Changes

== Introduction ==

== Introduction ==

#'''Glia''' - for insulation of electrical signals.

#'''Glia''' - for insulation of electrical signals.

Glia form oligodendrocytes in the CNS, and [[PNS Structure - Anatomy & Physiology#The_Schwann_Cell|schwann cells]] in the PNS. Glial cells surround the axons furthest away form the lumen of the neural tube. Because this part is myelinated, it appears ''white'' and is called the '''marginal layer'''. Axons closer to the lumen of the neural tube, which the glial cells do not surround appear ''grey''. This layer is called the '''intermediate layer'''. The ventricular layer persists, so that the spinal cord develops with these three layers.  

Glia form oligodendrocytes in the CNS, and [[PNS Structure - Anatomy & Physiology#Schwann Cell|schwann cells]] in the PNS. Glial cells surround the axons furthest away form the lumen of the neural tube. Because this part is myelinated, it appears ''white'' and is called the '''marginal layer'''. Axons closer to the lumen of the neural tube, which the glial cells do not surround appear ''grey''. This layer is called the '''intermediate layer'''. The ventricular layer persists, so that the spinal cord develops with these three layers.  

In the brain, further layers are added to increase complexity. This is possible because in the anterior of the embryo, neuroblasts (cells of the ventricular layer) retain their ability to undergo mitosis after leaving the ventricular layer. This difference is achieved by increased hydrostatic forces acting on the neuroblasts.  

In the brain, further layers are added to increase complexity. This is possible because in the anterior of the embryo, neuroblasts (cells of the ventricular layer) retain their ability to undergo mitosis after leaving the ventricular layer. This difference is achieved by increased hydrostatic forces acting on the neuroblasts.  

[[Image:Neual crest.png|thumb|right|250px|Mithril 2010  Cross section scheme of vertebrate embryo. 1 – neural crest, 2 – neural tube, 3 – dorsal somite, 4 – notochord. Both paths of the neural crest cells migation marked by arrows: red one is the dorsolateral path, blue one signs the ventromedial path.]]  

[[Image:Neual crest.png|thumb|right|250px|Mithril 2010  Cross section scheme of vertebrate embryo. 1 – neural crest, 2 – neural tube, 3 – dorsal somite, 4 – notochord. Both paths of the neural crest cells migation marked by arrows: red one is the dorsolateral path, blue one signs the ventromedial path.]]  

The dorsal neural fold is the site where the neural ectoderm meets the epidermal ectoderm. Signaling at the border between these two tissues leads to the induction of the Neural Crest Cells, which then delaminate (epithelial to mesenchymal transition), migrate to their target sites and differentiate based on the local environmental cues (thus, their fate is plastic). The delamination and migration of neural crest cells occurs during the embryonic stage of neurulation, but is species specific and may occur while the neural tube is still open (mouse and opossum) or closed (chick). In several frog species analyzed, this migration occurs at all phases of neurulation so is not standard for any particular group of vertebrates.  Migratory pathways are highly conserved across all vertebrate groups, with streams going toward the frontonasal region and branchial regions in the cranial region (with the regions lateral to the negatively numbered rhombomeres of the hindbrain free of neural crest streams). These streams are directed based on inhibitory signals in the mesenchyme (mesodermal in origin) lateral to the neural tube (rhombomere 3) or are physically blocked, such as lateral to rhombomere 5 with the otic vesicle. Apoptotic signals also play a role in the migration of these cells. The peripheral nervous system (PNS) develops from the neural crest.   They differentiate once they reach their destination. Other non - neural tissues also develop from the neural crest. Though previously thought that only the cranial neural crest streams can form skeletal (mesodermal types) of derivatives and the trunk only neural types, recent work over the last decade has shown the extreme plasticity in cell fates of these cells along the anterior-posterior axis. Generally:  

The dorsal neural fold is the site where the neural ectoderm meets the epidermal ectoderm. Signaling at the border between these two tissues leads to the induction of the Neural Crest Cells, which then delaminate (epithelial to mesenchymal transition), migrate to their target sites and differentiate based on the local environmental cues (thus, their fate is plastic). The delamination and migration of neural crest cells occurs during the embryonic stage of neurulation, but is species specific and may occur while the neural tube is still open (mouse and opossum) or closed (chick). In several frog species analyzed, this migration occurs at all phases of neurulation so is not standard for any particular group of vertebrates.  Migratory pathways are highly conserved across all vertebrate groups, with streams going toward the frontonasal region and branchial regions in the cranial region (with the regions lateral to the negatively numbered rhombomeres of the hindbrain free of neural crest streams). These streams are directed based on inhibitory signals in the mesenchyme (mesodermal in origin) lateral to the neural tube (rhombomere 3) or are physically blocked, such as lateral to rhombomere 5 with the otic vesicle. Apoptotic signals also play a role in the migration of these cells. The peripheral nervous system (PNS) develops from the neural crest. The cells differentiate once they reach their destination. Other non - neural tissues also develop from the neural crest. Though previously thought that only the cranial neural crest streams can form skeletal (mesodermal types) of derivatives and the trunk only neural types, recent work over the last decade has shown the extreme plasticity in cell fates of these cells along the anterior-posterior axis. Generally:  

'''Anterior Neural Crest''' forms:  

'''Anterior Neural Crest''' forms:  

'''Sensory Ganglion of Cranial Nerves''' - epithelial to mesenchymal transition.  

'''Sensory Ganglion of Cranial Nerves''' - epithelial to mesenchymal transition.  

[[Category:Developmental_Biology]] [[Category:Nervous_System_-_Anatomy_&_Physiology]] [[Category:A&P_Done]] [[Category:David_Hogg_reviewing]]

[[Category:Developmental_Biology]] [[Category:Nervous_System_-_Anatomy_&_Physiology]] [[Category:A&P_Done]] [[Category:David_Hogg_reviewing]]