Embryological development of human Nervous system

1. Development of Nervous
System
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2. • Neuroanatomy is the study of the structural aspects of the
nervous system, usually correlated with function.
• nervous system and endocrine system control activity of the
body.
• Nervous system is the chief controlling and coordinating
system of the body.
• Human nervous system is responsible for judgement,
intelligence and memory.
• It is the most complex system of the body.
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Berhanu. k (MSc)
1. 1 What is Neuroanatomy?
. Introduction to Neuroscience

3. 3
Berhanu. k (MSc)
Structural divisions
Central nervous system (CNS)
o Brain (within cranial cavity)
o spinal cord
Peripheral nervous system (PNS)
ocranial nerves (nerves that
extend from brain)
ospinal nerves (nerves that
extend from spinal cord)
oAssociated ganglia (clusters of
neuron cell bodies located
outside the CNS)
Organization of the Nervous System

4. 4
Berhanu. k (MSc)
Functional classification

5.  Parts of brain
1. Forebrain (prosencephalon)
 contain two distinct regions named:
 Telencephalon (cerebrum), made up of two cerebral
hemispheres.
 Diencephalon (thalamencephalon), consists of:
 Thalamus
Hypothalamus
Metathalamus
 Epithalamus
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Berhanu. k (MSc)

6. 2. Midbrain (mesencephalon)
3. Hindbrain (rhombencephalon)
 This part includes:
Pons
cerebellum
medulla oblongata
brain stem: consists;
Midbrain
Pons
Medulla oblongata
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Berhanu. k (MSc)

7. Development of Nervous System
• Understanding of development of the nervous system helps make
sense of its adult configuration and organization.
• Similarly, congenital malformations of nervous system are more easily
understood in light of its embryological development;
such malformations provide clues that aid in the understanding of
normal development.
• The ectoderm germ cell layer gives rise to three major structures:
• surface ectoderm, the epidermis of the skin including hair, nails,…
• neural tube (future brain and spinal cord
• neural crest
• Human nervous system starts out embryonically as a simple, tubular,
ectodermal structure.
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Neuroectoderm

8. • During the beginning of 3rd week of embryonic development, a longitudinal
band of ectoderm thickens to form the neural plate.
• The neural plate begins to fold inward, forming a longitudinal neural groove in
the midline flanked by a parallel neural fold on each side.
• The neural groove deepens, and the neural folds approach each other in the
dorsal midline.
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9. • At the beginning of the 4th week, the two folds begin to fuse midway
along the neural groove at a level corresponding to the future cervical
spinal cord.
 this starts the formation of the neural tube.
• Fusion proceeds cranially and caudally, and the entire neural tube is
closed by the end of the 4th week. This process is referred to as primary
neurulation.
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11.  As the neural tube closes, it progressively separates from the ectodermal surface,
leaving behind groups of cells from the crest of each neural fold. These neural
crest cells develop into a variety of cell types.
 The neural tube develops into the entire CNS; its cavity becomes the ventricular
system of the brain and the central canal of the spinal cord.
 The sacral spinal cord forms by a slightly different mechanism.
 After the neural tube closes, a secondary cavity extends into the solid
mass of cells at its caudal end during the fifth and sixth weeks, in a process
of secondary neurulation.
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12. Primary Vesicles of the neural tube
• During the 4th week, three bulges or vesicles are apparent at the
cephalic end of the neural tube.
• From rostral to caudal these are:
• Prosencephalon (Greek for “front brain,” or forebrain), This develops into
the forebrain.
• Mesencephalon (Greek for “midbrain”), it becomes the midbrain of the
adult brainstem
• Rhombencephalon (named for its rhomboidal shape) which merges
smoothly with the caudal (future spinal) portion of the neural tube.
The rhombencephalon (hindbrain) becomes the rest of the brainstem
and the cerebellum
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13. 13
 Primary vesicles at the end of the 4th week.
 A, Lateral view of the neural tube, showing vesicles and flexures.
 B, Schematic longitudinal section

14. • The three primary vesicles are associated with two bends or
flexures.
• cervical flexure, occurs b/n rhombencephalon and spinal cord; but
it straightens out later in development, thus rendering it of little
significance in the adult.
• Cephalic (or mesencephalic) flexure, occurs at the level of the
future midbrain and persists in the adult b/n the axes of brainstem
and forebrain.
This is the bend that is not present in animals that walk on four
legs but exists in humans and other animals that walk on two
legs.
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15. Five Secondary Vesicles of the neural tube
• During the 5th week, five secondary vesicles can be distinguished.
• prosencephalon gives rise to:
• Telencephalon (end-brain); becomes the cerebral hemispheres
of the adult brain.
• Diencephalon (in-between-brain); becomes thalamus,
hypothalamus, retina and several other small structures
• Mesencephalon remains undivided; it develops into the midbrain,
• rhombencephalon gives rise to:
• Metencephalon; “behind-brain,” becomes the pons and the
cerebellum.
• Myelencephalon; “medulla-brain” becomes the medulla
• In addition, a pontine flexure appears in the dorsal surface of
the brainstem b/n the metencephalon and the
myelencephalon. 15

16. 16
Secondary vesicles during the sixth week.
A, Lateral view of the neural tube, showing vesicles and
flexures.
B, Schematic longitudinal section.

