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Published online Nov 8. doi: /jgen The fetal stage is critical for skeletal muscle development, as well as adipose and connective tissue. MyomiRs represent an integral part of skeletal muscle development, i.e. .. confirmed as a direct target of miR-1 (Alteri et al., , Jash et al. SimplePie reported: A feed could not be found at A feed with an invalid mime type may.


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In summary, maternal under-nutrition during early to mid-gestation reduces muscle fiber numbers and during late-gestation decreases muscle fiber sizes in sheep and cattle; in rodents and pigs, the mid to late gestation are important for muscle fiber formation.

Maternal over-nutrition and fetal skeletal muscle development Besides maternal nutrient deficiency, maternal over-nutrition also affects fetal skeletal muscle muscular development 2013, mainly enhancing intramuscular adipogenesis and fibrogenesis.


In ruminant animals and muscular development 2013, adipogenesis begins around mid-gestation, which overlaps with myogenesis 1819 During fetal skeletal muscle development, a small portion of the progenitor cells differentiate into adipocytes, which form intramuscular fat and marbling in offspring Enhanced intramuscular fat accumulation is detrimental for health because increased intramuscular fat leads to skeletal muscle insulin resistance due to the paracrine effect of intramuscular adipocytes 48 - 50pre-disposing to Type 2 diabetes.

However, in animal production, enhancement of intramuscular fat accumulation or marbling improves meat quality; the amount of marbling is crucial for the flavor and juiciness of meat 73and is determined by muscular development 2013 number and size of intramuscular adipocytes Maternal over-nutrition elevates the expression of adipogenesis markers in skeletal muscle of mid-gestation fetuses A subsequent study also demonstrated that maternal over-nutrition resulted in increased number and size of adipocytes inside skeletal muscle of fetal sheep at late-gestation Postnatally, the increased adipocytes and total triglyceride content were also observed in offspring sheep of over-nourished mothers 4.

Besides myofibers and adipocytes, mesodermal progenitor cells can also differentiate into fibroblasts.


These cells synthesize connective tissue which forms endomysium, perimysium and epimysium in fetal skeletal muscle during late gestation Maternal over-nutrition increases the collagen content and cross-linking of skeletal muscle, heart and large intestine of fetuses, suggesting an important role of maternal nutrition during pregnancy in fetal fibrogenesis 74 - Similar increase in collagen and cross-linking was also observed in skeletal muscle of offspring with maternal over-nutrition 4.

In summary, maternal over-nutrition enhances intramuscular adipogenesis and fibrogenesis, increasing intramuscular fat and connective tissue content in offspring muscle. MiRNAs are single-strand RNA molecules of nucleotides in length 77which play a crucial role in developmental processes by regulating the expression of target mRNA The target mRNA transcripts of miRNAs include genes which play important roles in proliferation and differentiation So far, thousands of miRNAs have been discovered; thus, miRNAs have become one of the most abundant categories of gene regulatory molecules in multicellular organisms MiRNA and skeletal muscle development: InmiR was the first miRNA shown to play an important role in skeletal muscle development by muscular development 2013 the expression of connexin43, a gap junction protein required for skeletal muscular development 2013 fusion MyoD induces the transcription of miR 86which promotes myogenic differentiation 87 BMP-2, which is known to inhibit myogenesis, represses the expression of miR by inhibiting its maturation process Besides miR, miR-1 also promotes myogenic differentiation MicroR, a miRNA up-regulated during muscle differentiation, is likewise very important in muscle development MiR induces myoblast proliferation MEF2 transcription factor, a critical regulator of myogenesis, induces the expression of miR-1 and miR MiR also regulates connective tissue growth factor, a key mediator of fibrosis MiR-1 and miR in zebrafish control the expression of muscle genes and regulate sarcomeric actin organization The expression of miR increases during myogenic differentiation of C2C12 cells, as visualized by a GFP-related retroviral vector system In addition, miR promotes myogenesis by down-regulating the homeobox protein Hox-A11, an inhibitor of muscle differentiation MiRb regulates Pax3 protein level and ensures myogenic differentiation MiR has also been shown to induce myoblast differentiation by down-regulating Pax7