Effect Of Monochromatic Light Stimuli During Embryogenesis On Muscular Development And Mechanism In Broilers

Poultry are light sensitive species, and light had become an important environmentalfactor that influences the poultry performances. Artificial illumination has been widely usedto promote avian productive performance in modern poultry industry. The hatching period offast-growing broilers takes21days, which almost takes one third of its life span (calculatedby a42-day feeding period). Therefore, broiler performance promoted by light stimuli duringembryogenesis might be important for poultry industry. This dissertation consists of4experiments. The effect of monochromatic light stimuli during embryogenesis on breastmuscle growth, chemical composition, meat quality, blood hormone levels, muscle fibermorphology, satellite cell mitotic activity, along with expression of growth-related factors andmyogenic regulatory factors in pectoral muscle of broilers was systematically investigated byusing the methods of absorption spectrometry, radioimmunoassay, enzyme-linkedimmunoassay, immunohistochemistry, and real time quantitative RT-PCR. These data willprovide a scientific data for poultry hatchery.Trial1focused on the effect of monochromatic light stimuli during embryogenesis onmuscular growth, chemical composition, and meat quality of breast muscle in male broilers.Fertile broiler eggs (Arbor Acres, n=1320) were preweighed and randomly assigned to oneof three treatment groups in3modified incubators:1) control group (in dark condition),2)monochromatic green light group (560nm), and3) monochromatic blue light group (480nm).The monochromatic lighting systems sourced from light-emitting diode lamps and wereequalized at the intensity of15lx at eggshell level. After hatching,120male chicks from eachgroup were placed in6replicates with20birds each. All the birds were housed under whitelight (30lx at bird-head level) with a light schedule of23L:1D. At21,35, and42d of age,BW and breast muscle weight in the green light group were significantly increased comparedwith birds in the blue or dark groups (P <0.05). The breast muscle weight and breast musclepercentages in birds incubated under green light were significantly elevated by50.39g (11.78%) and54.07g (12.70%) than those in dark condition or blue group at42d of marketage (P <0.05), respectively. In the green light group, feed intake during0⁴2d was higherthan that in the other2treatment groups (P <0.05); feed conversion ratio during0³5d and0⁴2d were lower than that in the dark condition (1.66vs.1.73during0³5d;1.80vs.1.88during0⁴2, P <0.05, respectively). No significant differences in the contents of breastmoisture, CP, crude fat and crude ash among all groups were observed (P>0.05). Green lightstimuli tended to increase cooking loss (P=0.08), and L^*value of24-h meat color (P=0.09).These results suggested that green light stimuli during embryogenesis enhanced the posthatchBW of male broilers, increased breast muscle growth, and improved the feed coversion ratio,but it did not cause any noticeable changes in breast chemical composition or overall meatquality characteristics.Trial2was aimed to investigate whether the regulation of chicken embryonic orposthatch growth by green light stimuli during incubation is associated with the changes ofexdocrine axes by measuring dynamical changes in plasma growth hormone (GH),insulin-like growth factor-I (IGF-I), triiodothyronine (T₃), thyroxine (T₄), and testosteroneconcentrations at different ages of embryogenesis and posthatching chicks. Fertile broilereggs (Arbor Acres, n=880) were preweighed and randomly assigned to1of2treatments in2modified commercial incubators:1) control group (in dark condition), and2) monochromaticgreen light group (560nm). Plasma samples were obtained from15d of embryogenesis (E15)to42d of posthatch (H42) according to sampling protocol. Compared to the control group,significantly higher plasma GH levels in embryos and chicks incubated under green lightwere observed from E19to H5(P <0.05). Green light stimuli during incubation significantlyincreased plasma IGF-I levels of embryos from E17to E19, and of chicks from H3to H35compared with those incubated under dark condition (P <0.05). No significant differenceswere found (P>0.05) in plasma T3, T4and testosterone in embryos or hatched chicksbetween the2groups, although green light group showed a trend in increasing plasma T3levels in hatched chicks from H7(P=0.09) to H21(P=0.06). These data suggested thatsomatotropic axis hormone GH and IGF-I might be the most important contributor to chickgrowth promoted by green light stimuli during embryogenesis.Tiral3was carried out to assess whether the regulation of chicken embryonic orposthatch growth by green light stimuli during incubation was associated with the changes ofgrowth-related factors in pectoral muscle by measuring dynamical changes in mRNAexpression of GHR, IGF-I, IGF-IR and MSTN at different ages of embryogenesis andposthatching chicks. Compared to the birds in control group, green light stimuli duringincubation significantly up-regulated pectoral muscle GHR mRNA levels from E17to E19, and MSTN mRNA levels of embryos from E17to E19and of chicks from H5to H7(P <0.05), respectively. No significant differences were observed in pectoral muscle IGF-I andIGF-IR mRNA levels of embryos between green light group and control group (P>0.05).However, green light stimuli significantly up-regulated muscle IGF-I (from H3to H1) andIGF-IR (from H5to H7) mRNA levels of posthatch chicks (P <0.05). These resultssuggested that green light stimuli during incubation promote posthatch muscle growth byup-regulating the expression of these growth-related factors and MSTN in late embryos andearly posthatch stage.Tiral4was conducted to further investigate the morphological and molecular basis ofthis phenomenon by measuring changes in BW, pectoral muscle weight, satellite cell mitoticactivity, and the developmental expression of MyoD and MyoG mRNA in pectoral muscle inembryos and early posthatch broilers. At7d of age (H7), BW and pectoral muscle weight inthe green light group were significantly increased by14.33g and1.46g compared with birdsin the blue or dark groups (P <0.05). Green light stimuli during embryogenesis significantlyincreased myofiber cross-sectional areas of pectoral muscle on H5to H7(P <0.05). Index ofsatellite cell mitotic activity was also higher in the green light group on H1and H3(P <0.05).Besides, in green light group, higher MyoD mRNA levels from E17to H3, and highermygenin mRNA levels from H1to H5were observed in pectoral muscle of embryos orbroiler (P <0.05), respectively. These data suggested that green light stimuli duringembryogenesis promoted posthatch muscle growth by increasing satellite cell mitotic activity,enhancing myoblast proliferation and differentiation in late embryos and early posthatchstage.In summary, monochromatic green light stimuli during embryogenesis stimulated thesecretion of somatotropic axis hormone GH and IGF-I in late embryos and posthatch chicks,upregulated pectoral muscle growth-related factors (GHR and MSTN) and muscle regulatoryfactors (MyoD and MyoG) mRNA expression, increased satellite cell mitotic activity,enhanced myoblast proliferation and differentiation in late embryos and early posthatch stage,and improved subsequently pectoral muscle growth of posthatch broilers.