8.29.11+-+Formation+of+Primary+Germ+Layers

Formation of the Primary Germ Layers (Gastrulation); the trilaminar Embryo; Neurulation, Formation of the Body cavity, Origins of the Heart and Circulatory system

 * 1) Be able to define the following terms**
 * **Primitive Streak**: The primitive streak is what, during early stages of development, will establish bilateral symmetry, determine site of gastrulation and initiate germ layer formation. Temporary structure, helps to develop the notochord. Composed of a thickened ridge of epiblast cells
 * **Primitive Groove:** a depression in the primitive streak
 * **Primitive Knot**: the cranial end of the primitive streak
 * **Primitive Pit**: cranial end of the primitive groove in the primitive knot
 * **Mesenchyme**: embryonic connective tissue (not only mesodermal). migratory. capable of developing into different types of connective tissue
 * **Oropharyngeal Membrane**: One of the two points where the mesoderm does not separate the the endoderm and the ectoderm. Eventually becomes the mouth
 * **Cloacal Membrane**: The other point where the endoderm and the ectoderm meet directly. Eventually becomes the anus
 * **Neural Plate:** creation of neural plate is induced by the notochord and is the first step of neuralation (process by which neural tube is transformed into structures which will develop into CNS). Ectoderm is induced to thicken to form the neural plate
 * **Neural Groove:** stage when neural plate invaginates along central axis to form a longitudinal median neural groove with neural folds along each side.
 * **Neural Fold**: Folds which form along the neural groove which are first signs of brain development
 * **Neural Tube:** Created once folds along the neural groove merge and the newly formed neural tube separates from the surface ectoderm. the remainder of the ectoderm fuses into a continuous surface. The neural tube will eventually become the CNS. neurulation is now complete
 * **Neural Crest:** cells from the neural tube which seperate bilaterally and spread out, eventually forming the PNS.
 * **Sacrococcygeal Teratoma:** a birth defect that can arise when the primitive streak cells do not disappear. prognosis usually good and can be removed surgically
 * **Intraembryonic Mesoderm (IEM):** Mesenchyme cells leave the primitive streak and form the IEM, which eventually develops into the mesoderm
 * **Extraembryonic Mesoderm (EEM):** Mesoderm layer which surrounds the yolk sac [RIGHT? NOT SURE ENTIRELY ON THIS....]
 * **Axial mesoderm (the notochord):** a cellular road induced by the primitive streak. serves as primordial axis of embryo, gives rigidity, serves as basis for development of axial skeleton, indicates future site of vertebral bodies
 * **paraxial mesoderm:** region of IEM which forms the most medial of three layers to the neural tube. eventually forms somites which eventually become dermis, skeletal muscle and vertebrae**.**
 * **Intermediate Mesoderm:** Next most medial layer of IEM which is nephrogenic.
 * **Lateral Plate Mesoderm**: Most lateral layer of IEM which is parietal and vixceral mesoderm (will form linings of body cavities)
 * **Prochordal Mesoderm:** portion of mesoderm which develops into the heart
 * **Intraembryonic Coelom:** Cavitation which occurs in the Lateral Plate mesoderm. IEC becomes the body cavity [RIGHT? NOT SURE ON THIS]
 * **Splanchnic (visceral) and Somatic (parietal) layers of lateral plate mesoderm:** Two layers formed by the intraembryonic coelom in the lateral mesoderm. the dorsal half forms the somatic layer which becomes the parietal lining of body cavities. the ventral half becomes the visceral lining of the body cavities.
 * **Somite**: Paraxial mesoderm differentiates and forms paired somites which eventually give rise to the axial skeleton and associated musculature and adjacent dermis
 * **Angiogenic cells:** cells which support development of arteries
 * **Vasculogenesis and Angiogenesis:** angiogenesis is the growth of new blood vessels from existing blood vessels. vasculogenesis is the creation of new blood vessels from scratch.
 * **Endocardial heart tubes**: sets of angiogenic cells from the prochordal mesoderm which split into 2 solid tubes which canalizes into primitive heart and circulatory system. already beating at 3 weeks [DOES NOT FEEL VERY COMPLETE...]
 * **septum transversum:** prochordal mesoderm also forms the septum transversum which becomes the future diaphragm. DOES NOT FEEL VERY COMPLETE...]


 * 2) Be able to describe the following concepts**
 * **The sequence of gastrulation (conversion of bilaminar embryo to a trilaminar embryo):** Gastrulation is the process by which the three germ layers and axial orientation are etablished in embryos. The bilaminar embryonic disc is converted into a trilaminar embryonic disc. First stage is the appearance of the primitive streak in the epiblast. shortly after its formation, the first wave of mesenchymal cells leave its deep surface and form the true endoderm, displacing the hypoblast cells in the roof the the yolk sac, forming the ventral portion of the embryonic plate in the trilaminar embryo. The second wave of mesenchymal cells form the IEM. The remaining epiblast forms the intraembryonic ectoderm. The primitive streak induces the formation of the notochord and neural plate before it diminishes and disappears. [ANYONE HAVE A GOOD DIAGRAM OF GASTRULATION? NATALIE? ANYONE?]
 * **Origin of the three primary germ layers (ectoderm, mesoderm and endoderm):** The mesoderm forms when cells from the epiblast drift down between the epiblast and the ectoderm. Cells from the epiblast displace the hypoblast forming the endoderm. The cells remaining in the epiblast form the ectoderm. [SURELY MORE HERE?]
 * **The clinical significance of the primitive streak**: The primitive streak is merely a temporary structure, to induce the notochord and the neural plate. Once its job is complete it is supposed to disappear. If this fails to happen, it can be problematic, since the primitive streak is pluripotent, it can create tumors such as sacrococcygeal teratoma.
 * **Induction (in general terms)**: Refers to the process whereby the differentiation of one group of cells (or tissue) is controlled by the influence of a second group of cells. Three types...
 * Induction by diffusion: one cell releases a factor and only cells with proper receptors, within realistic diffusible distance, will be induced
 * Induction by deposition: one cell releases a signal into an extracellular matrix and only if a migratory cell comes near it will it be induced
 * Induction by contact mediation: one cell possess one type of inducing cells and another has receptors. if they touch, induction occurs
 * **Formation of the neural tube and neural crest from the neural plate**: The neural plate initially forms over the notochord from the ectoderm . The neural plate is also induced by the notochord. The neural plate begins to invaginate to form the neural groove, which has neural folds on either side. Eventually these folds come together and the neural groove separates and forms the neural tube. Alongside the neural groove, portions of ectoderm become neural crest, which separates along with the neural tube and eventually splits into two sections. The neural crest becomes the PNS and the neural tube becomes the CNS.
 * **Derivatives of the Ectoderm**: The ectoderm eventually becomes the epidermis, the CNS and PSN.
 * **Derivatives of the different regions of the mesoderm (in general terms):** The mesoderm separates into the paraxial mesoderm (most medial) becomes the somites which become the vertebrae, skeletal muscle and the dermis. The intermediate mesoderm is nephrogenic. The lateral plate mesoderm will become the linings of the body cavities.
 * **Formation and extent of the intraembryonic coelom**
 * **Formation of the endocardial heart tubes**
 * **Canalization**
 * **When does the primitive circulatory system first become functional**: After about 3 weeks.