Lecture 16: Nervous Development I Flashcards
Describe the stages of the neural tube wall from that of a simple cuboidal epithelium to a stratified epithelium
- Early neural epithelium: simple cuboidal
- Neural plate: simple columnar epithelium
- Early neural tube wall: pseudostratified epithelium, single layer of columnar cells with nuclei at varying heights, limiting membranes=basal lamina (external limiting membrane)
- Late neural tube wall: stratified epithelium
Describe and explain the importance of the orientation of the metaphase plate during proliferation of cells within the neural tube
If metaphase plate is perpendicular to inner margin of neural tube (next to lumen), two resulting daughter cells will remain proliferative
If metaphase plate is parallel to inner margin…
- Daughter cell closest to lumen will: remain proliferative
- Daughter cell further from lumen will: Express Notch receptor, Become postmitotic, Move to the external limiting membrane, Become a neuroblast (pre-neuron)
Describe the hierarchy of basic cell lineages in the development of the nervous system and note when mitosis ceases in each lineage and differentiation begins
- Neuroepithelium refers to the initial epithelial layer of the neural tube
- Bipotential progenitor cell refers to the fact that his cell has been restricted to one of the two pathways: neuronal lineage progenitor or glial lineage progenitor
- Neuronal lineage cells are committed to developing into neurons; they are postmitotic
- Glial lineage cells have a number of paths leading to types of glial cells
Describe the experiments where one grafts a secondary notochord, removes the notochord, and slits the neural plate on one side of the floor plate
- Grafting an extra notochord near the neural tube induces a secondary floor plate.
- In the absence of a notochord, a very incomplete floor plate forms, and nerve fibers exit from multiple sites around the spinal cord.
- Slitting the neural plate on one side of the floor plate removes the wall of the neural tube from the influence of the notochord, allowing the disorganized exit of nerve fibers from that part of the spinal cord.
Describe the signaling pathways and factors that establish the dorsal-ventral axis of the developing neural tube. What roles do the notochord and non-nervous ectoderm play?
Ventral Signaling
- The notochord induces formation of the floor plate of the neural tube via sonic hedgehog (Shh)
- Sonic hedgehog produced by the floor plate induces the formation of motor neurons
Dorsal Signaling
- Ectoderm flanking the neural plate uses BMPs to induce snail-2 in the future neural crest and later to maintain Pax-3 and Pax-7 to create a dorsalizing effect
- The expression of Pax-3 and Pax-7 is suppressed by Shh from the floor plate to suppress the dorsalizing effect in the basal plate
Describe the relationship between rhombomeres, cranial nerves, and pharyngeal arches.
The cranial nerves, which have a highly ordered pattern by which they supply structures derived from the pharyngeal arches and other structures in the head, have an equally highly ordered origin with respect to the rhombomeres (which are just a highly ordered expression of different transcription factors that show a morphological reflection)
What pharyngeal arches are associated with which cranial nerves?
Cranial Nerve V: first pharyngeal arch
Cranial Nerve VII: second pharyngeal arch
Cranial Nerve IX: third pharyngeal arch
How is the segmented nature of spinal nerves related to the somitic mesoderm pattern?
- Segmented nature of spinal nerves is due to pattern of somatic mesoderm along neural tube
- Motor neurons can penetrate anterior mesoderm of somites but not posterior mesoderm of somites
What is the role of the isthmic organizer, and what signaling/transcription factors are involved?
It is a signaling center!
It’s signaling molecule is FGF-8/Wnt to induce expression of En-1, En-2, Pax-2, Pax-5
How does shh determine the midbrain dorsoventral axis?
Shh restricts ventral expression of molecules, such as Pax-7, which are characteristic of the alar plate.
What role do prosomeres P1-P3 play in the forebrain patterning?
- P1-P3 define the diencephalon
- P2-P3 define dorsal and ventral thalamus
What is the relationship between Shh and holoprosencephaly?
Similar to the spinal cord, the ventral forebrain is induced and organized by shh, secreted by midline axial structures. In the absence of shh signaling in this area, the tissues of the ventral forebrain are greatly reduced, leading to midline fusion of the optic vesicles and a general reduction of the growth of the midface region. This situation results in a type of anomaly called holoprosencephaly (no proencephalon), which in extreme cases is accompanied by cyclopia (orbits of eyes are not formed into two cavities)
Describe steps in the outgrowth of a motor neuron and compare with a sensory neuron
Motor axon outgrowth:
- Axon grow out from the motor neuroblasts located in the basal plate of the spinal cord
- Boundary caps created by neural crest cells maintain separation between central nervous system components and peripheral nervous system components
Formation of a sensory neuron:
- Cell bodies of sensory neurons are derived from neural crest cells
- Cell bodies of sensory neurons form sensory spinal ganglia
- Axons grow from these cell bodies toward the spinal cord and the periphery
- Boundary caps maintain separation here as well
What are growth cones?
- Characterized by expanded region of cytoplasm with filopodia
- Growth cones advance via extension/resorption cycles that involve actin microfilaments
How do environmental factors effect growth?
They can cause chemoattraction, contact attraction, chemorepulsion, contact repulsion
- some environments give growth cones signals to continue extending forward, whereas other environments give “stop” signals or “turn” signals. The sensitivity of growth cones to their environment is so great that they may be able to discriminate a concentration difference of as little as one molecule across the surface.
