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==Introduction==

==Introduction==

Development of the Central Nervous System (CNS) includes development of the brain, spinal cord, optic and auditory systems, as well as surrounding supporting cells including [[Ependymal_Cells#Ependymal_Cells|ependymal cells]], [[Astrocytes#Astrocytes|astrocytes]], [[Astrocytes#Oligodendrocytes|oligodendrocytes]] and [[Astrocytes#Microglial_Cells|microglia]]. Information within this page will exclude development of the [[PNS_Structure_-_Anatomy_%26_Physiology|Peripheral Nervous System]] (PNS) which includes nerve and ganglia formation. For further information on PNS development, please PNS link above.

Development of the Central Nervous System (CNS) includes development of the brain, spinal cord, optic and auditory systems, as well as surrounding supporting cells including [[Ependymal Cells|ependymal cells]], [[Astrocytes#Astrocytes|astrocytes]], [[Oligodendrocytes|oligodendrocytes]] and [[Microglial Cells|microglia]]. Information within this page will exclude [[PNS Structure - Anatomy & Physiology|development of the Peripheral Nervous System]] (PNS) which includes nerve and ganglia formation.

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For the appropriate development of the CNS a number of important basic requirements must be achieved. For example, the appropriate haemopoeitic cells must differentiate into specific cells (either neurons or glia) and any axons must extend to appropriate positions in order to form synapses. Development of the CNS is a complex process and complete coordination is required to facilitate the correct procedure of cell differentiation and the establishment of the correct intracellular connections. A range of neurotrophic factors regulate these interactions including FGF, nerve growth factor, laminin, brain derived neurotrophic factor and neurotrophin 3.

For the appropriate development of the CNS a number of important basic requirements must be achieved. For example, the appropriate haemopoeitic cells must differentiate into specific cells (either [[Neurons - Anatomy & Physiology|neurons]] or glia) and any axons must extend to appropriate positions in order to form synapses. Development of the CNS is a complex process and complete coordination is required to facilitate the correct procedure of cell differentiation and the establishment of the correct intracellular connections. A range of neurotrophic factors regulate these interactions including FGF, nerve growth factor, laminin, brain derived neurotrophic factor and neurotrophin 3.

==Developmental Origins==

==Developmental Origins==

The embryological origin of the CNS is from the '''neural ectoderm''' which is a ridge of tissue in the centre of the early embryo once the basic [[Blastocyst_Embryonic_Development_-_Anatomy_%26_Physiology|blastocyst]] has undergone a degree of differentiation. The neural ectoderm is formed via the thickening of the ectoderm and it's interaction with underlying basic neural tissues including the notochord. This interaction results in the formation of the neural plate, termed the neuroectoderm. Below is a diagram of the completed initial stages of development to provide an indication of the anatomy of the development process.

The embryological origin of the CNS is from the '''neural ectoderm''' which is a ridge of tissue in the centre of the early embryo once the basic [[Blastocyst Embryonic Development - Anatomy & Physiology|blastocyst]] has undergone a degree of differentiation. The neural ectoderm is formed via the thickening of the ectoderm and its interaction with underlying basic neural tissues including the notochord. This interaction results in the formation of the neural plate, termed the neuroectoderm. Below is a diagram of the completed initial stages of development to provide an indication of the anatomy of the development process.

[[File:Gray19 with color.png|thumb|centre|500px|Neurulation]]

[[File:Gray19 with color.png|thumb|centre|500px|Neurulation]]

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==Neural Development==

==Neural Development==

In the early embryo as shown in the images above, the neural tube is formed and represents the initial stages of the formation of the brain and spinal cord. As the embryo develops, the neural tube continues to develop in conjunction with the formation of the vertebrae of the spine.  

In the early embryo as shown in the images above, the neural tube is formed and represents the initial stages of the formation of the brain and spinal cord. As the embryo develops, the neural tube continues to develop in conjunction with the formation of the vertebrae of the spine.  

===Development of Vertebrae===

===Development of Vertebrae===

Formation of the cells that will be the precursors to the spinal vertebrae align in an intersegmental fashion to the neural cells of the spinal cord, in a form of bridging pattern. Therefore each neural cell has a spinal vertebrae precursor cell between itself and the adjacent neural cell such that the spinal cell effectively overlaps the edges of two neural cells. This ensures that as the spine and spinal cord develop the spinal nerves become aligned to the spaces between the vertebrae that will eventually become seperated by intervertebral discs.  

Formation of the cells that will be the precursors to the spinal vertebrae align in an intersegmental fashion to the neural cells of the spinal cord, in a form of bridging pattern. Therefore each neural cell has a spinal vertebrae precursor cell between itself and the adjacent neural cell such that the spinal cell effectively overlaps the edges of two neural cells. This ensures that as the spine and spinal cord develop the spinal nerves become aligned to the spaces between the vertebrae that will eventually become separated by intervertebral discs.  

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===Development of the Brain===

===Development of the Brain===

Development of the brain begins with a series of expansions of the neural tube at the cranial end, giving rise to the internal spaces of the brain, the ventricles and aqueducts. The initial expansion of the neural tube is in the [[Midbrain_-_Anatomy_%26_Physiology|mesencephalon]] area, or mid brain area. This is then followed by expansions in the [[Hindbrain_-_Anatomy_%26_Physiology|rhombencephalon]] (hind brain) and in the [[Forebrain_-_Anatomy_%26_Physiology|proencephalon]] (forebrain).  

