University:
Kentucky State University
Course:
BIO 111 | Principles of Biology
Academic year:

2022
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446

Pages:

25
Author:

nesifenqof5i

Chapter 13 Neural tube formation and patterning The vertebrate ectoderm, the outer germ layer covering the late- stage gastrula, has three major responsibilities 1- One part of the ectoderm will become the neural tube, the presumptive neural tissue induced by the prechordal plate and notochord during gastrulation. • Neural tube, the precursor of the central nervous system (CNS)- composed of brain and spinal cord. The vertebrate ectoderm, the outer germ layer covering the late- stage gastrula, has three major responsibilities 2- Another part of the ectoderm will become the surface ectoderm, primarily epidermis: the outer layer of the skin (which is the largest organ of the vertebrate body). The vertebrate ectoderm, the outer germ layer covering the late- stage gastrula, has three major responsibilities 3- Between the compartments forming the epidermis and the central nervous system lies the presumptive neural crest. • The process by which the three ectodermal regions (neural tube, epidermis, and neural crest) are made physically and functionally distinct from one another is called neurulation. • Neurula: an embryo undergoing those processes. • The specification of the ectoderm is accomplished during gastrulation, primarily by regulating the levels of BMP experienced by the ectodermal cells. Transforming the neural plate into a tube: birth of the central nervous system • The cells of the neural plate are characterized by expression of Sox family of transcription factors (Sox1, 2, and 3). Functions of the Sox transcription factors: 1. Activate the genes that specify cells to be neural plate. 2. Inhibit the formation of epidermis and neural crest by blocking the transcription and signaling of BMPs. Transforming the neural plate into a tube: birth of the central nervous system There are two principal modes of neurulation: • In primary neurulation, the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate into the body, and separate from the surface ectoderm to form an underlying hollow tube. • In secondary neurulation: the neural tube arises from the aggregation of mesenchyme cells into a solid cord that subsequently forms cavities that coalesce to create a hollow tube Transforming the neural plate into a tube: birth of the central nervous system • In many vertebrates, primary and secondary neurulation are divided spatially in the embryo such that primary neurulation forms the anterior portion of the neural tube, while the secondary neurulation forms the posterior portion. Transforming the neural plate into a tube: birth of the central nervous system • Junctional neurulation: the formation of the neural tube in the transition zone between the primary and secondary neural tubes. It involves a combination of mechanisms involved in both primary and secondary neurulation. Transforming the neural plate into a tube: birth of the central nervous system Primary neurulation • Chick embryo undergoing neurulation at bout 24 hrs. Transforming the neural plate into a tube: birth of the central nervous system Primary neurulation • Primary neurulation can be divided into four distinct but spatially and temporally overlapping stages: 1- Elongation and folding of the neural tube. • Cell divisions within the neural plate are preferentially in the anterior- posterior (rostal- caudal) direction. Transforming the neural plate into a tube: birth of the central nervous system Primary neurulation • Primary neurulation can be divided into four distinct but spatially and temporally overlapping stages: 2- Bending of the neural plate. • The bending of the neural plate involves the formation of the hinge regions where the neural MHP: medial hinge point plate contacts the surrounding tissue. • Neural folds: thickened edges of the neural plate that move upward during neurulation and migrate toward the midline and eventually fuse to form the neural tube. Transforming the neural plate into a tube: birth of the central nervous system Primary neurulation • Primary neurulation can be divided into four distinct but spatially and temporally overlapping stages: 3- Convergence of neural folds • The plate bends around the DLHP regions as their cells become wedge- shaped. • Another motive force for the bending of the neural plate, is the continued convergence of the surface ectoderm which pushes toward the midline of the embryo. DLHPs: dorsolateral hinge points Transforming the neural plate into a tube: birth of the central nervous system Primary neurulation • Primary neurulation can be divided into four distinct but spatially and temporally overlapping stages: 4- Closure of the neural tube. • The neural tube closes as the neural folds are brought into contact with one another at the dorsal midline. Transforming the neural plate into a tube: birth of the central nervous system Regulation of hinge points • The epithelial cells in the MHP and DLHP regions have a “wedge- shaped” morphology, one that is wider basally than apically. The hinge points (MHP, DLHPs) are formed by the action of: 1- Apical constriction. • The contraction of actinomyosin complexes at the apical border of the cell reduces the size of the apical half of the cell relative to the basal compartment, a process known as apical constriction. Transforming the neural plate into a tube: birth of the central nervous system Regulation of hinge points The hinge points (MHP, DLHPs) are formed by the action of: 2- basal thickening with retention of the nucleus within the basal portion of the cells. 3- The cell packing in the neural folds. How does the packing of the cells in the neural fold occur? How does this packing contribute to the formation of DLHPs? Transforming the neural plate into a tube: birth of the central nervous system Secondary neurulation • Secondary neurulation takes place in the most posterior region of the embryo during tailbud elongation. Transforming the neural plate into a tube: birth of the central nervous system Secondary neurulation • Neural tube formation in the transition zone is referred to as junctional neurulation. Transforming the neural plate into a tube: birth of the central nervous system Secondary neurulation • The junctional neural tube in the chick is a mosaic of both ventral mesenchyme cells and dorsal neural ectodermal cells. • Neural plate cells undergo an epithelial- to- mesenchymal transition and ingress into the underlying mesenchyme pool. Patterning the central nervous system • The early development of most vertebrate brains is similar. • This shows the development of chick brain. neural groove Head fold Anterior Telencephalic vesicles retinal layer pigmented layer Chick embryo at the end of day 3 Patterning the central nervous system The anterior- posterior axis • In mammals (humans), even before the posterior portion of the tube has formed, the most anterior portion of the tube is undergoing tremendous changes. oblongata Patterning the central nervous system The anterior- posterior axis • The neural epithelium of the hindbrain is segmented. • The segmental pattern of rhombencephalon specifies the places where certain nerves originate. • Periodic swellings called rhombomeres divide the rhombencephalon into smaller compartments. Patterning the central nervous system The anterior- posterior axis • The neural crest cells derived from the rhombomeres form ganglia: clusters of neuronal cell bodies whose axons form a nerve. • The cranial nerves are generated from rhombomeres. • Neurons originating from rhombomere 2 (r2) ganglia form the fifth (trigeminal) cranial nerve. • Neurons originating from rhombomere 4 (r4) form the seventh (facial) and eighth (vestibuloacoustic) cranial nerves. • Neurons originating from rhombomere 6 (r6) form the ninth (glossopharyngeal) cranial nerve.

Chapter 13 Neural Tube Formation and Patterning

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