Opportunities and challenges in applying genomics to the study of oogenesis and folliculogenesis in farm animals

Abstract

Ovarian oogenesis and folliculogenesis are complex and coordinated biological processes which require a series of events that induce morphological and functional changes within the follicle, leading to cell differentiation and oocyte development. In this context, the challenge of the researchers is to describe the dynamics of gene expression in the different compartments and their interactions during the follicular programme. In recent years, high-throughput arrays have become a powerful tool with which to compare the whole population of transcripts in a single experiment. Here, we review the challenges of applying genomics to this model in farm animal species. The first limitation lies in limited the availability of biological material, which makes the study of the follicle compartments (oocyte, granulosa cells and thecal cells) or early embryo much more difficult. The concept of observing all transcripts at once is very attractive but despite progress in sequencing, the genome annotation remains very incomplete in non-model species. Particularly, oogenesis and early embryo development relate to the high proportion of unknown expressed sequence tags. Then, it is important to consider post-transcriptional and translational regulation to understand the role of these genes. Ultimately, these new inferred insights will still have to be validated by functional approaches. In addition to in vitro or ex vivo functional approaches, both 'natural mutant' ewe models and RNA interference represent, at the moment, the best hope for functional genomics. Advances in our understanding of reproductive physiology should be facilitated by gene expression data exchange and translation into a better understanding of the underlying biological phenomena.

Transcriptome profiling of sheep granulosa cells and oocytes during early follicular development obtained by Laser Capture Microdissection

“…These percentages take into account the percentage of undetected genes (10 to 24%: Figure 6) which could be partly due to the relative specificity of heterologous hybridizations and the high percentage of missing genes (37 to 45%: genes reported in the previous studies that were missing in the bovine Affymetrix chip used here). This underlines the fact that genomics of livestock animals have to face with incomplete arrays and/or incomplete genome annotations [54]. …”

In vivo collection of follicular fluid and granulosa cells from individual follicles of different diameters in cattle by an adapted ovum pick-up system

“…However, the internal position of the ovaries and the small size of the follicles complicate the recovery of GC in vivo. Therefore, their availability for analysis remains limited [6]. To overcome this technical limitation, most studies use follicular fluid (FF) and GC collected from slaughterhouse ovaries [7-11] or after ovariectomy [12-14].…”

Implications of miRNA expression pattern in bovine oocytes and follicular fluids for developmental competence

“…In conclusion, our data identified a 46 few miRNAs in the follicular fluid and in the ooplasm that modulate the oocyte 47Recently, microRNAs (miRNAs), which regulate gene expression at the mRNA level, 65 have been associated with folliculogenesis and oogenesis [10, 11]. MiRNAs, which 66 range in size from 18 to 25 nucleotides (nt), have been found in the different 67 compartments of ovarian follicles, including granulosa cells [12, 13], theca cells [14], 68 follicular fluid and the oocyte itself [15]. Studies on the role of miRNAs during follicle 69 development in humans [16][17][18], mice [19, 20], cattle [10, 21, 22], pigs [23] and 70horses [24] suggest that they regulate the cellular differentiation processes which 71 occur during follicular development.…”