Organogenesis

Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the tissues and internal organs of the organism.

The cells of each of the three germ layers undergo differentiation, a process where less-specialized cells become more-specialized through the expression of a specific set of genes. Cell differentiation is driven by cell signaling cascades. Differentiation is influenced by juxtacrine, paracrine, and autocrine signaling. Juxtacrine signaling is exchanged between adjacent cells. Paracrine signals are sent by one cell and received by neighboring cells over short distances. Autocrine signaling is due to intracellular signals being produced and received by the same cell. These signaling pathways allow for cell rearrangement and ensure that organs form at specific sites within the organism at the correct time during development. The organogenesis process can be studied using embryos of various model organisms like zebrafish, xenopus, chickens, and mice. Studying human embryos is difficult and faces ethical concerns, but can be done using organoids developed in vitro without the use of a human embryo. Organoids are simplified organ structures made from adult stem cells, embryonic stem cells, or induced pluripotent stem cells. These stem cells differentiate into various organs due to interactions with other cells or the matrix they are grown on.