3-DIMENSIONAL RECONSTRUCTIONS OF THE DEVELOPING FOREBRAIN IN RAT EMBRYOS

Using a computerized three-dimensional reconstruction technique with s erially sectioned rat embryos, changes in the size and form of the for ebrain were studied on Embryonic Days (E) 12 (1 day after closure of t he neural tube), E15, E18, and E21 (2 days before birth). During this time, the forebrain changes from a relatively simple tubular structure with thin walls surrounding a large ventricular system to a thick-wal led brain with a highly convoluted but reduced ventricular system. On E12, the two components of the forebrain, the telencephalon and the di encephalon, cannot be distinguished. Considering the forebrain as a wh ole (the embryonic prosencephalon), its volume continually increases b etween E12 and E21 due to the generation, differentiation, and maturat ion of neurons and glia. Attention was paid to changes in the sizes of the ventricles, the neuroepithelium, and the parenchyma. Volumes of t he ventricles and the surrounding neuroepithelium rapidly expanded fro m E12 to E18 and then decreased by E21, while the volume of the parenc hyma continually increased. Differential growth of the telencephalon a nd that of the diencephalon were compared between E15 and E21. The exp ansion of the telencephalon was much larger than that of the diencepha lon. In the telencephalon, the volumes of the lateral ventricles and t he surrounding neuroepithelium increased between E15 and E18 and decre ased by E21, while in the diencephalon the volumes of the third ventri cle and its surrounding neuroepithelium continually declined between E 15 and E21. That observation is compatible with previous work showing that the majority of diencephalic structures develop earlier than thos e in the telencephalon. It is important to note that volume changes in the ventricles and the neuroepithelium are maintained in ''lock-step, '' suggesting a close relationship between the size of the ventricle a nd the size of the neuroepithelium. (C) 1994 Academic Press, Inc.