• The Primary Germ Layers
• Notochord Formation
• The Events of Neurulation
• Induction of Neural Tissue by Chordamesoderm
• Hensen's Node Induces Neural Axis
• Cell Lineages At Neurula Stage
• Late Neurula

The following diagram shows the general arrangement of tissues in a cross-section of the human embryo after gastrulation.

• After gastrulation, three germ layers are evident: Ectoderm, mesoderm, endoderm
• Next the presumptive notochordal tissue will separate from the endoderm to form the notochord
• As notochord formation is occuring, neurulation will separate the ectoderm into the epithelial ectoderm and the neural ectoderm with the latter being transformed into the neural tube by the end of the process

At this point the presumptive notochordal tissue will separate from the endoderm and re-organize as the notochord below the neural ectoderm as the neural ectoderm begins folding to form the neural tube (Sulik et al, 1994. Developmental Dynamics 201: 260-278).

• The future neural ectoderm overlies notochordal process (chordamesoderm)
• Folding of the neural ectoderm results in the neural tube
• The neural tube is the precursor of the brain & spinal cord

While the neural tube is forming, other embryonic tissues are undergoing changes too as the embryo itself is elongating. The following gives an idea of some of these changes and, more importantly, sets the stage for some of the developmental processes (e.g., limb formation) that we will cover in future lectures.

• Neural ectoderm thickens
• Neural folds form producing Neural Groove
• Neural folds contact
• Cells rearrange forming Neural Tube with overlying Ectoderm
• Special cells: Bottle Cells act as driving force of neurulation

Let's look at a movie of this dynamic process. Note that as the neural tube is forming, many other events are occuring as well. The somites are forming and the embryo is elongating and flexing. Click on the link. Download Movie

• During gastrulation special region of mesoderm underlies overlying ectoderm
• Chordamesoderm is future notochord
• Signals emitted by chordamesoderm have not been identified

In the previous lecture on gastrulation, the role of the node (Hensen's Node) in organizing the overall body plan was discussed briefly. Historically, this special region of tissue has been shown to establish the vertebrate body plan in everything from frogs to zebra fish to mice (Nieto, 1999. Cell 98:417-425). Experimentally this organizing ability is shown by the ability of transplanted nodal tissue to induce a second embryonic axis as revealed in the next diagram.

• These classic experiments were done in chicken embryos
• Remove HN: No neural tissue development
• Transplant HN to host embryo: 2nd Neural Axis induced
• Primary Induction: Induces primary embryonic axis
• These experiments have also been done in the mouse showing that the node is an embryonic organizer in mammals (Beddington, 1994. Development 120: 603-612. Beddington & Robertson, 1998. Trends Genet. 14: 277-284).
• More recent studies have shown however, that the mammalian node is not a classic organizer because it can't act alone to induce the embryonic axis but requires other anterior germ tissues to be fully effective (Tam & Steiner, 1999. Development 126: 5171-5179). Thus progressive cellular interactions are more important rather than a single inductive event that characterizes the organizer activity of the Hensen's Node region in lower vertebrates.

For a detailed review of neurulation check out: Smith & Shoenwolf, 1997. Neurulation: Coming to Closure. Trends in Neurosci. 20: 510-517.

• Neural Tube: One layer of cells surrounding a lumen & covered by external limiting membrane
• Neural Crest have separated out--begun to migrate
• Neural Tube thickens & folds to form brain
• New layers of nerve cells will appear in brain & spinal cord (neurogenesis)