Development of the nervous system

1
Development of the nervous system

• Develop from ectoderm
• Neural plate appears at beginning of 3rd week
• Thickened ectodermal layer in mid dorsal line
• Neural groove
• Formed from the folding plate
• Neural folds
• Elevated sides of neural groove
• Neural tube
• Fusion of neural folds
• Neural Crest cells
• Migrate on either side of neural tube
• diagram

2
Neural Plate

• Originally single layer of columnar cells
• Rapidly becomes pseudostratified
• Neuroepithelial cells of the pseudostratified
  layer continue to divide differentiate into
  different zones of the neural tube
• Neuroblast
• Spongioblast

3
Neuroepithelial cells

• Neuroblasts
• Apolar
• Bipolar
• Multipolar
• Pseudo-unipolar

4
Neuroepithelial cells (cont)

• Spongioblasts
• Glioblasts
• Astroblasts
• Astrocytes
• Protoplasmic
• Fibrous
• Oligodendroblast
• Oligodendrocytes
• Ependymal cells

5
Neural Tube

• 6th week -3 concentric zones/layers
• Inner ependymal layer
• Middle nucleated mantle layer
• Future gray matter of CNS
• Outer non nucleated marginal layer
• Fibrous mesh
• Supports axons dendrites
• Contains glial cells that have migrated from
  mantle layer
• White matter of CNS
• Central Canal

6
Cephalic end of neural tube

• 3 dilatations
• Prosencephalon (forebrain)
• Mesencephalon (midbrain)
• Rhombencephalon (hindbrain)
• 2 flexures (ventral bends)
• Cervical flexure
• At junction of hindbrain spinal cord
• Cephalic flexure
• Located in midbrain region

7
3 dilatations (at 5 weeks)

• Prosencephalon ?
• Anterior portion (telencephalon or endbrain)
• Formed by a mid portion and two lateral out
  pocketings ? primative cerbral hemispheres
• Posterior portion (diencephalon)
• Characterized by outgrowth of the optic vesicles
• Mesencephalon ? has undergone little ?
• Rhombencephalon ?
• Anterior portion (metencephalon)
• Later forms the pons and cerebellum
• Posterior portion (myelencephalon)
• Later become the medulla oblongata

8
Lumen/Central canal

• Lumen of spinal cord (central canal) is
  continuous with brain vesicles
• Allowing free circulation of CSF
• Cavity of rhombencephalon
• 4th ventricle
• Cavity of diencephalon
• 3rd ventricle
• 3rd ? 4th ventricle (aqueduct of Sylvius)
• Becomes very narrow
• Lateral ventricle 3rd ventricle
• Communicate via interventricular foramina of Monro

9
Neural tube-spinal cord level

• Rhomboid shaped-4 longitudinal grooves
• Dorsal sulcus (SD)
• Roof plate is above (only ependymal layer)
• Ventral sulcus (SV)
• Floor plate is below (only ependymal layer)
• 2 lateral sulci
• Sulcus limitans (SL)
• Alar plate between SL roof plate (all 3 layers)
• Sensory neurons ascending pathways
• Basal plate between SL floor plate (all 3
  layers)
• Motor neurons descending pathways
• Central canal still lined by neuroepithelium

10
Development of Spinal Cord (SC)

• Wall of neural tube thickens quickly
• In 4th week typical three layers are present
• Continued growth causes narrowing of central
  canal
• Alar plate thickens, roof plate is obliterated
• Ependymal layers unite ? dorsal median septum
• Reduces extent of central canal
• Basal plate thickens but do not fuse ? ventral
  median fissure

11
Development of the Spinal Cord

• Mantle layer containing neuroblasts ? in mass ?
  gray matter
• On each side of the midplane the gray substance
  forms bulky masses
• Dorsal, lateral, ventral columns (horns)
• Marginal layer
• Neuroepithelial cells, glial cells, axons of
  nerve cells
• Thickening of this layer due to nerve fibers
  deposition of myelin around these fibers

12
Spinal Ganglia

• Pseudo-unipolar neurons in spinal ganglia are
  derived from the neural crest cells
• Each neural crest cell develops two processes ?
  bipolar neuroblast
• Two processes grow fuse at base forming a T
  shaped structure with a central process (axon)
  peripheral process (dendrite)
• Central branches enter spinal cord (dorsal roots
  of spinal N)

13
Positional changes of SC

• In 3rd month the SC extends the entire length of
  the embryo spinal nerves pass through the IVF
  at level of origin.
• Vertebral column meninges grow faster than SC,
  the above relationship does not last
• Terminal end of SC gradually shifts to a higher
  level
• At 6th month the end of SC is at level of S1
• At birth the end of SC is at L2-3
• In adult the end of SC is at lower end of L1