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18. • Within each of the brain vesicles, the neural canal is expanded into a
cavity called a primitive ventricle w/c become the definitive
ventricles of the mature brain.
• The cavity of the rhombencephalon becomes the fourth ventricle,
• the cavity of the mesencephalon becomes the cerebral aqueduct (of
Sylvius),
• the cavity of the diencephalon becomes the third ventricle, and
• the cavity of the telencephalon becomes the paired lateral
ventricles of the cerebral hemispheres.
• After the closure of the caudal neuropore, the developing brain
ventricles and the central canal of the more caudal spinal cord are
filled with cerebrospinal fluid.
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19. 19
Summary

20. Anomalies
 Spina bifida: it is failure of closure of the caudal neuropore the
developing neural tube.
 The various types of spina bifida are commonly associated anomalies of the
vertebral arch, meninges and spinal cord.
Spina bifida occulta: Evidenced by a tuft of hair in the lumbosacral region.
least severe with unfused vertebral arch and the spinal cord is intact.
Spina bifida with meningocele: occurs when the meninges protrude through
a vertebral defect and form a sac filled with CSF. The spinal cord remains in its
normal position.
Spina bifida with meningomyelocele: Occurs when the meninges and spinal
cord protrude through a vertebral defect and form a sac filled with CSF.
Spina bifida with myeloschisis: This condition is the most severe type of
spina bifida, causing paralysis from the level of the defect caudally. This
variation presents clinically as an open neural tube
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22. Cranium bifida: Cranium bifida occurs when the bony skull fails to
form properly, thereby creating a skull defect, usually in the occipital
region.
Cranium bifida with meningocele: occurs when the meninges
protrude through the skull defect and form a sac filled with CSF.
Cranium bifida with meningoencephalocele: occurs when the
meninges and brain protrude through the skull defect and form a
sac filled with CSF.
Cranium bifida with meningohydroencephalocele: occurs when
the meninges, brain, and a portion of the ventricle protrude
through the skull defect.
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Anomalies…..

23. Neural crest cells
 As the neural folds elevate and fuse, cells at the lateral border or crest of the
neuroectoderm begin to dissociate from their neighbors. This cell population is the
neural crest cells
 Neural crest cells undergo migration throughout the embryo (both cranial and
trunk regions) and ultimately differentiate into a wide variety of adult cells and
structures.
 Cranial neural crest cell
 Cranial neural crest cells differentiate into the following adult cells
and structures:
• Pharyngeal arch skeletal and connective tissue components
• Bones of neurocranium
• Pia and arachnoid matters
• Parafollicular (C) cells of thyroid gland
• Aorticopulmonary septum of heart
• Odontoblasts (dentin of teeth)
• Sensory ganglia of cranial nerve (CN) V, CN VII, CN IX, and CN X
• Ciliary (CN III), pterygopalatine (CN VII), submandibular (CN VII), and otic
(CN IX) parasympathetic ganglia. 23

