Summary

1. Epigenesis is the progressive development of increasingly complex tissues and structures as a result of ongoing differentiation. See Figure 1.5
2. Cellular differentiation in metazoans represents the selective expression of a particular subset of genes, as regulated by transcription factors. See Figure 1.6
3. In C. elegans, cell division allocates the transcription factors that were unevenly distributed in the egg (a maternal effect), and consequently each resulting cell has a unique mix of transcription factors, therefore a unique complement of gene expression, and therefore a particular fate depending on mitotic lineage. See Figure 1.10
4. In other species, this mosaic specification of cell fate through mitotic lineage has been supplanted by conditional specification of cell fate through cell-cell interactions. This means that embryos of most species display extensive self-regulation, with cells directing each other’s fates, as when the optic cup induces formation of a cornea and lens. See Figure 1.22
5. In vertebrates, the organizer, a group of cells of the dorsal lip of the blastopore, releases at least three signals (noggin, chordin, follistatin) that block BMP signaling to induce neural formation. The organizer cells migrate to become mesoderm and, eventually, the notochord. See Figure 1.23
6. The nervous system arises when mesodermal tissue blocks BMP stimulation of TGFβ receptors in overlying ectoderm so the tissue follows a neural fate rather than becoming epidermis. See Figure 1.27
7. The organizer induces the dorsal ectoderm to form the neural tube, the future central nervous system, and the neural crest, the future peripheral nervous system. See Figure 1.28
8. The induction of an organizer in the dorsal lip of the blastopore is itself a result of β-catenin expression in the underlying endodermal tissue, which is an indirect result of maternal effects. See Figure 1.29
9. In insects, mesoderm secretes the chordin homologue sog to block BMP-like signaling in ventral ectoderm, inducing expression of the proneural genes AS-C, which are necessary, but not sufficient, for the cells to take a neural fate. See Figure 1.32
10. Within a rosette of AS-C–expressing cells, each exerts lateral inhibition to suppress proneural gene expression of its neighbors through a system of Notch-Delta signals. See Figure 1.33
11. The cell that produces the most Delta activates the Notch receptors of its neighbors to suppress their expression of AS-C, and then it delaminates to become a neuroblast, which will give rise to the segmental ganglion in that region. See Figure 1.36