14
Myelencephalon

• Lies between pontine cervical flexure
• Forms medulla oblongata
• Transitions from SC differences
• Lateral walls everted
• Loss of serially segmented repetition of cord
• Addition of lateral rows of cranial nerves
• V, VII, IX, X, XI
• Loss of sharp demarcation btw. gray white
• Nerve fibers crossing break up gray matter
• This mixture ? reticular formation
• figure

15
Myelencephalon

• Some areas form discrete nuclei
• Roof Plate
• Stretched into broad thin layer of ependymal
  cells
• Vascular pia ependymal roof ? tela choroidea
• Rostrally tela choroidea infolds as vascular tuft
• Project into the 4th ventricle ? choroid plexus
  (produces CSF)
• 3 foramina formed due to reabsorption
• Lateral pair ? Luschka
• Medial ? Madendie
• Floor Plate
• Ependymal cells elongate ? septum-like raphe

16
Myelencephalon (cont)

• Alar Plate (sensory)
• Sensory nerves enter from CN VII, IX, X
• Collect into solitary tract located in marginal
  layer
• Alar plate neuroblasts form 3 sensory nuclei
• Somatic afferent (lateral)
• Receive input from ear (CN VIII) head (CN V)
• Special visceral afferent (intermediate)
• Receive input from taste buds via facial N (CN
  VII)
• General visceral afferent (medial)
• Receive input from GI tract, heart via vagus N
  (CN X)
• Olivary n migrate to basal plate area
• Receive afferent input from various sources
  project output to the cerebellum

17
Myelencephalon (cont)

• Basal Plate (motor) Differentiates before alar
• Basal plate neuroblasts form 3 motor nuclei
• General visceral efferent (lateral)
• Parasympathetic
• Dorsal motor nucleus of vagus (CN X)
• Inferior salvitory nucleus (CN IX)
• Special visceral efferent (intermediate) (CN IX,
  X, XI)
• Nucleus Ambiguus
• Supply muscles of 3rd 4th pharyngeal/branchial
  arches
• Somatic efferent (medial)
• Hypoglossal Nucleus
• Supply muscles of tongue (CN XII)
• Decending corticospinal fibers ? pyramids

18
Metencephalon

• Extends from isthmus ? pontine flexure
• Similar to Myelencephalon with addition of
• Cerebellum dorsally
• Chief coordination center of unconscious stimuli
  related to body position movement
• Pons ventrally
• Main conduction pathway between spinal cord
  cerebral cerebellar cortices

19
Metencephalon (cont)

• Roof plate
• Rostral to cerebellum
• Superior medullary velum
• Thin sheet of white substance
• Caudal to cerebellum
• Inferior medullary velum
• Ependymal cells
• Balance lost in substance of cerebellum
• Floor Plate
• Ependymal cells elongate ? septum-like raphe

20
Metencephalon (cont)

• Alar Plates
• Become very prominent forms cerebellum
• Somatic afferent (lateral)
• Sensory nuclei receive fibers from
• Trigeminal (CN V)
• Small portion of Vestibulocochlear (CN VIII)
• Special visceral afferent (intermediate)
• General visceral afferent (medial)

21
Metencephalon (cont)

• Basal Plate (motor)
• General visceral efferent (lateral)
• Axons supply submandibular sublingual glands
• Special visceral efferent (intermediate)
• Contain N of trigeminal (CN V) facial (CN VII)
• Supply muscles of 1st 2nd pharygeal/branchial
  arches
• Somatic efferent (medial)
• Gives rise to N of abducens (CN VI)
• Marginal layer expands extensively ?Pons
• Many fibers pass uninterrupted through pons
• Pyramidal tracts

22
Cerebellum

• Formed from thickening of dorsolateral parts of
  alar plates
• Bend medially forming rhombic lips (RL)
• Approach each other in midline near mesencephalon
• As a result of further deeping of the pontine
  flexure, RL become compressed in a cephalo-caudal
  direction ? cerebellar plate
• Small midline portion ? vermis
• Two lateral portions ? hemispheres
• Transverse fissure seperates nodule from vermis
  lateral flocculus from the hemispheres

23
Cerebellum (cont)

• Cerebellar plate
• Initially consists of 3 layers
• Neuroepithelial
• Number of cells migrate to surface ? external
  granule layer
• Retain ability to divide ? proliferative zone on
  surface
• Gives rise to granule, basket, cells
• Mantle
• During 2nd 3rd month cells migrate into
  marginal layer organizing cerebellar cortex
• Between 3rd 5th month cortex grows faster than
  deeper layers forming main lobes fissures
• Many cells of primitive mantle layer ? neuroglia
  deep nuclei
• marginal

24
Mesencephalon

• Morphologically the most primitive of the brain
  vesicles
• Least modified from the primitive neural tube
• Primarily associated with reflexes of eye head
  in response to visual stimuli
• Caudal part involved with reflexes of acoustic
  centers

25
Mesencephalon (cont)

• Alar Plate
• 2 longitudinal elevations divided by transverse
  groove
• 2 Superior/anterior colliculi (visual reflexes)
• 2 Inferior/posterior colliculi (auditory relay)
• Colliculi formed by waves of neuroblasts
  migrating into overlying marginal zone ? become
  stratified

