well describes the development of nervous system from basic to advanced concept including neural tube defects. the concepts are presented in graphical form for easy understanding of concepts.
This is the first lecture about the anatomy of the brainstem discussing the definition of the brainstem and the anatomical relations along with the external and internal parts (in general) and listing the major functions of brainstem. Then describing the medulla oblongata with its location, external and internal functions at different levels of sections. Lastly, this lectures discusses the most important clinical syndromes affecting the medulla oblongata.
well describes the development of nervous system from basic to advanced concept including neural tube defects. the concepts are presented in graphical form for easy understanding of concepts.
This is the first lecture about the anatomy of the brainstem discussing the definition of the brainstem and the anatomical relations along with the external and internal parts (in general) and listing the major functions of brainstem. Then describing the medulla oblongata with its location, external and internal functions at different levels of sections. Lastly, this lectures discusses the most important clinical syndromes affecting the medulla oblongata.
USMLE neuroanatomy neuroanatomy 019 CNS development .pdfAHMED ASHOUR
The development of the CNS is a fascinating process that occurs during embryonic development and continues into early childhood.
Disruptions or abnormalities during this process can lead to a wide range of neurological disorders and developmental disabilities.
Understanding the mechanisms underlying CNS development is critical for advancing our knowledge of brain development and for developing new therapies for neurological disorders.
Fate of embryonic layers and structures develope from themNizadSultana
Fate of embryonic layers and structures develope from it. Embryonic layers ectoderm mesoderm and endoderm form different structures during embryonic development.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Slide 1: Title Slide
Extrachromosomal Inheritance
Slide 2: Introduction to Extrachromosomal Inheritance
Definition: Extrachromosomal inheritance refers to the transmission of genetic material that is not found within the nucleus.
Key Components: Involves genes located in mitochondria, chloroplasts, and plasmids.
Slide 3: Mitochondrial Inheritance
Mitochondria: Organelles responsible for energy production.
Mitochondrial DNA (mtDNA): Circular DNA molecule found in mitochondria.
Inheritance Pattern: Maternally inherited, meaning it is passed from mothers to all their offspring.
Diseases: Examples include Leber’s hereditary optic neuropathy (LHON) and mitochondrial myopathy.
Slide 4: Chloroplast Inheritance
Chloroplasts: Organelles responsible for photosynthesis in plants.
Chloroplast DNA (cpDNA): Circular DNA molecule found in chloroplasts.
Inheritance Pattern: Often maternally inherited in most plants, but can vary in some species.
Examples: Variegation in plants, where leaf color patterns are determined by chloroplast DNA.
Slide 5: Plasmid Inheritance
Plasmids: Small, circular DNA molecules found in bacteria and some eukaryotes.
Features: Can carry antibiotic resistance genes and can be transferred between cells through processes like conjugation.
Significance: Important in biotechnology for gene cloning and genetic engineering.
Slide 6: Mechanisms of Extrachromosomal Inheritance
Non-Mendelian Patterns: Do not follow Mendel’s laws of inheritance.
Cytoplasmic Segregation: During cell division, organelles like mitochondria and chloroplasts are randomly distributed to daughter cells.
Heteroplasmy: Presence of more than one type of organellar genome within a cell, leading to variation in expression.
Slide 7: Examples of Extrachromosomal Inheritance
Four O’clock Plant (Mirabilis jalapa): Shows variegated leaves due to different cpDNA in leaf cells.
Petite Mutants in Yeast: Result from mutations in mitochondrial DNA affecting respiration.
Slide 8: Importance of Extrachromosomal Inheritance
Evolution: Provides insight into the evolution of eukaryotic cells.
Medicine: Understanding mitochondrial inheritance helps in diagnosing and treating mitochondrial diseases.
Agriculture: Chloroplast inheritance can be used in plant breeding and genetic modification.
Slide 9: Recent Research and Advances
Gene Editing: Techniques like CRISPR-Cas9 are being used to edit mitochondrial and chloroplast DNA.
Therapies: Development of mitochondrial replacement therapy (MRT) for preventing mitochondrial diseases.
Slide 10: Conclusion
Summary: Extrachromosomal inheritance involves the transmission of genetic material outside the nucleus and plays a crucial role in genetics, medicine, and biotechnology.
Future Directions: Continued research and technological advancements hold promise for new treatments and applications.
Slide 11: Questions and Discussion
Invite Audience: Open the floor for any questions or further discussion on the topic.
Richard's entangled aventures in wonderlandRichard Gill
Since the loophole-free Bell experiments of 2020 and the Nobel prizes in physics of 2022, critics of Bell's work have retreated to the fortress of super-determinism. Now, super-determinism is a derogatory word - it just means "determinism". Palmer, Hance and Hossenfelder argue that quantum mechanics and determinism are not incompatible, using a sophisticated mathematical construction based on a subtle thinning of allowed states and measurements in quantum mechanics, such that what is left appears to make Bell's argument fail, without altering the empirical predictions of quantum mechanics. I think however that it is a smoke screen, and the slogan "lost in math" comes to my mind. I will discuss some other recent disproofs of Bell's theorem using the language of causality based on causal graphs. Causal thinking is also central to law and justice. I will mention surprising connections to my work on serial killer nurse cases, in particular the Dutch case of Lucia de Berk and the current UK case of Lucy Letby.
A brief information about the SCOP protein database used in bioinformatics.
The Structural Classification of Proteins (SCOP) database is a comprehensive and authoritative resource for the structural and evolutionary relationships of proteins. It provides a detailed and curated classification of protein structures, grouping them into families, superfamilies, and folds based on their structural and sequence similarities.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Introduction:
RNA interference (RNAi) or Post-Transcriptional Gene Silencing (PTGS) is an important biological process for modulating eukaryotic gene expression.
