Stress responses in avian embryos

American Zoologist, Dec 1997 by Epple, August, Gower, Barbara, Busch, Marc Ten, Gill, Tejendra, Et al

In order to gain insight into the role of the allantoic barrier in stress responses and metabolite transfers, we injected two different amino acids (600 and 530 Wg/egg into the allantoic fluid): Asparagine, and valine, a branched amino acid with a very different structure.

Despite a large increase of the VAL concentration in the allantoic fluid (385%) following the injection, this substance did not cause a disturbance of the metabolite spectrum in allantoic fluid and plasma (Table 6). However, the concentrations of 10 amino compounds, including VAL, increased in the amniotic fluid. Since the concentrations of 9 of these substances (3-methylhistidine being the exception) did not decrease in the allantoic fluid, an origin of these substances from a source outside the allantois is likely. ASN injections caused an array of changes in all three fluids, with statistically significant increases or decreases of 19 metabolites (Tables 7a, 7b). At the current state of knowledge, a detailed discussion of these results would be premature. However, it is noteworthy that (1) the VAL and ASN concentrations of the plasma were not increased at the time of study (8 min p.i.), even though they had drastically increased in the allantoic fluid (406% and 275%, respectively); (2) 10 other metabolites had significantly decreased in the plasma (Tables 7a, 7b); (3) VAL and ASN increased in the amniotic fluid (297% and 1011%, respectively) despite no increases of their plasma levels. These observation raise the question of either a transfer via a portaltype vascular system, or transudation across the allantois/amnion barrier. Overall, it appears that ASN was unable to cross the allantoic/general blood barrier in appreciable quantities, but was nevertheless a much stronger stressor than VAL or 0.1 ml ethanol.

DISCUSSION

The data summarized in Table 1 show that much more work on the role of the allantois in endocrine regulations is needed. The stress-related changes in allantoic catecholamines show that this organ can serve as a temporary depot, and that it is highly regulated (Epple et al., 1992). However, we don't understand the significance of presence or absence of other messenger substances in the allantoic fluid. Are these compounds taken up from the circulation only when their titers exceed certain levels? Do they arrive via transfer from the blood, or via the kidneys? Are some of them produced by the allantois itself? Are they temporarily stored like the catecholamines, or permanently retained? Are catecholamine and steroid conjugates (Gill et al., 1983; Gill et aL, 1994) of the allantoic fluid inactivated metabolites, or can they be reused? What does the absence of circulating messengers, such as free steroids and thyroid hormones (Table 1), in the allantoic fluid indicate? Which, if any, peptide hormones pass the blood/allantois barrier? Clearly, the cannulated chicken allantois (Epple et al., 1992) offers unique possibilities to answer these questions, and to study endocrine responses of the embryo.