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Growth hormone secretion is under the control of a pair of hypothalamic factors, growth hormone releasing hormone and somatostatin. The growth hormone secretagogue receptor (GHSR) and its endogenous ligand represent a novel third method regulating the release of growth hormone. Early chicken embryonic development has been proposed to be independent of GH. However, recent evidence shows that peripheral GH secretion has paracrine/autocrine functions during embryonic development. In the current study, we used the reverse-transcriptase polymerase chain reaction to determine the expression pattern of the GHSR during embryonic development and the effects of in ovo recombinant human (rh) IGF-I administration on its expression pattern. Eggs were injected once with 100 ng rhIGF-I in 10 mM acetic acid, and 0.1% BSA per embryo on embryonic day 3. Total RNA was isolated from whole embryos on embryonic day (E) 0-6 (n=6 per day), thoracic/abdominal halves of the embryos on E7- E8 (n= 6 per day) and Pectoralis muscle on E9-E20 (n= 4 per day). We found that GHSR expression was low during E0-E4, followed by an increase on E5 and remained constant through E17. GHSR expression then increased on E18 before reducing on E20. A similar pattern was found in the rhIGF-I treated embryos with the exception of a significant increase in GHSR expression on E8. These data indicate that the GHSR may be active in regulating GH secretion during early embryonic development, and upregulation of the GHSR gene following IGF-I administration may have an important role in the determination of postnatal muscle growth.

Data are presented on the pattern of organ growth in the Common Tern, a semi-precocial bird species with a high growth rate, and are compared with data from some bird species within the altricial-precocial spectrum. The data show that a high growth rate is linked to a growth pattern characterized by a rapid early development of the digestive organs and a late development of pectoral muscles and feathers. These results suggest that the growth rate is most likely restricted by the capacity to ingest and digest food. It also appears that the organ growth pattern is more closely related to the growth rate than to the degree of precocity.

Body weight, pectoralis muscle weight, pectoralis protein and DNA concentration, and plasma GH and IGF-I concentrations of broiler chicks (BrBr), bantam chicks (BaBa) and reciprocal crosses between them (BaBr and BrBa) were measured between 0 and 42 days after hatching. At hatch, body weight and pectoralis weight of the two types of chicks from broiler eggs (BrBr and BaBr) were equal to each other but greater than the two types of chicks from bantam eggs (BaBa and BrBa), which were not different from each other. BrBr chicks grew more rapidly than crossbreds and BaBa chicks grew more slowly. Weights of the reciprocal crosses (BaBr and BrBa) were markedly different at day of age, but converged by day 14. The increase in pectoralis muscle mass of BrBr chicks exceeded that of the reciprocal crosses which in turn exceeded that of BaBa chicks. The increase in pectoralis DNA content and protein content followed the same pattern. The DNA unit size, as expressed by the protein:DNA ratio, was markedly lower in pure bantam chicks from 14 to 42 days of ages, whereas the unit size did not differ between the intermediate sized reciprocal crosses and the large bodied broiler chicks. Differences in muscle mass were primarily achieved by differences in the number of DNA units although a difference in unit size was also a factor. There were no clear relationships between growth and plasma growth hormone or insulin-like growth factor I concentrations. Thus while satellite cell proliferation is primarily responsible for genotypic differences in muscle mass, the plasma growth hormone-IGF-I axis does not appear to be regulating their proliferation.

The influence of sex and genotype on growth of chick embryos was studied using eggs from commercial broiler hens mated with broiler strain males (BrBr) or with bantam males (BaBr) and using eggs from bantam females mated to bantam males (BaBa) or to broiler strain males (BrBa). Male BrBr embryos were first significantly heavier than females at 9 days of incubation. The influence of sire's genotype was expressed between 6 and 16 days of incubation but not at 20 days. Dam's genotype, and the associated difference in egg weight, significantly affected embryo weight by 12 days of incubation and by 20 days it alone influenced embryo weight. Plasma insulin-like growth factor-I (IGF-I) levels of BrBr and BaBr embryos did not differ at 8, 10, 12, or 14 days of incubation, nor were there sex differences. There were no genotype differences in whole body protein concentration, DNA concentration or protein:DNA ratios at 4 or 8 days of incubation. At 12 days the pectoralis major DNA concentration of BaBa embryos was significantly higher than that of all other groups. Pectoralis muscle protein concentration of BrBa embryos was significantly lower than those of BaBr embryos at 12 days of incubation, but by 16 days it was highest. The protein:DNA ratios of embryos developing in bantam eggs were significantly lower than those in broiler eggs at 12 days. In summary, genotypic and sex differences in body weight and muscularity that develop after hatching appear to be predetermined by differences in embryonic growth patterns, but there is no clear relationship between embryonic growth, plasma IGF-I concentration or the concentration of DNA or protein in pectoralis muscle.