- Gastrulation
- Gastrulation is the formation of the trilaminar embryonic disc (Gastrula) through the migration of epiblast cells
- Epiblast cells accumulate and elongate at the center of the epiblast to form the primitive streak
- Epiblast cells detach and migrate through the primitive pit in between the epiblast and hypoblast to form an intermediate layer known as the intraembryonic mesoderm. The proliferation of cells on both sides of the primitive pit forms the mesoderm.
- Epiblast cells also migrate from the primitive streak and primitive node towards the hypoblast. The hypoblast is replaced by epiblast cells to form the endoderm
- The original epiblast becomes the ectoderm. The remaining epiblast cells differentiate after the migration of cells for the endoderm, mesoderm, and notochord.
- Role of the primitive streak
- Role
- Establishes bilateral symmetry
- Establishes the site for gastrulation
- Initiates germ layer formation
- Creates left-right and craniocaudal body axes
- Role
- The normal fate of the primitive streak
- After the 4th week, the primitive streak regresses and becomes an insignificant structure in the sacrococcygeal region of the embryo.
- It degenerates and disappears by the end of the 4th week
- Congenital anomaly involving the primitive streak
- Sacrococcygeal teratoma – teratoma formed from primitive streak remnants in the sacrococcygeal region
- Functions of the notochord
- Defines the primordial longitudinal axis of the embryo and provides rigidity
- Provides signals necessary for the development of the CNS and axial musculoskeletal structures
- Contributes to the nucleus pulposus of IV discs
- Process of primary neurulation
- Primary neurulation is the formation of the neural tube and neural crest, the precursors of the Central Nervous System and Peripheral Nervous System.
- The notochord signals the medial ectoderm, located directly above it, to differentiate into neural cells. This region of differentiated neural cells is known as the neural plate
- The medial axis of the neural plate invaginates forming the neural groove with neural folds on both sides of the neural groove.
- The neural folds fuse at week 2 to form the neural tube, which detaches from the ectoderm.
- The dorsal part of the neural tube forms the specialized neural crest cells. As the folds fuse, these cells dissociate and undergo an epithelial-to-mesenchymal transformation. They deposit between the neural tube and the surface ectoderm.
- The neural tube communicates with the amniotic cavity cranially and caudally through an opening known as the neuropore. The anterior neuropore closes on days 24-25. The posterior neuropore closes on days 26-27.
- Neurulation is completed during the 4th week
