Compartmental analysis and glomerular filtration in chick embryos.

We have developed microtechniques that allow the determination of compartmental fluid distribution and glomerular filtration rate in chick embryos during three significant developmental periods: phase 1, the developmental period when the mesonephros alone is functioning; phase 2, periods of simultaneous meso/metanephric kidney function; and phase 3, the period during late development when the metanephros completely replaces the degenerated mesonephros. Water content of tissues is greater in younger embryos (89.4 +/- 0.2%, day 10) compared with older animals (78.3 +/- 0.5%, day 18). Although all major tissue components show an absolute increase in mass during this period, the embryo proper increases at five times the rate of the extraembryonic tissues. Glomerular filtration rate increases during development from 0.61 +/- 0.08 ml/h at day 10 to 2.31 +/- 0.11 ml/h at day 18. Glomerular filtration rate scales to body mass with an allometric exponent identical to adult birds only if total tissue mass (embryo + membranes) is considered. Our data suggest that significant errors in allometry will be encountered when scaling measurements are made on embryonic or fetal amniotes without taking into consideration the extraembryonic tissues.