ZP2, 3, 4), the components of MPF (CDK1 and Cyclin B1) and others are missing from the list of genes over expressed in OO + CCs. Page 15 of 20 From these results, it can be argued that either these over expressed genes have functional redundancy with those missing genes or the expression of the latter is completed prior to CCs removal at GV stage and conse- quently they are detected as equally as those cultured without their CCs. One plausible explanation inline of the latter argument is the fact that bovine oocytes are transcriptionally active during folliculogenesis and tran- scriptional activity decreases at later stages of follicular development [84]. Additionally, our present microarray data analysis between GV and MII oocytes (data not shown) reveals that only GDF9 and CDK1 are slightly over expressed (fold change = 2.46) at MII relative to GV stage. Interestingly, while BMP15 and TGFB2 are over expressed at GV stage, ZP2, 4 and Cyclin B1 are equally expressed between the two stages. Interestingly several genes have been found to be over expressed in MII stage relative to GV stage including MPV17, ATP6V1D, TMEM127, NUDT14, UQCR, EXOSC6, MAPK10, CSNK1D, DBNL, FILIP1L, YIF1A and ARH- GAP27. Considering the transcriptional activity of bovine oocytes, the over-expression of transcripts at MII stage compared to the GV stage need further investiga- tion in terms of transcriptional regulation of genes dur- ing oocyte maturation. The absence of oocyte during in vitro maturation alters the gene expression profile of CCs Notable interspecies differences exist whether OSFs are mandatory for FSH induced CCs expansion in vitro. In rat, the presence of OSFs are crucial for in vitro CCs expansion as oocytectomized complexes (CCs - OO) failed to expanded their cumulus oophorus [45]. In cat- tle and pig, CCs expansion doesn’t depend on the pre- sence of these factors as CCs - OO expanded as equally as the intact ones [46,47]. In the present study the results of IPA showed that some of the genes under expressed due to removal of oocyte before in vitro maturation are classified into cellular growth and prolif- eration (VEGFA, GADD45A, FOS, EGR1, HAS2), cell cycle (CCND2, CDCA8, CDK6), and gene expression (FOSB, TGFB2, ATF3) functional groups (Figure 9). These genes are also mapped into a complex gene network that includes genes involved in cellular devel- opment such as Cyclin D, HPSE, JUNB and others (Figure 9). Some of these genes are not well character- ized and their direct role in the biology of CC is poorly understood. However, results of previous studies in dif- ferent species have shown that some of these genes are involved in cumulus expansion and oocyte maturation. For instance, genes from FOS family have been impli- cated as regulators of cell proliferation, differentiation and transformation and IGFPB proteins stimulate the growth promoting effects of IGF1 which in turn is