Ectoderm, mesoderm, and endoderm are the major structures arising out of the primary germ layers.

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. These three germ layers (of the embryo differentiate and further specialize to form the various organs of the body. 

Organs develop from the germ layers through the process by which a less-specialized cell becomes a more specialized cell type. During cell differentiation, the embryonic stem cells express specific sets of genes that will determine their ultimate cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will change into epidermal cells. The ectoderm is the outermost layer which gives rise to the epidermis as well as the organs that make up the nervous system (brain, nerves, spinal cord).

In vertebrates, one of the primary steps during organogenesis is the formation of the neural system. The ectoderm forms epithelial cells and tissues, as well as neuronal tissues. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were disrupted, then the entire ectoderm would differentiate into neural tissue. The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube. In further development, the neural tube will give rise to the brain and the spinal cord.

The mesoderm, which is the middle layer, develops into the circulatory system (blood cells, vessels and heart) as well as the various connective tissues (bone, muscles, tendons and dermal layer) of the animal body and the kidney. A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites, with spaces between them. The somites will further develop into the ribs, lungs, and segmental (spine) muscle.

The endoderm, outermost layer of the germ layer, forms the epithelial lining of the whole of the digestive tube (except part of the mouth and pharynx) and the end part of the rectum. It also forms the lining cells of all the glands which open into the digestive tube. Additionally, it forms internal organs such as the stomach, colon, liver, pancreas, urinary bladder, epithelial parts of the trachea, the lungs, pharynx, thyroid, parathyroid, and intestines.

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Key Points

• Molecules signal cells in the ectoderm called growth factors to form the neural plate, which rolls up to form a structure called the neural tube; the neural tube will eventually develop into the brain and spinal cord.

• The differing expression of various genes controls the differentiation of the mesoderm into connective tissue, as well as the ribs, spine, skeletal muscle, and lungs.

• The endoderm forms the lining of the digestive tract, as well as the linings of all the glands that will empty into the digestive tract; it also forms a wide variety of internal organs.

Key Terms

organogenesis: the formation and development of the organs of an organism from embryonic cells

ectoderm: outermost of the three tissue layers in the embryo of a metazoan animal, which will produce the epidermis (skin) and nervous system of the adult

mesoderm: one of the three tissue layers in the embryo of a metazoan animal, which will produce many internal organs of the adult such as the muscles, spine and circulatory system

endoderm: one of the three tissue layers in the embryo of a metazoan animal, which will produce the digestive system and other internal organs of the adult