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 hindbrain - spinal cord

(b) Cephalic flexure 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

originate from 1. ectodermal placodes and 2. neural crest cells • Ectodermal placodes include 1. Nasal placode 2. otic placode 3. four Epibranchial placodes

dorsal to the pharyngeal (branchial) arches & contribute to ganglia for nerves of the pharyngeal arches (V, VII, IX, and X).

Parasympathetic (visceral efferent) ganglia

• derived from neural crest cells

• their fibers are carried by cranial nerves III, 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