28) Why would the cells lining the esophagus only live 2-3 days on average?

From days 0 to 14, the human embryo develops into a bilaminar disk of ectoderm and endoderm, with the endoderm forming the lining of the yolk sac. The endoderm is the scaffold for the future digestive tract.

Through the neurulation process, the neural plates evolve to neural tube and neural crest cells. The neural tube is the precursor for the spinal cord and brain. The neural crest cells, placed between the dorsal neural tube and the overlying epidermis, migrate out to form the peripheral nervous system by week 4.

On day 15, the third embryonic layer, the mesoderm, appears and provides the substrate for the connective tissue, angioblasts, smooth muscle, and serosal layers of the gut. By day 21, the mesoderm is thickened and forms longitudinal masses called the paraxial mesoderm. By day 28, the paraxial mesoderm fragments progressively from cranial to caudal into cubes of tissue called somites. This process ends with the formation of 33 to 35 somites by day 31 of embryo development.