Embryology of Nervous System New13

Dr.Ardhiyanti PR

Medical School of Al Azhar Islamic University

Mataram 2013

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1) Describe the formation and differentiation of the neural tube.

2) Mention the derivatives of the neural crest. 3) Mention the development of the brain vesicles4) Discuss the derivatives of both basal (motor)

and alar (sensory) laminas in the brain stem.5) Define the development of the brain6) Recognize the development of the cerebellum.7) Describe the formation and development of

nervous cells.8) Report its congenital anomalies.

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The central nervous system (CNS) appears at the beginning of the third week.

It is developed from the ectoderm dorsal to the notochord as a plate of thickened ectoderm, the neural plate, in front of the primitive node.

The lateral margins of the neural plate become raised to form the neural folds, while the median part becomes depressed to form the neural groove.

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Anterior Neuropore: closes at day 25 (18-20 somite stage) and becomes the lamina terminalis.Failure of the anterior neruropore to close results in anencephaly.

Posterior Neuropore: closes at day 27.Failure of the posterior neuropores to close results in spina bifida.

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The neural tube gives rise to the central nervous system: It is divided into:

a craniala cranial enlarged part that forms the brain

a caudal tubular part which forms the spinal cord.

Its cavity gives rise to the ventricles of the brain and the central canal of the spinal cord.

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Some cells at the at the top of the neural fold become specialized, on each side to form the neural crest.

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Tissues derived from the neural Tissues derived from the neural crest:crest:

The (cells) neurons of the dorsal root ganglia.

The (cells) neurons of the sensory ganglia of the cranial nerves.

The (cells) neurons of the autonomic ganglia.

The cells of the supra renal medulla (Chromaffin cells).

Schwann cells (neurolemmal sheath cells).

Pia and arachnoid matter.Pigment cells of the skin (melanocytes).Odontoblasts.Skeletal and connective components of

the pharyngeal arches.Parafollicular cells.

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Vesicle Development:1- the primary vesicles:

The three primary brain vesicles (forebrain, midbrain &hindbrain) and two associated flexures:

a. Cephalic flexure is located between the Prosencephalon and the rhombencephalon.

b. Cervical flexure is located between the rhombencephalon and the future spinal cord,

develop during the 4th week.

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2- Secondry Vesicles:A. Prosencephalon (forebrain): It is associated with the appearance of the optic vesicles.-It gives rise to the telencephalon and the diencephalon.B. Mesencephalon (midbrain)It remains as the mesencephalon.C. Rhombencephalon (hindbrain): It gives rise to the metencephalon, which forms the pons and the cerebellum and the myelencephalon which forms the medulla oblongata.

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Primary Vesicle Primary Division Subdivision Adult Structures

Forebrain vesicle

Prosencephalon (forebrain)

Telencephalon Cerebral hemisphere, basal ganglia, hippocampus

    Diencephalon Thalamus, hypothalamus, pineal body, infundibulum

Midbrain vesicle

Mesencephalon (midbrain)

Mesencephalon (midbrain)

Tectum, tegmentum, crus cerebri

Hindbrain vesicle

Rhombencephalon (hindbrain)

Metencephalon Pons, cerebellum

    Myelencephalon Medulla oblongata

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. Five secondary brain vesicles (with four ventricles) become visible in week 6 of development; the brain vesicles are the primordial of the five major brain divisions:

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a lateral diverticulum called the optic vesicle appears on each side of the forebrain. That part of the forebrain that lies rostral to the optic vesicle is the telencephalon, and the remainder is the diencephalon.

The telencephalon now develops a lateral diverticulum on each side of the cerebral hemisphere, and its cavity is known as the lateral ventricle.

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The diencephalon, which develops from the median portion of the prosencephalon, is thought to consist of a roof plate and two alar plates.

The cavity of the diencephalon forms the greater part of the third ventricle.

Its roof shows pineal body. The remainder of the roof forms the choroid plexus of the third ventricle.

In the lateral wall of the third ventricle, the thalamus & hypothalamus arise as thickening of the alar plate on each side.

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The rhombencephalon is made up of the: pons, cerebellum & medulla oblongata.

The rhombencephalon consists of the myelencephalon, the most caudal of the brain vesicles, gives rise to medulla oblongata,& the metencephalon, which extends from the pontine flexure to the rhombencephalic isthmus, gives rise to pons &cerebellum.

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- The medulla oblongata develops from the caudal part of rhombencephalon.

