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Lecture 18

DEV2011: Lecture 18 summary

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Monash University

LECTURE 18 Neurulation: Neurulation is the stage of organogenesis in vertebrate embryos, during which the neural tube is transformed into the primitive structures that will later develop into the central nervous system. The process begins when the notochord induces the formation of the central nervous system (CNS) by signaling the ectoderm germ layer above it to form the thick and flat neural plate. The neural plate folds in upon itself to form the neural tube, which will later differentiate into the spinal cord and the brain, eventually forming the central nervous system. Different portions of the neural tube form by two different processes, called primary and secondary neurulation, in different species. Primary Neurulation: In primary neurulation, the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate, and pinch off from the surface to form a hollow tube. During primary neurulation, the original ectoderm is divided into three sets of cells: 1. The internally positioned neural tube, which will form the brain and spinal cord 2. The externally positioned epidermis of the skin 3. The neural crest cells. The neural crest cells form in the region that connects the neural tube and epidermis, but then migrate elsewhere; they will generate the peripheral neurons and glia, the pigment cells of the skin, and several other cell types. Shortly after the neural plate has formed, its edges thicken and move upward to form the neural folds, while a U-shaped neural groove appears in the center of the plate, dividing the future right and left sides of the embryo. The neural folds migrate toward the midline of the embryo, eventually fusing to form the neural tube beneath the overlying ectoderm. The cells at the dorsalmost portion of the neural tube become the neural crest cells. The Four Stages of Primary Neurulation: 1. Formation of the neural plate 2. Shaping of the neural plate 3. Bending of the 4. Closure of the neural groove to form the neural tube Formation and Shaping of the Neural Plate: The process of neurulation begins when the underlying dorsal mesoderm (and pharyngeal endoderm in the head region) signals the ectodermal cells above it to elongate into columnar neural plate cells. Their elongated shape distinguishes the cells of the prospective neural plate from the flatter pre-epidermal cells surrounding them. As much as 50% of the ectoderm is included in the neural plate. The neural plate is shaped by the intrinsic movements of the epidermal and neural plate regions. The neural plate lengthens along the anterior-posterior axis, narrowing itself so that subsequent bending will form a tube (instead of a spherical capsule). Bending of the Neural Plate: The bending of the neural plate involves the formation of hinge regions where the neural tube contacts surrounding tissues. In these regions, the presumptive epidermal cells adhere to the lateral edges of the neural plate and move them toward the midline. Closure of the Neural Tube: The neural tube closes as the paired neural folds are brought together at the dorsal midline. The folds adhere to each other, and the cells from the two folds merge. In some species, the cells at this junction form the neural crest cells. In birds, the neural crest cells do not migrate from the dorsal region until after the neural tube has been closed at that site. In mammals, however, the cranial neural crest cells (which form facial and neck structures) migrate while the neural folds are elevating (i.e., prior to neural tube closure), whereas in the spinal cord region, the crest cells wait until closure has occurred. Secondary Neurulation: In secondary neurulation, the neural tube arises from a solid cord of cells that sinks into the embryo and subsequently hollows out (cavitates) to form a hollow tube. Secondary neurulation involves the making of a medullary cord and its subsequent hollowing into a neural tube. Anterior-Posterior Patterning: The anterior segment of the neural tube forms the three main parts of the brain: the forebrain, midbrain, and the hindbrain. The 3 and 5 Vesicle Stage: Primary Vesicles Secondary Vesicles Adult Structures Forebrain Vesicle Telencephalon Cerebral hemispheres, (Prosencephalon) consisting of the cortex and medullary center, basal ganglia, lamina terminalis, hippocampus, the corpus striatum, and the olfactory system Diencephalon Thalamus, epithalamus,
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