Central nervous system
Development of the brain
Prof. Dr. Malak A. Al-yawer
Department of Anatomy/ Embryology Section
Learning objectives
 At the end of this lecture, the medical student will be able to
 Define the primary brain vesicles and state their fates
 Name the flexures that present in the cephalic end of neural
tube
 State the role of basal and alar plates in the formation of
different parts of the brain
 Describe the embryonic origin of olfactory system
 State the embryonic origin of different types of commissures
 Define the embryonic origin of cranial nerves and state the
embryonic origin of their motor nuclei, sensory ganglion and
parasympathetic ganglion
 State some clinical correlates
Primary brain vesicles
 Three dilations( primary brain vesicles):
(a) Prosencephalon (forebrain)
(b) Mesencephalon (midbrain)
(c) Rhombencephalon (hindbrain)
 Two flexures:
A. Cervical flexure
spinal cord
(b) Cephalic flexure
hindbrain -
midbrain
 Rhombencephalic isthmus
mesencephalon -rhombencephalon
Rhombencephalon (Hindbrain)
 consists of the
(a) Metencephalon
 pontine flexure -rhombencephalic
isthmus
 later forms the pons and
cerebellum
(b) Myelencephalon
 most caudal of the brain vesicles
 forms the medulla oblongata
 Pontine flexure
Myelencephalon
 differs from the spinal cord
in that its lateral walls are
everted
 the sulcus limitans can be
clearly distinguished.
 The basal plate contains
motor nuclei
 The alar plate contains
three groups of sensory
relay nuclei
 The roof plate
 Tela choroidea
 Choroid plexus
Metencephalon
1. pons
 Basal plate / 3 groups of
motor neurons
 Alar plate / 3 groups of
sensory nuclei
 Marginal layer of its basal
plates expands & makes a
bridge for nerve fibers
 Pontine nuclei originate in the
alar plates of the
metencephalon and
myelencephalon
Metencephalon
2. Cerebellum
 rhombic lips
 cerebellar plate
 shows vermis and 2 hemispheres (12
week embryo)
 A transverse fissure separates the
nodule from the vermis and the
lateral flocculus from the
hemispheres .
 Flocculonodular lobe is
phylogenetically the most primitive
part of the cerebellum.
Mesencephalon (Midbrain )
Basal plate / 2 groups of
motor nuclei
Marginal layer of its basal
plate enlarges and forms the
crus cerebri
Alar plates
 Initially / 2 longitudinal
elevations separated by a
shallow midline depression
 With further development, a
transverse groove divides each
elevation into superior &
inferior colliculi
Prosencephalon
1. Diencephalon
 develops from the median portion of the
prosencephalon
 characterized by outgrowth of the optic
vesicles and forms the
 optic cup and stalk
 pituitary
 thalamus
 hypothalamus
 epiphysis
Roof plate
 choroid plexus of 3rd ventricle
 Its most caudal part develops into the pineal
body ( epiphysis)
 Alar plates / lateral walls of the
diencephalon.
 Hypothalamic sulcus
Prosencephalon
2. Telencephalon
consists of
 2 lateral outpocketings
( cerebral hemispheres)
 a median portion(lamina
terminalis) / used by
commissures
• Insula: the depressed area between the frontal and
temporal lobes. At the time of birth it is completely
covered.
• During the final part of fetal life, the surface of the
cerebral hemispheres grows so rapidly that a great many
convolutions (gyri) separated by fissures and sulci appear
on its surface
Olfactory system
epithelial– mesenchymal
interactions between
1. neural crest cells and
ectoderm of the
frontonasal prominence olfactory placodes /
primary sensory neurons
2. same crest cells and the
floor of the telencephalon
- olfactory bulbs/
secondary neurons
By the 7th week, these
contacts are well
established.
Commissures
• Anterior commissure
 first to appear
 connecting the olfactory bulb and related
brain areas of one hemisphere to those of
the opposite side
• Hippocampal commissure ( fornix
commissure )
 second to appear
 arise in the hippocampus and converge on
the lamina terminalis to the mamillary
body and the hypothalamus
• Corpus callosum
 appears by the 10th week of development
 connects the non olfactory areas of the
right and the left cerebral cortex
• Posterior and habenular commissures /
below and rostral to the stalk of the pineal
gland.
• Optic chiasma
 rostral wall of the diencephalon
 contains fibers from the medial halves of
the retinae.
Cranial Defects
Holoprosencephaly (HPE)
• Loss of midline tissue
has resulted in a
midline cleft lip, lack of
nasal tissue, and eyes
that are too close
together
• The loss of midline
tissue causes the lateral
ventricles to merge into
a single chamber.
 Drawings illustrating various types of skull defects
 meningocele (B)
 Meningoencephalocele(C)
 Meningohydroencephalocele(D)
 usually occur in the occipital region, but may occur in the
frontonasal region.
 Origin of these defects is due to abnormal neural tube closure/ can
be prevented by maternal use of folic acid
Hydrocephalus
• is due to an obstruction of
the aqueduct of Sylvius
(aqueductal stenosis).
• This prevents the CSF of
the lateral and third
ventricles from passing
into the 4th ventricle and
from there into the
subarachnoid space, where
it would be resorbed.
• As a result, fluid
accumulates in the lateral
ventricles and presses on
the brain and bones of the
skull.
Microcephaly
• a cranial vault is
smaller than normal
• due to poor growth of
the brain
• frequently associated
with mental
retardation.
Cranial nerves
• 4th week of development/ nuclei
for all 12 cranial nerves are
present.
• All except the olfactory (I) and
optic (II) nerves arise from the
brainstem, and of these only the
oculomotor (III) arises outside the
region of the hindbrain.
• The hindbrain is divided into eight
rhombomeres (r1–r8) - give rise
to motor nuclei of cranial nerves
IV, V, VI, VII, IX, X, XI, and XII.
• Motor neurons for cranial nuclei
are within the brainstem, while
sensory ganglia are outside of the
brain.
Cranial nerve sensory ganglia
Parasympathetic (visceral
efferent) ganglia

1.
2.
•
1.
2.
3.
• derived from neural crest
cells
• their fibers are carried by
cranial nerves III, VII, IX, and
X
originate from
ectodermal placodes and
neural crest cells
Ectodermal placodes include
Nasal placode
otic placode
four Epibranchial placodes
dorsal to the pharyngeal
(branchial) arches &
contribute to ganglia for
nerves of the pharyngeal
arches (V, VII, IX, and X).
Summary
 The cephalic end of the neural tube shows three dilations (primary
brain vesicles)
 The cephalic end of the neural tube forms two flexures: cervical and
cephalic flexures
 Rhombencephalic isthmus separates the mesencephalon from the
rhombencephalon
 Distinct basal and alar plates, representing motor and sensory
areas, respectively, are found on each side of the midline in the
rhombencephalon and mesencephalon
 In the prosencephalon, however, the alar plates are accentuated
and the basal plates regress
 Differentiation of the olfactory system is dependent on epithelial–
mesenchymal interactions
 All cranial nerves except olfactory (I) and optic (II) nerves arise
from the brainstem
 Cranial nerve sensory ganglia originate from ectodermal placodes
and neural crest cells
 Parasympathetic (visceral efferent) ganglia are derived from neural
crest cells