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. thumb|590x590px|The [[endoderm of vertebrates produces tissue within the lungs, thyroid, and pancreas. The mesoderm aids in the production of cardiac muscle, skeletal muscle, smooth muscle, tissues within the kidneys, and red blood cells. The ectoderm produces tissues within the epidermis and aids in the formation of neurons within the brain, and melanocytes.]]
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.
Organs produced by the germ layers
The endoderm is the inner most germ layer of the embryo which gives rise to gastrointestinal and respiratory organs by forming epithelial linings and organs such as the liver, lungs, and pancreas. The mesoderm or middle germ layer of the embryo will form the blood, heart, kidney, muscles, and connective tissues. Pander later expanded on this idea in the 1820's by discovering the three germ layers and their mechanism of development into organs. He noted that each germ layer is differentiated due to interactions with surrounding tissues. Folds form in the germinal sheet of cells and usually form an enclosed tube which you can see in the development of vertebrates neural tube. Splits or pockets may form in the germinal sheet of cells forming vesicles or elongations. The lungs and glands of the organism may develop this way. There are three kinds of neurulation: primary, secondary, and junctional neurulation. The development of the neural tube will give rise to the brain and spinal cord. Vertebrates develop a neural crest that differentiates into many structures, including bones, muscles, and components of the central nervous system. Neural crest cells transition from sheet-like epithelial cells to mobile mesenchymal cells. This transition allows them to migrate to other areas of the embryo. The coelom of the body forms from a split of the mesoderm along the somite axis.
See also
- Ectoderm
- Embryogenesis
- Endoderm
- Eye development
- Gastrulation
- Germ layer
- Germ line development
- Gonadogenesis
- Heart development
- Histogenesis
- Limb development
- List of human cell types derived from the germ layers
- Mesoderm
- Morphogenesis
- Organoid