Changes in cell cycle parameters and cell number in the rat midbrain during organogenesis.

We employed 5-bromo-2'-deoxyuridine (BrdU) labeling to identify in vivo changes in the cell cycle patterns of the rat midbrain during the major period of midbrain organogenesis, gestational days (gd) 11 to 16. We also used quantitative stereology to determine changes in absolute cell numbers during these gestational time points. Between gd 12 and 16, the length of S-phase did not change significantly while the fraction of cycling cells decreased from 73 to 11%. The average cell cycle length was determined to be 15 h on gd 12 and 17 h on gd 16, the difference not being statistically significant. The cell number in the midbrain increased from 1.3E5 cells on gd 11 to 1.7E7 cells on gd 16. On gd 12 and gd 13, there was a significant negative correlation between litter position and midbrain cell number, the effect diminishing on later days of gestation. The combined use of quantitative stereology and flow cytometry to study brain development represents a novel application that allows for simultaneous evaluation of changes in cell proliferation kinetics and the resulting effect of those kinetic changes on embryonic midbrain development.