The Morphogenic Mapping of the Brain and the Design of the Nervous System

Figure 1

(a) The outer layer of the gridded sphere is shown, labeled as a fate map of the nervous system. The cap at top represents the brain. The lower portion of the sphere is divided in half, making two sides: the spine half and the peripheral half. (b) The cap from (a) is shown in isolation and rotated 180 degrees. (c) With upward movement toward the apex, the centerline of the dorsal half of the cap moves upward and begins to fold inward. (d) The dorsal half, consisting of nose buds, eye buds, ear buds, corpus callosum, and thalamus, continues folding in as the blastopore forms at top. (e) Involution begins at the blastopore, beginning with apex, consisting of frontal and parietal lobes along with infolded nose and eye buds. (f) The involuted apex continues moving inward, pushes against the interior, and turns 180 degrees. (g) The whole cap has turned inside and curling continues, creating a forward curl at front and the trailing medulla oblongata at back. (h) Continued pressure from turning inside out causes inflation at outer edge of curve. (i) The brain is turned upside down, and temporal lobes push forward because of pressure from behind. This pressure also causes creation of the Sylvian fissure, calcarine fissure, and parietooccipital sulcus folds. The rest of the layer continues to flow inward, as spinal half is formed lengthwise into the spinal column and the peripheral half pulls apart, fans out, enters through the blastopore, and becomes the radiating and branching peripheral nervous system (pictured in Figure 3). (j) The nose, eye, and ear buds extrude from the interior because of pressure from behind, becoming olfactory, optic, and cochlear receptors (pictured right side up).