- It contains:- in its base the pyramids (corticospinal

tracts).Sulcus limitans separates Alar (sensory) lateral in position from basal (motor) plates:

1. Alar plate (sensory) neuroblasts: give rise to the following:

a. Solitary nucleus, which forms special visceral afferent (taste-SVA) column.

b. Spinal nucleus trigeminal, which forms the general somatic afferent (GSA) column.

c. Cochlear and vestibular nuclei - They form the special somatic afferent

(SSA) column.d. gracile and cuneate nucleie. Inferior olivary nuclei 20

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2. Basal plate (motor) neuroblasts: give rise to the following:a. Hypoglossal nucleus, which forms the general

somatic efferent (GSE) column.b. Nucleus ambiguous, which forms the special

visceral efferent (SVE) column (CN IX, CN X, and CN XI). muscles arising from pharyngeal arches.

c. Dorsal motor nucleus of the vagal nerve (CN X) and the inferior salivatory nucleus of the glossopharyngeal nerve (CN IX), which form the general visceral efferent (GVE) column. Parasympathetic

Roof plate:- forms the roof of the fourth ventricle.- is the tela choroidea, a layer of ependymal cells

covered with pia mater.- is invaginated by pial vessels to form the choroid

plexus of the fourth ventricle.22

The metencephalon gives rise to the pons and cerebellum.

Cerebellum: The dorsolateral

parts of the alar plates bend medially and form the rhombic lips

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1. Alar plate (sensory) neuroblasts:

give rise to the following:

a. Solitary nucleus, which forms the SVA column (taste) of CN VII.

b. Cochlear and vestibular nuclei, which form the SSA column of CN VIII.

c. Spinal nucleus of trigeminal nerve, which forms the GSA column of CN V.

d. Pontine nuclei: which consist of cerebellar relay nuclei (pontine gray).

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- The midbrain develops from the walls of the mesencephalic vesicle.

- It contains the cerebral aqueduct, which develops from the mesencephalic cavity.

A. Alar plate neuroblasts form the cell layers of the superior colliculi and the nuclei of the inferior colliculi.

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B. Basal plate neuroblasts: give rise to the following:1. Trochlear and oculomotor nuclei of

CN IV and CN III, which form the GSE column.

2. Edinger-Westphal nucleus of CN III, which forms the most rostral cell group of the GVE column.

3. Substantial nigra.4. Basis pedunculi (crus cerebri)- contains corticobulbar, corticospinal and

corticopontine fibers, derived from the cerebral cortex of the telencephalon

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Neural tube first lined by single layer of cells

Proliferate to form several layers◦ Matrix layer◦ Mantle layer◦ Marginal layer

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Stages in the formation of a nerve cell are:◦ Apolar neuroblast◦ Bipolar neuroblast◦ Unipolar neuroblast◦ Multipolar neuroblast◦ Axon and dendrites

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Neuroglial cells are also formed from germinal cells of the ependymal layer

Glioblasts migrate in to mantle and marginal layer as medulloblasts

They differentiate either into astroblasts or oligodendroblasts

Microglial cells are mesodermal in origin

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Nerve fibers which remain with in the brain and spinal cord receives support from and are ensheathed by neuroglial cells

Peripheral nerves special sheath called the neurolemma derived from schwann cells

Myelin of the CNS derived from oligodendrocytes

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results from failure of the cephalic part of the neural tube to close (failure of the anterior neuropore to close), the lamina terminalis fails to develop.

occurs when: The brain fails to develop, A rudimentary brain stem is

usually present The vault of the skull does not

form. Occurs once in every 1000-

1500 births. is the most common serious

birth defect seen in stillborn fetuses. 34

In cases of neural tube defects as in anencephaly: Serum alpha-fetoprotein (AFP) levels increase in amniotic fluid and maternal serum.

up to 70% of these cases can be prevented by having women take 400 μg of folic acid per day before and during pregnancy.

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Hydrocephalus: is a dilation of the ventricles due to an

excess of CSF. may result from blockage of CSF

circulation or overproduction of CSF. Aqueductal stenosis is the most common

cause of congenital hydrocephalus; it may be transmitted by an X-linked trait or may be caused by cytomegalovirus infection or toxoplasmosis.

Communicating hydrocephalus results from obstruction distal to the ventricles.

Non communicating hydrocephalus results from obstruction within the ventricle system (e.g., aqueductal occlusion).

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Hydrocephalus is typically characterized by:

1) The cranial sutures are widely separated, and the anterior fontanelle is much enlarged.

2) Enlargement of the head. 3) The veins of the scalp

are distended .4) The eyes look downward.

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include the following variations: cranial meningocele,

meningoencephalocele and meningohydroencephalocele.

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Microcephaly- The cranial vault is

smaller than normal.- It is due to defect in

brain development which may be due to genetic disease, postnatal infections or exposures to drugs and other teratogens.

- Impaired mental development occurs in more than half the cases.

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Thank You41