| Sheep are large precocial species, and the maximum myofibre complement of a muscle is achieved prior to birth,especially in the second half of gestation in the case of sheep. The potential of growth in muscle depends on the muscle fibre number formed during the prenatal stage. In the present study, first, we examined the patterns of proliferation of myofibre in these two sheep breeds during the second half of gestation .Then, using the commercial and standardized sheep transcriptome-wide oligo DNA microarray (Agilent), we first analyzed the transcriptomic profiles of longissmuss dorsi muscle from fetuses of Texel and Ujumqin sheep, sampled at d70, d85, d100, d120 and d135 of gestation respectively. The main findings are followings:1. Dynamic changes of body weight and individual muscle weight during the fetal development in Texel and Ujumqin sheep.Weights of fetus and the individual muscle tissue in Texel sheep were significantly higher than Ujumqin sheep at the stage of 70,85 and 100d (P<0.05 or P<0.01), whereas there were no differences between them at the stage of 120 and 135d (P>0.05). There was specific profile of each muscle tissue between two sheep breeds during the skeletal muscle development. Weights of the semimembranosus, longissimus dorsi, quadriceps femoris, gluteus medius and flexor digitorum superficialis muscle in Texel sheep were significantly higher than Ujumqin sheep at 85 day of gestation. Until the stage of 100d, the Texel sheep had the preponderance over the Ujumqin sheep and the differences between them became ever greater than else. When developing at 120 and 135 day of gestation, there were no differences in the remaining 11 muscle tissues except that the infraspinatus in Texel sheep grew faster than Ujumqin sheep (P<0.05). The preponderance of development and growth in Texel at 85 and 100 day of gestation underlied their fast postnatal growth of muscle.There were distinct patterns of hyperplasia of muscle fibre between Texel and Ujumqin sheep at the histological level during the second half of gestation. First, there were obvious hyperplasia in musculus longissimus dorsi, gastrocnemius, semitendinosus, gluteus medius and triceps brachii in these two sheep breeds. However the timing of proliferation in myofibre was different between Texel and Ujumqin sheep. Second, there were hyperplasia in semimembranosus, quadriceps femoris and biceps femoris in Texel sheep, but not observed in Ujumqin sheep. Third, there were proliferation of myofibre in adductor and infraspinatus in Ujumqin sheep, but not observed in Texel sheep. Finally, there was no obvious hyperplasia of myofibre observed in supraspinatus in both sheep breeds during the second half of gestation. However, the two sheep breeds had the same patterns of myofibre cross-section area during the skeletal muscle development.2. Profiles of gene expression in sheletal muscle between Texel and Ujumqin sheep during the second half of gestation.2.1 GO analysis showed that in general, the categories of biological processes involved in myogenesis were similar in Texel and Ujumqin sheep. Mainly, they included the following biological processes: metabolism, transport, cell communication, response to external stimulus, cell death, morphogenesis, regulation of transcription , regulation of cellular process, cell proliferation, cell differentiation. However, Texel and Ujumqin sheep quite differred in certain biological processes for differential genes across the development times. For example, genes related to DNA, RNA , regulation of gene expression and cell proliferation were enriched significantly in the Texel gene set, while genes involved in fat anabolic metabolism, muscle development, immune response, morphogenesis and cell differentiation were enriched significantly in Ujumqin gene set.We demonstrated that there were varietal specific gene expression profiles derived from differentially expressed genes in each sheep breed across the five developmental stages. There are 3 and 4 kind of profiles in Texel and Ujumqin sheep, respectively.Also there were more uptrend genes and profiles in Texel, compared with Ujumqin sheep. It suggested that the additional up-regulated genes might be part of target genes of myostatin. We identified two different gene network associated with myogenesis in Texel and Ujumqin sheep across the development stages.Network in Texel sheep is involved in function of"RNA Post-Transcriptional Modification","Cellular Development"and"Skeletal and Muscular System Development and Function", while the one in Ujumqin sheep focuses on"Lipid Metabolism","Small Molecule Biochemistry"and"Skeletal and Muscular System Development and Function".2.2 At the each developmental stage between the two breed, comparisons showed that differentially expressed genes were enriched in various signaling pathways or networks, genes in which probably are targets of myostatin in skeletal muscle development, suggesting that perturbation of myostatin can affect several biological processes during the myogenesis. The cellular functions of the differentially expressed probes that matched annotated genes revealed characteristic differences in various categories between two breeds at each developmental stages. The diversity of transcription profiles in prenatal skeletal muscle underlies the phenotype variation in these two sheep groups.This present study revealed that there were intrinsic differences in skeletal muscle development on histological level as well as on transcriptomic level between Texel and Ujumqin sheep. Next works are to elucidate the putative networks involved in skeletal muscle development on protein level and to determine direct or indirect target genes of myostatin in the context of development.
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