43.6A: Organogenesis
- Describe the process of organogenesis in vertebrates
Organogenesis
Organogenesis is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism. Organs form from the germ layers through the differentiation: the process by which a less-specialized cell becomes a more-specialized cell type. This must occur many times as a zygote becomes a fully-developed organism. During differentiation, the embryonic stem cells express specific sets of genes which 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 differentiate into epidermal cells. Therefore, the process of differentiation is regulated by cellular signaling cascades.
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 that lies on either side of the vertebrate neural tube will develop into the various connective tissues of the animal body. 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 mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.
The endoderm consists, at first, of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube (except part of the mouth and pharynx) and the terminal part of the rectum (which is lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and air cells of the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus. Additionally, the endoderm forms internal organs including the stomach, the colon, the liver, the pancreas, the urinary bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.
Key Points
- Cells in the ectoderm are signaled by molecules 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
- neural plate : a thick, flat bundle of ectoderm formed in vertebrate embryos after induction by the notochord