Figure 2.

Type 1 EMT in development and embryogenesis. Following fertilization, the zygote undergoes several rounds of cleavage to form a dense spheroid structure, which then undergoes blastulation to from a hollowed blastula. During gastrulation, TGF-β provides inductive EMT signals that elicits the formation of an invaginating structure that generates the three multipotent germ layers, namely the mesoderm, endoderm, and ectoderm. The mesoderm gives rise to muscle, skeleton, and connective tissue, while the endoderm gives rise to internal organs, such as the liver, colon, and stomach. The outer ectodermal layer generates the epidermis and ocular lens, as well as produces the mammary gland. The ectoderm also gives rise to the primordial nervous system via neurulation and neural crest formation, a process that dependent upon EMT induced by TGF-β. Finally, TGF-β stimulation of EMT during organogenesis is essential for the faithful development of the kidney, and of the endocardial cushion and subsequent atrioventricular valve formation. The role of TGF-β in regulating type 1 EMT during mammary gland branching and invasion is discussed in the text.