16.16: Regulating Gene Expression in Cell Development - Gene Expression for Spatial Positioning
- Describe the role genes play in development and ensuring proper spatial positioning
Genes provide positional information
Gastrulation leads to the formation of the three germ layers that give rise, during further development, to the different organs in the animal body. This process, known as organogenesis, is characterized by rapid and precise movements of the cells within the embryo.
Organogenesis
Organs form from the germ layers through the process of differentiation. 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 (the outer tissue layer of the embryo) will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. The process of differentiation is regulated by cellular signaling cascades. Scientists study organogenesis extensively in the lab in fruit flies (Drosophila) and the nematode Caenorhabditis elegans. Drosophila have segments along their bodies, and the patterning associated with the segment formation has allowed scientists to study which genes play important roles in organogenesis along the length of the embryo at different time points. The nematode C.elegans has roughly 1000 somatic cells and scientists have studied the fate of each of these cells during their development in the nematode life cycle. There is little variation in patterns of cell lineage between individuals, unlike in mammals where cell development from the embryo is dependent on cellular cues.
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.
Key Points
- Organogenesis results in the formation of the various organs in the body; however it will only occur if specific sets of genes are expressed to determine ultimate cell type.
- The ability of specific cells to migrate to the the edge of the ectoderm is highly regulated by specific gene expression and allows for differentiation into epidermal cells; in contrast, the cells which remain in the center will develop into the neural plate.
- The expression of specific sets of genes will also regulate the reorganization of the mesoderm into distinct groups of cells, called somites, which develop into the ribs, lungs, spine muscle and notochord.
Key Terms
- gastrulation : the stage of embryo development at which a gastrula is formed from the blastula by the inward migration of cells
- organogenesis : the formation and development of the organs of an organism from embryonic cells
- somite : one of the paired masses of mesoderm distributed along the sides of the neural tube that will eventually become dermis, skeletal muscle, or vertebrae