ABSTRACT
This Atlas focuses on the development of the human brain during the late first trimester, and is Volume 4 in the Atlas of Human Central Nervous System Development series. Volume 1 (Bayer and Altman, 2002) provides a record of the development of the spinal cord from the 4th gestational week (GW4) to the 4th postnatal month. Volume 2 (Bayer and Altman, 2004a) records brain development during the third trimester, with the specimens ranging in age from GW37 to GW26. Volume 3 (Bayer and Altman, 2005) presents brain development during the second trimester, from GW24 to GW13.5. The specimens dealt with in the present volume cover the period from GW11 to GW7.5. The major theme of Volume 2 is the maturation of the brain’s settled and enduring neuron populations. The major theme of Volume 3 is the migration, sojourning, and settling of neuronal populations. Prominent migratory streams and sojourning structures are evident by GW13.5 and many of these are still present at GW24. The major theme of the present volume is late neurogenesis and early neuronal differentiation. The structures of central interest are the visually distinguishable divisions, or mosaics, of the germinal neuroepithelium that generate different populations of neurons and neuroglia. The major theme of the forthcoming Volume 5, which will deal with brain development during the early first trimester (GW3-GW7), will be neuroepithelial expansion along the expanding brain ventricles, and early neurogenesis. As in Volumes 2 and 3, the specimens are presented in a reverse, older-to-younger order. We do that for both heuristic and pedagogical reasons: proceeding from the familiar and better known to what is less familiar and often uncertain or hypothetical.