Development of the brain begins with a series of expansions of the neural tube at the cranial end, giving rise to the internal spaces of the brain, the ventricles and aqueducts. The initial expansion of the neural tube is in the [[Midbrain - Anatomy & Physiology|mesencephalon]] area, or mid brain area. This is then followed by expansions in the [[Hindbrain - Anatomy & Physiology|rhombencephalon]] (hind brain) and in the [[Forebrain - Anatomy & Physiology|proencephalon]] (forebrain).  

[[Image:WIKIVETbraindifferentiation.jpg|thumb|right|300px|Brain Differentiation - © Sophie Stenner]]

[[Image:WIKIVETbraindifferentiation.jpg|thumb|right|300px|Brain Differentiation - © Sophie Stenner]]

[[File:4 week embryo brain.jpg|thumb|150px|centre|Embryonic Brain Development]]

[[File:4 week embryo brain.jpg|thumb|150px|centre|Embryonic Brain Development]]

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The development of the brain continues with the forebrain out-pouchings developing into hemispheres and growing dorsolaterally, forming the telencephalon. These hemispheres grow over the newewly formed diencephalon, mesencephalon and back to the metencephalon. During this stage other developmental landmarks are developed including the Cepahlic flexure, Pontine flexure, Cervical flexure and the myelencephalon. An out-pouching for the cerebellum is also present at this stage of development, together with a [[Cerebral_Spinal_Fluid_-_Anatomy_%26_Physiology|choroid plexus]] (for the production of CSF) and defined lateral ventricles.

The development of the brain continues with the forebrain out-pouchings developing into hemispheres and growing dorsolaterally, forming the telencephalon. These hemispheres grow over the newly formed diencephalon, mesencephalon and back to the metencephalon. During this stage other developmental landmarks are developed including the Cepahlic flexure, Pontine flexure, Cervical flexure and the myelencephalon. An out-pouching for the cerebellum is also present at this stage of development, together with a [[Cerebral Spinal Fluid - Anatomy & Physiology|choroid plexus]] (for the production of CSF) and defined lateral ventricles.

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===Caudal Spinal Cord Development===

===Caudal Spinal Cord Development===

In late development, the spinal cord extends along the entire length of the spinal canal (formed by the spinal vertebrae). The spinal cord then terminates at the 'conus medullaris' in the middle of the lumbar spine. In adults the spinal cord ends at the beginning of the lumbar psine. Lumbar and sacral nerves extend via the 'cauda equine'.

In late development, the spinal cord extends along the entire length of the spinal canal (formed by the spinal vertebrae). The spinal cord then terminates at the 'conus medullaris' in the middle of the lumbar spine. In adults the spinal cord ends at the beginning of the lumbar spine. Lumbar and sacral nerves extend via the 'cauda equina'.

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Therefore in neonates CSF taps must be caudal to the conus medullaris to avoid damaging the spinal cord.  

Therefore in neonates CSF taps must be caudal to the conus medullaris to avoid damaging the spinal cord.

 

==Neuronal Differentiation==

==Neuronal Differentiation==

===Embryonic Orientation===

===Embryonic Orientation===

An important element to the development of the CNS is the development and regulation of orientation within the embryo, i.e. which part is cranial, caudal, lateral etc. Within the neural tube this is controlled via '''HOX''' (Homeobox) transcription factors which ensure there is an orientation between rostral and caudal. Two HOX genes are expressed resulting in boundaries within the tissue that is expressing that particular gene. One HOX gene is expressed rostrally and the other along the length of the spinal cord. This ensures only appropriate neural development takes place for the given location and orientation.

An important element to the development of the CNS is the development and regulation of orientation within the embryo, i.e. which part is cranial, caudal, lateral etc. Within the neural tube this is controlled via '''HOX''' (Homeobox) transcription factors which ensure there is an orientation between rostral and caudal. Two HOX genes are expressed resulting in boundaries within the tissue that express that particular gene. One HOX gene is expressed rostrally and the other along the length of the spinal cord. This ensures only appropriate neural development takes place for the given location and orientation.

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The dorsal horn tissues eventually fuse allowing the formation of a 'median septum'. Ventral horn tissues expand more fully than the dorsal grey matter tissues and form a 'ventral fissure'.

The dorsal horn tissues eventually fuse allowing the formation of a 'median septum'. Ventral horn tissues expand more fully than the dorsal grey matter tissues and form a 'ventral fissure'.

===Neuronal Formation===

===Neuronal Formation===

Within the brain and the spinal cord the fundamental organisation of the neurons is into 'columns' or groupings of cells within both the sensory and motor regions of the brain and within the grey matter of the spinal cord. Groups of neurons with similar functions are able to form 'nuclei'. Cranial nerve nuclei can include both motor and sensory neurons and are able to undertake a wide variety of functions and tasks. Cranial nerves can also convey fibres from multiple cranial nerve nuclei. Within the spinal cord specific columns of neurones, or 'spinal tracts' convey specific information to high centres in the brain.

Within the brain and the spinal cord the fundamental organisation of the neurons is into 'columns' or groupings of cells within both the sensory and motor regions of the brain and within the grey matter of the spinal cord. Groups of neurons with similar functions are able to form 'nuclei'. Cranial nerve nuclei can include both motor and sensory neurons and are able to undertake a wide variety of functions and tasks. [[Cranial Nerves - Anatomy & Physiology|Cranial nerves]] can also convey fibres from multiple cranial nerve nuclei. Within the spinal cord specific columns of neurones, or [[Sensory Pathways - Anatomy & Physiology#Pathways|'spinal tracts']] convey specific information to high centres in the brain.

 

[[Category:Nervous System - Anatomy & Physiology]][[Category:Developmental Biology]]

[[Category:Nervous System - Anatomy & Physiology]][[Category:Developmental Biology]]

[[Category:To Do - A&P]]

[[Category:A&P Done]]