particular complement of cells. The neural tube cells give rise to the central nervous system, neural crest cells give rise to the peripheral and enteric nervous system, melanocytes, and facial cartilage, and the epidermal region will give rise to the epidermis, hair, nails, sebaceous glands, olfactory and oral epithelium, and eyes. Neurulation Neurulation occurs in two parts, primary and secondary neurulation. Both processes position neural crest cells between a superficial epidermal layer and the deep neural tube. During primary neurulation, the notochord cells of the mesoderm signal the adjacent, superficial ectoderm cells to reposition themselves into a columnar pattern to form cells of the ectodermal neural plate. As the cells continue to elongate, a group of cells immediately above the notochord change their shape, forming a wedge in the ectodermal region. These special cells are called medial hinge cells (MHPs). As the ectoderm continues to elongate, the ectodermal cells of the neural plate fold inward. The inward folding of the ectoderm by virtue of mainly cell division continues until another group of cells forms within the neural plate. These cells are termed dorsolateral hinge cells (DLHPs), and, once formed, the inward folding of the ectoderm stops. The DLHP cells function in a similar fashion as MHP cells regarding their wedge like shape, however, the DLHP cells result in the ectoderm converging. This convergence is led by ectodermal cells above the DLHP cells known as the neural crest. The neural crest cells eventually pull the adjacent ectodermal cells together, which leaves neural crest cells between the prospective epidermis and hollow, neural tube. Organogenesis All of the organs that rise from the ectoderm such as the nervous system, teeth, hair and many exocrine glands, originate from two adjacent tissue layers: the epithelium and the mesenchyme. Several signals mediate the organogenesis of the ectoderm such as: FGF, TGFβ, Wnt, and regulators from the hedgehog family. The specific timing and manner that the ectodermal organs form is dependent on the invagination of the epithelial cells. FGF-9 is an important factor during the initiation of tooth germ development. The rate of epithelial invagination in significantly increased by action of FGF-9, which is only expressed in the epithelium, and not in the mesenchyme. FGF-10 helps to stimulate epithelial cell proliferation, in order make larger tooth germs. Mammalian teeth develop from ectoderm derived from the mesenchyme: oral ectoderm and neural crest. The epithelial components of the stem