REVIEW Characterization and importance of microRNAs in mammalian gonadal functions M. M. Hossain & M. M. H. Sohel & K. Schellander & D. Tesfaye Received: 11 January 2012 / Accepted: 21 June 2012 # Springer-Verlag 2012 Abstract Recent progress in high throughput sequencing and bioinformatic analysis and other biochemical methods have fuelled our appreciation for the important role of microRNAs (miRNAs) in disease, fertility and development. These tiny RNAs were found to be potentially involved in various aspects of cellular processes of reproductive tissues by posttranscriptional regulation of protein coding genes. Mammalian gonads which exhibit strictly regulated spatio- temporal gene expression patterns are also known to express unique sets of miRNAs and genes involved in the miRNA biogenetic pathway. Studies on miRNAs and their associated processing enzymes have evidenced the contribution of these small regulatory RNAs to germ cell differentiation, post- meiotic male germ cell function and growth, and development and maturation of oocytes through pertaining tightly regulated gene expression. The existence, preferential and temporal expression of miRNAs and their processing machinery genes in different stages of testicular and ovarian cellular develop- ment have evidenced the potential role of miRNAs in testic- ular and ovarian physiology. MiRNAs are also found to be associated with functional regulation of gonadal somatic cells, namely Leydig cells and Sertoli cells in testis and granulosa cells/cumulus cells in the ovary in steroid synthesis. Here, we review the recent works on the involvement and diverse roles of miRNAs in the development and physiology of gonadal cells in mammalian reproduction. Keywords microRNAs . Gonads . Testis . Ovary . Fertility Introduction Gonads are the primary reproductive organ in sexual repro- duction that produce both gametes and steroid hormones. Successful gametogenesis in the gonads is a prerequisite for mammalian reproduction which involves complex molecu- lar mechanism for cellular functions as well as the interac- tion of different gonadal cell types, primarily the germ cells that develop into oogonia (primitive eggs) or spermatogonia (stem cell precursors of sperm) and the supporting somatic cells that are either the Sertoli cells and Leydig cells in the male or the granulosa cells and the theca cells in the female. A remarkable aspect of gametogenesis either in the testis or ovary is that germ cells exhibit strictly regulated spatiotem- poral gene expression for functional development of sperm or oocytes (Eddy 1998; Grimes 2004). Despite the fact that a large proportion of these transcripts are partially stored in translationally inactive free mRNA ribonucleoprotein par- ticles in mammalian haploid germ cells (Kleene 2003; Schmidt et al. 1999), much evidence has accumulated reporting the expression of miRNAs and their biogenesis pathway genes in germ cells and gonadal somatic cells (Hossain et al. 2009; Kotaja et al. 2006; Ro et al. 2007a, b; Buchold et al. 2010), which suggest the potential involvement of miRNAs in translational control during gametogenesis. MiRNAs are a class of small non-coding RNAs, which are 18–24 nucleotides long and are considered as one of the posttranscriptional suppressors through binding to their tar- get mRNAs by base-pairing and subsequently inducing either translational repression or mRNA destabilization (Kim 2005). They are estimated to comprise 1–5 % of animal genes, and thousands of miRNAs can be encoded by a genome at a time (Bentwich et al. 2005; Berezikov et M. M. Hossain Department of Animal Breeding and Genetics, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh M. M. H. Sohel : K. Schellander : D. Tesfaye (*) Institute of Animal Science, Animal Husbandry and Breeding Group, University of Bonn, Endenicher Allee, 53115 Bonn, Germany e-mail: tesfaye@itw.uni-bonn.de Cell Tissue Res DOI 10.1007/s00441-012-1469-6