The developing brain by trimester.

Transient anatomical entities play a role in the maturation of brain regions and early functional fetal networks. At the postmenstrual age of 7 weeks, major subdivisions of the brain are visible. At the end of the embryonic period, the cortical plate covers the neopallium. The choroid plexus develops in concert with it, and the dorsal thalamus covers about half the diencephalic third ventricle surface. In addition to the fourth ventricle neuroepithelium the rhombic lips are an active neuroepithelial production site. Early reciprocal connections between the thalamus and cortex are present. The corticospinal tract has reached the pyramidal decussation, and the arteries forming the mature circle of Willis are seen. Moreover, the superior sagittal sinus has formed, and at the rostral neuropore the massa commissuralis is growing. At the viable preterm age of around 24 weeks PMA, white matter tracts are in full development. Asymmetric progenitor division permits production of neurons, subventricular zone precursors, and glial cells. Myelin is present in the ventral spinal quadrant, cuneate fascicle, and spinal motor fibers. The neopallial mantle has been separated into transient layers (stratified transitional fields) between the neuroepithelium and the cortical plate. The subplate plays an important role in organizing the structuring of the cortical plate. Commissural tracts have shaped the corpus callosum, early primary gyri are present, and opercularization has started caudally, forming the lateral fissure. Thalamic and striatal nuclei have formed, although GABAergic neurons continue to migrate into the thalamus from the corpus gangliothalamicum. Near-term PMA cerebral sublobulation is active. Between 24 and 32 weeks, primary sulci develop. Myelin is present in the superior cerebellar peduncle, rubrospinal tract, and inferior olive. Germinal matrix disappears from the telencephalon, except for the GABAergic frontal cortical subventricular neuroepithelium.