26
Mesencephalon (cont)

• Basal Plate (motor nuclei)
• Somatic efferent (medial)
• Occulomotor (CN III) Trochlear (CN IV)
• General visceral efferent
• Edinger-Westphal (parasym) ? sphincter pupillary
  muscle
• Marginal layer ? crus cerebri (cerebral
  peduncles)
• Neural pathways decending from cerebral cortex ?
  pons SC
• Red nucleus (may have migrated from alar plate)
• Substantia Nigra
• figure

27
Diencephalon

• Caudal portion of prosencephalon (forebrain)
• Develops from the primitive roof alar plates
• (no basal plates)
• Surrounds third ventricle
• Roof plate ( thin plate of ependymal cells)
• Ependymal cells pia mater ? choroid plexus
• Forms part of epithalamus
• Synaptic region for correlation of olfactory
  impulses
• Habenular nuclei
• Caudal to choroid plexus evagination (7th week) ?
  epiphysis/pineal body (pineal gland)

28
Diencephalon

• Alar Plate
• Dorsal part ? balance of epithalamus
• Contain a group of nuclei ? habenular nuclei
• Lateral part ? Thalamus
• Each thalamus contains a of nuclei
• Relay sensory signals to cerebral cortex
• Ventral part ? hypothalamus
• Differentiate into a of nuclear areas
• Visceral functions
• Downward extension of hypothalamus (infundibulum)
  contributes to formation of pituitary gland
  (neurohypopysis/stalk posterior lobe)
• Neuroglia cells nerve fibers from hypothalamus

29
Pituitary/Hypophysis

• Develops from two different sources
• Ectodermal outpocketing of the stomodeum
• Rathkes Pouch
• Appears around 3rd week grows dorsally
• By end of 2nd month it loses connection with
  stomodeum lies close to the infundibulum
• Gives rise primarily to the anterior lobe
  (adenohypophysis)
• Downward extension of the hypothalamus
• Indundibulum gives rise to stalk posterior lobe

30
Telencephalon

• Medial portion ? Lamina Terminalis
• Original rostral end of the neural tube that
  remains as the medial band
• Two lateral outpockets ? cerebral hemispheres
• Become prominent during the 6th week expands
  rapidly until by about 6th month they overgrow
  the diencephalon mesencephalon

31
Telencephalon

• Divisible into three portions
• Corpus Striatum
• Continuous with the thalamus
• Rhinencephalon
• Paleopallium
• Archipallium
• Hippocampal formation
• Neopallium
• Largest part represented primarily by whats
  visible externally

32
Corpus Striatum

• Appears during the 6th week as a prominent
  swelling in floor of each hemisphere
• Due to proliferating mantle layer cells ? caudate
  lentiform nuclei
• These nuclei are seperated by band like tract of
  fibers called the internal capsul
• Elongates as cerebral hemispheres grow
• Caudal portion forms tail of the caudate
• Will eventually form the basal ganglia

33
Rhinencephalon

• Appears as a swelling on ventral surface of each
  hemisphere during 6th week
• Enlarge into distinct olfactory lobes
• Each lobe divided into a rostral bulb a caudal
  tract region

34
Neopallium

• Predominant part of the forebrain
• Comprises almost all of the hemispheres except
  the hippocampal formation (archipallium)
• As each hemisphere expands it is thrown into
  complex convolutions seperated by numerous
  fissures

35
Histogenesis of Cortex

• In wall of pallium differentiated cells of the
  following layers typical of the neural tube
• Ependymal
• Marginal
• Mantle
• During the third month neuroblasts from this
  layer start to migrate to the marginal layer ?
  primordial cortex
• By the 6th month cortical layers are demarcated
  (usually 6)
• Final differentiation of the outer layers is not
  completed until mid-childhood

36
Congenital Malformations

• Cranium Bifidum
• Hydrocephaly
• Agenesis of Corpus Callosum
• Holoprosencephaly
• Macrocephaly
• Microcephaly

37
Cranium Bifidum

• Open cranium with cyst
• Caused by ossification defect in bones of skull
  (most common is squamous part of occipital bone)
• meningocele
• Cyst contains meninges
• meningoencephalocele
• Cyst contains brain meninges
• Meningohydroencephalocele
• Cyst contains brain, meninges, ventricular space

38
Hydrocephaly

• Abnormal accumulation of cerebralspinal fluid
  within the ventricular system
• Majority of cases is due to obstruction of the
  aqueduct of Sylvius (aqueductal stenosis)
• Fluid buildup in lateral 3rd ventricles
• Reduces volume of brain tissue in anterior
  middle fossa

39
Arnold-Chiari malformation

• Caudal displacement herniation of cerebellar
  structures through the foramen magnum
• Defect occurs in virtually every case of spina
  bifida cystica is usually accompanied by
  hydrocephalus