The 3rd Week of Development PDF

Semester 1 The 3rd week of Development Dr. Hend M. Hassan The Third Week of Development Events: 1. Primitive streak formation (15th – 16th days). 2. Gastrulation. 3. Notochord formation. 4. Differentiation of the three germ layers....

Semester 1 The 3rd week of Development Dr. Hend M. Hassan The Third Week of Development Events: 1. Primitive streak formation (15th – 16th days). 2. Gastrulation. 3. Notochord formation. 4. Differentiation of the three germ layers. 5. Neurulation. 1. Primitive Streak ▪ Primitive streak is a thickened part of the epiblast in the midline at the caudal end. ▪ The elevated cranial end of the primitive streak forms a primitive node (primitive knot) that will depress to form the primitive pit. ▪ A narrow depressed central area of primitive streak called primitive groove develops. 1 Semester 1 The 3rd week of Development Dr. Hend M. Hassan 2. Gastrulation A process of formation of the trilaminar embryonic disc. The formation of the primitive streak is the indicator of the start of gastrulation. Process: 1. Epiblasts migrate through the primitive groove: 2. The early migratory cells will replace the hypoblasts to form the endoderm. 3. The next migratory cells will be placed between the endoderm and the epiblasts to form the mesoderm. 4. The remaining epiblasts will form the ectoderm. So, the epiblasts are the source of the 3 germ layers. 2 Semester 1 The 3rd week of Development Dr. Hend M. Hassan 3. Notochord Formation A solid cord formed from the migration of epiblasts through the primitive pit in the midline cranially. Notochord is the core around which the vertebral column is formed. Fate: Degenerates inside the bodies of the vertebrae and remains as nucleus bulbosus in the intervertebral discs. 3 Semester 1 The 3rd week of Development Dr. Hend M. Hassan 4. Differentiation of the three germ layers 4 Semester 1 The 3rd week of Development Dr. Hend M. Hassan 5