Organogenesis

Organogenesis is the phase of development, following gastrulation, during which the ectoderm, mesoderm and endoderm interact and their cells differentiate to build organs. Cells switch on particular sets of genes, change shape, migrate and fold into tubes and buds, guided by signaling molecules passed between neighboring tissues, a process called induction. In humans, the rudiments of all major organs form during the embryonic period, from about the third to the eighth week after fertilization, although most organs keep growing and maturing until birth and beyond.

One of the first events is neurulation, which turns ectoderm into the neural tube. Mesoderm beside the tube divides into blocks called somites that form the vertebrae, skeletal muscles and dermis, and the heart begins as a simple tube that starts beating early in week 4. Folding of the flat embryo turns the endoderm into a gut tube, from which the lungs, liver and pancreas bud off. Limb buds appear in weeks 4 to 5, and fingers separate when the cells between them die by apoptosis.

Because so much is being built, this is when the embryo is most vulnerable to teratogens and when most major birth defects arise. Researchers study organogenesis in fruit flies, zebrafish and mice, and copy parts of it to grow organoids in the lab.