24. Trunk neural crest cells
Trunk neural crest cells extend from somite 6 to the most
caudal somites and migrate in a dorsolateral, ventral, and
ventrolateral direction throughout the embryo.
Trunk neural crest cells differentiate into the following adult
cells and structures:
• Melanocytes
• Schwann cells
• Chromaffin cells of adrenal medulla
• Dorsal root ganglia
• Sympathetic chain ganglia
• Prevertebral sympathetic ganglia
• Enteric parasympathetic ganglia of the gut (Meissner and
Auerbach; CN X)
• Abdominal/pelvic cavity parasympathetic ganglia
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25. Placodes
 Placodes are localized thickenings of surface ectoderm. They give rise
to cells that migrate into underlying mesoderm and develop into
sensory receptive organs of cranial nerves (CN I and CN VIII) and lens
of the eye. There are three placodes:
 Nasal (olfactory) placode: differentiate into neurosensory cells that give
rise to the olfactory nerve (CN I) and induce formation of olfactory bulbs.
 Otic placode: give rise to the otic vesicle, which forms the following:
 Utricle, semicircular ducts, and vestibular ganglion of CN VIII
 Saccule, cochlear duct (organ of Corti), spiral ganglion of CN VIII
 Vestibulocochlear nerve (CN VIII)
 Lens placode; gives rise to the lens and is induced by the optic vesicles.
Optic vesicles, cups, and stalks: are derivatives of diencephalon.
They give rise to the retina, iris, ciliary body, optic tract (CN II), optic
chiasm.
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26. HISTOGENESIS OF THE NEURAL TUBE
Initially, the wall of the neural tube is composed of a thick,
pseudostratified columnar neuroepithelium.
The precursors of most cell types of future CNS are produced by
proliferation in the layer of neuroepithelial cells.
Neuroblasts (young neurons) form all neurons found in the CNS.
Glioblasts (spongioblasts) are formed after cessation of
neuroblast formation. These cells differentiate into the macroglia
of the CNS—astrocytes and oligodendrocytes
ependyma (ependymal epithelium) form when neuroepithelial
cells cease producing neuroblasts and glioblasts, they
differentiate into ependymal cells.
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27. Layers of the early neural tube
A.Ventricular Zone
The early neural tube consists of
neuroectoderm arranged in a
pseudostratified columnar arrangement.
First wave of proliferation and
differentiation of the neuroectoderm gives
rise to neuroblasts, which migrate into the
intermediate zone.
Second wave of proliferation and
differentiation of the neuroectoderm gives
rise to glioblasts, which migrate into the
intermediate zone and marginal zone.
Neuroectoderm that remains in the
ventricular zone gives rise to
ependymocytes, tanycytes, and choroid
plexus cells.
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28. B. Intermediate Zone/Mantle layer
contains neuroblasts, which differentiate into neurons with dendrites
and axons.
also contains glioblasts, which differentiate into astrocytes and
oligodendrocytes.
 forms the gray matter of the spinal cord.
divided into the alar plate, associated with sensory (afferent) functions,
and the basal plate, associated with motor (efferent) functions.
C. Marginal Zone
contains axons from neurons within the intermediate zone
 also contains glioblasts, which differentiate into astrocytes and
oligodendrocytes.
forms the white matter of the spinal cord.
 Microglial cells (microglia), which are scattered throughout the gray and white
matter, are small cells that are derived from mesenchymal cells. Microglia
originate in the bone marrow and are part of the mononuclear phagocytic cell
population.
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30. Differentiation Of Spinal Cord
 Starting at the end of 4th week, neurons in the
mantle layer of spinal cord become organized into four
plates; connected at sulcus limitans laterally
• a pair of dorsal or alar plates and a pair of
ventral or basal plates.
 Alar (sensory) plate
a dorsolateral thickening of the intermediate
zone of the neural tube.
gives rise to sensory neuroblasts of dorsal
horn (general somatic afferent [GSA] and
general visceral afferent [GVA] cell regions).
receives axons from the dorsal root ganglia
that become dorsal (sensory) roots.
 becomes the dorsal horn of the spinal cord.
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• Embryonic spinal cord during
the sixth week; spinal
ganglion (SG) cells, send their
central processes into the
spinal cord to terminate on
alar plate (AP) cells;
• many basal plate (BP) cells
become motor neurons,
whose axons exit in the
anterior roots.
Neurons of the alar plate develop into association neurons and
afferent nuclei, and groups of these nuclei form the dorsal gray
columns. These neurons receive synapses from afferent
(incoming) fibers from the sensory neurons of the dorsal root
ganglia.

32. B. Basal (motor) plate
a ventrolateral thickening of the intermediate zone
of the neural tube.
 gives rise to motor neuroblasts of the ventral and
lateral horns (general somatic efferent [GSE] and
general visceral efferent [GVE] cell regions).
 project axons from motor neuroblasts, which exit
the spinal cord and become the ventral (motor)
roots.
becomes the ventral horn of spinal cord.
Sulcus limitans is a longitudinal groove in the lateral
wall of neural tube that separates the alar and basal
plates. It disappears in the adult spinal cord but is
retained in the rhomboid fossa of the brain stem.
Roof plate is nonneural roof of central canal, which
connects the two alar plates.
Floor plate is nonneural floor of central canal, which
connects the two basal plates.
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33. As the pontine flexure develops, the walls of the neural tube spread apart to form
a diamond-shaped cavity so that only a thin membranous roof remains the
primitive fourth ventricle. Thus alar and basal plates come to lie in the floor of
fourth ventricle.
The result is that in the corresponding part of the adult brainstem (rostral
medulla and caudal pons), sensory nuclei are located lateral, rather than
posterior, to motor nuclei.
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The dorsal-ventral arrangement of sensory and motor areas in the
spinal cord (A) becomes a lateral-medial arrangement in the
brainstem (B)

34. THE END
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