<P> During the process of gastrulation, a special type of cells called bottle cells invaginates a hole on the surface of the blastula which is called the dorsal lip of the blastopore . Once this lip has been established, the bottle cells will extend inward and migrate along the inner wall of the blastula known as the roof of the blastocoel . The once superficial cells of the animal pole are destined to become the cells of the middle germ layer called the mesoderm . Through the process of radial extension, cells of the animal pole that were once several layers thick divide to from a thin layer . At the same time, when this thin layer of dividing cells reaches the dorsal lip of the blastopore, another process occurs termed convergent extension . During convergent extension, cells that approach the lip intercalate mediolaterally, in such a way that cells are pulled over the lip and inside the embryo . These two processes allow for the prospective mesoderm cells to be placed between the ectoderm and the endoderm . Once convergent extension and radial intercalation are underway, the rest of the vegetal pole, which will become endoderm cells, is completely engulfed by the prospective ectoderm, as these top cells undergo epiboly, where the ectoderm cells divide in a way to form one layer . This creates a uniform embryo composed of the three germ layers in their respective positions . </P> <P> Once there is an embryo with three established germ layers, differentiation among these three layers proceeds . The next event that will take place within the ectoderm is the process of neurulation, which results in the formation of the neural tube, neural crest cells and the epidermis . It is these three components of the ectoderm that will each give rise to a particular set of cells . The neural tube cells will become the central nervous system, neural crest cells will become the peripheral and enteric nervous system, along with melanocytes, facial cartilage and the dentin of teeth, and the epidermal cell region will give rise to epidermis, hair, nails, sebaceous glands, olfactory and mouth epithelium, as well as eyes . </P> <P> Neurulation proceeds by primary and secondary neurulation, both positioning neural crest cells between a superficial epidermal layer and a deep neural tube . During primary neurulation, the notochord cells of the mesoderm signal the adjacent, superficial ectoderm cells to reposition themselves in 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 (MHP). Now, 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 form within the neural plate . These cells are termed dorsolateral hinge cells (DLHP), 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 . </P> <P> 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 cells for continuously growing teeth form from tissue layers called the stellate reticulum and the suprabasal layer of the surface ectoderm . </P>

What is the structure that signals the other germ layers to continue to develop