Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson   OUTLINE OF THIRTEENTH LECTURE: Feb 15, 2006, by Corey Johnson Neurulation and axial structures I The chordamesoderm, which differentiates into the notochord, is at the center of a number of embryonic structures. It plays a role in the development of: - Ectoderm Neural tube Neural crest Pituitary gland - Mesoderm Somites - Endoderm The notochord is the first axial structure: a structure that lies along the plane of bilateral symmetry. The neural tube is also axial, so are vertebrae. Somites, which are blocks of mesoderm that lie next to the neural tube, are sometimes called paraxial mesoderm because they are next to the axis. Neurulation is the production of the neural tube. The process of neurulation accomplishes: 1) the generation of the neural tube 2) the creation of the neural crest lineage 3) defines the surface ectoderm (which will become mostly epidermis) Where neurulation takes place is defined by the presence of the notochord. As we've discussed previously, the notochord induces the neural tube to form in the overlying ectoderm: primary induction. Grafting a notochord (or its precursor, the primary organizer) to an embryo produces a second neural tube. Notochord grafting experiments have shown that much of the ectoderm is competent to form neural ectoderm. The opposite experiment confirms the notochord induces neural ectoderm: ectoderm, which had not come in contact with the notochord, was grafted to a ventral location and it never became neural ectoderm. However, after some time ectoderm looses its competence to form neural ectoderm. So, in the embryo the notochord is necessary and sufficient to induce the ectoderm to become neural ectoderm. Induction also occurs among ectoderm cells. Once the ectoderm is induced to become neural ectoderm, it can also induce its neighbors to become neural ectoderm. This is sometimes called planar induction. This is, as opposed to vertical induction where different tissue types interact. Regional specification (regionalization) occurs along the A-P axis. The forebrain forms at the front, spinal chord at the back, and various bits lie between. This is what Saxen and Toivonen demonstrated, experimentally (gastrulation lecture 02/10/06) Two types of Neurulation: 1) Primary Neurulation - epithelium rolls, folds, or bends into a tube Steps include: a) columnarization (is that a word?) of epithelium. It becomes columnar. This defines the neural plate b) rising of the lateral edges to form the neural folds c) convergent extension of the neural plate forming the neural groove and production of a "hinge" to facilitate bending d) closure of the folds to form the neural tube animation Some NT's are round, others diamond/triangle shaped, others are tall and narrow. They are formed by slight variations of the same general mechanism. In round tubes, all cells have apical constrictions. In triangular shapes, only small groups of cells become wedge-shaped. neural tube closure The lumen of the neural tube is called the neurocoel. The unfused ends are the anterior (cranial) and posterior (caudal) neuropores. These ordinarily fuse, but in mammals, failure of the neural tube to close completely is associated with spina bifida caudally and anencephaly or exencephaly cranially. The neural tube is bilaterally symmetrical with right and left halves similar. The dorsal and ventral aspects of the NT are different functionally and structurally. The ventral region is called the floorplate. Some of these cells are actually derived from the organizerÉ they snuck in when everyone was watching the tube form. Neural Crest Forms from the crests of the neural folds. This corresponds to the border between neural and surface ectoderm. Grafting quail neural plate to a chick non-neural ectoderm region results in the formation of neural crest from both tissues. 2) Secondary Neurulation - cavitation of mesoderm Occurs in the posterior region of the chick and some mammals Steps include: a) mesenchymal cells coalesce into a rod: the medullary chord b) cells of the rod become epithelial c) the lumen forms comparison between primary and secondary neurulation Fish neurulation Used to be considered equivalent to secondary neurulation, but more recently some have argued that it is more like primary neurulation and much like Xenopus. Parts of the Xenopus tube form like fish. The lumen forms after the neural folds have come together. The cells of the neural folds grow close together and intercalate. Then the cavity (neurocoel) forms. So this is a form of cavitation that occurs in primary neurulation. Teleosts (zebrafish anyway) form a thickened epithelium, the neural plate. Instead of neural folds, the center of the neural plate sinks into the embryo and the edges (what would become the folds in another vertebrate) migrate toward the center: intercalation. This structure is called the neural keel. After the rod is formed, it cavitates. What defines primary vs. secondary neurulation is not whether cavitation forms but whether the starting material is ectoderm or mesoderm. OK, so you're all wondering... do fish undergo secondary neurulation in their caudal end? It hasn't been researched, or at least not published. This could be your PhD thesis!

 

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