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# Embryology - 3rd Week ## Formation of Mesoderm The bilaminar embryonic disk is converted to a trilaminar disk due to the formation of a mesodermal germ layer between ectoderm and endoderm (gastrulation). This process occurs through these steps: 1. Ectodermal cells in the caudal part of the bila...

# Embryology - 3rd Week ## Formation of Mesoderm The bilaminar embryonic disk is converted to a trilaminar disk due to the formation of a mesodermal germ layer between ectoderm and endoderm (gastrulation). This process occurs through these steps: 1. Ectodermal cells in the caudal part of the bilaminar disk multiply and migrate to the midline, forming a primitive streak. 2. The primitive streak consists of a narrow groove with slightly bulging areas on both sides. 3. Cells from the primitive streak migrate between the endodermal and ectodermal germ layers to form a mesodermal germ layer. 4. Mesodermal cells migrate to either side of the middle line until they meet in front of the prochordal plate. ## Formation of Notochord The notochord is a temporary supporting structure formed by: 1. **Formation of Hensen's knot:** * A thickening appears at the cephalic end of the primitive streak. * It consists of a small centrally located depression called a primitive pit, surrounded by a slightly elevated area. 2. **Formation of notochordal process**: * Cells of the primitive pit invaginate between the ectoderm and endoderm, and move along the midline anteriorly until reaching the prochordal plate. * This forms a cord of cells called the notochord process. The notochord separates the ectoderm and endoderm except in the region of the prochordal plate, which gives the embryo rigidity and influences the CNS (Neurulation) and vertebral column formation. ## Formation of the Neural Tube The neural plate, a thickening of the ectodermal layer at the cephalic end of the embryo, is formed due to the inductive influence of the notochord and adjacent mesoderm. * The neural plate invaginates along its longitudinal axis. * The margins of the neural plate rise rapidly and form neural folds. * Neural folds meet in the midline and fuse to form the neural tube. * This tube is separated from the ectoderm and subsequently forms the brain and spinal cord. ## Neural Crest Cells During the folding of the neural plate, cells differentiate at the crest of the neural plate to form neural crest cells. These cells migrate to the mesoderm. Neural crest cells are of ectodermal origin but have mesodermal behavior. **Diagram Description:** The image contains diagrams illustrating these developmental processes. Diagrams show a developing embryo splitting into multiple layers. Different layers are highlighted in various colors. Arrows and labels indicate the movement of cells and parts of the embryo.