It is highly conserved process of posttranscriptional gene silencing by which double stranded RNA (dsRNA) causes sequence-specific degradation of mRNA sequences.
dsRNA-induced gene silencing (RNAi) is reported in a wide range of eukaryotes ranging from worms, insects, mammals and plants.
This process mediates resistance to both endogenous parasitic and exogenous pathogenic nucleic acids, and regulates the expression of protein-coding genes.
What are small ncRNAs?
micro RNA (miRNA)
short interfering RNA (siRNA)
Properties of small non-coding RNA:
Involved in silencing mRNA transcripts.
Called “small” because they are usually only about 21-24 nucleotides long.
Synthesized by first cutting up longer precursor sequences (like the 61nt one that Lee discovered).
Silence an mRNA by base pairing with some sequence on the mRNA.
Discovery of siRNA?
The first small RNA:
In 1993 Rosalind Lee (Victor Ambros lab) was studying a non- coding gene in C. elegans, lin-4, that was involved in silencing of another gene, lin-14, at the appropriate time in the
development of the worm C. elegans.
Two small transcripts of lin-4 (22nt and 61nt) were found to be complementary to a sequence in the 3' UTR of lin-14.
Because lin-4 encoded no protein, she deduced that it must be these transcripts that are causing the silencing by RNA-RNA interactions.
Types of RNAi ( non coding RNA)
MiRNA
Length (23-25 nt)
Trans acting
Binds with target MRNA in mismatch
Translation inhibition
Si RNA
Length 21 nt.
Cis acting
Bind with target Mrna in perfect complementary sequence
Piwi-RNA
Length ; 25 to 36 nt.
Expressed in Germ Cells
Regulates trnasposomes activity
MECHANISM OF RNAI:
First the double-stranded RNA teams up with a protein complex named Dicer, which cuts the long RNA into short pieces.
Then another protein complex called RISC (RNA-induced silencing complex) discards one of the two RNA strands.
The RISC-docked, single-stranded RNA then pairs with the homologous mRNA and destroys it.
THE RISC COMPLEX:
RISC is large(>500kD) RNA multi- protein Binding complex which triggers MRNA degradation in response to MRNA
Unwinding of double stranded Si RNA by ATP independent Helicase
Active component of RISC is Ago proteins( ENDONUCLEASE) which cleave target MRNA.
DICER: endonuclease (RNase Family III)
Argonaute: Central Component of the RNA-Induced Silencing Complex (RISC)
One strand of the dsRNA produced by Dicer is retained in the RISC complex in association with Argonaute
ARGONAUTE PROTEIN :
1.PAZ(PIWI/Argonaute/ Zwille)- Recognition of target MRNA
2.PIWI (p-element induced wimpy Testis)- breaks Phosphodiester bond of mRNA.)RNAse H activity.
MiRNA:
The Double-stranded RNAs are naturally produced in eukaryotic cells during development, and they have a key role in regulating gene expression .
2. SESSION OBJECTIVES
By the end of the session each participant
should be able to:
1.Describe the embryology of the nervous
system
2.Outline the congenital anomalies of the CNS
3.Appreciate the origin of the different
congenital anomalies and plan to prevent
them in “real life”
2
3.
4. 4
Terminology
• Congenital means exist since birth, whether
clinical evidences are obvious or not
obvious.
• Anomaly means a deviation from the
normal.
• Malformation means faulty development of
6. Subdivisions of the PNS
Peripheral nervous system (PNS)
a) Somatic (voluntary) nervous system
(SNS)
b) Autonomic (involuntary) nervous
systems (ANS)
c) Enteric nervous system (ENS)
11. • Neural crest becomes
peripheral nervous
system (PNS)
• Neural tube becomes
central nervous system
(CNS)
• Somites become spinal
vertebrae.Somites
12
12. • The nervous system develops from the
neural plate.
–The neural plate is a thickened area of the
embryonic ectoderm
• The neural plate then differentiates to form
the neural tube, neural folds and neural
crest.
–The neural tube differentiate into the CNS
(brain + spinal cord)
–The neural crest will give rise to cells that
form the peripheral and autonomic nervous
system
13. • Neurulation is the formation of the neural tube
– Begins in the region of the 4th
-6th
somites
– The cranial 2/3 of the neural plate the future
brain
– The caudal 1/3 of the neural plate spinal cord
– Neural canal “the lumen of the neural tube
communicates with the amniotic fluid
– the walls of the neural tube thickens to form the
brain and the spinal cord
– The neural canal is converted into the ventricular
system of the brain and the central canal of the
spinal cord
15. Development of the spinal
cord
• The lateral walls of the neural tube thickens
gradually reduce the size of the neural canal
central canal of the spinal cord is formed
• The wall of the neural tube is composed of the
neuroepithelium constitute the ventricular
zone gives rise to all neurons and microglial
cells in the spinal cord.
17. • Proliferation and differentiation of the
neuroeithelial cells produce thick walls and thin
roof and floor plates
• Differential thickening of the lateral walls of the
spinal cord shallow groove on each side
sulcus limitans
– This groove separate the alar plate “ the dorsal
side” from the basal plate “ the ventral part”
– Cell bodies in the alar plates forms the dorsal
gray columns
– Cell bodies in the basal plate form the ventral
and lateral gray horns
19. Development of spinal ganglia
• The axons of cells in the spinal ganglia are at first
bipolar the 2 processes unite in a T-shape
fashion unipolar
• The unipolar neurons in the spinal ganglia “dorsal root
ganglia” are derived from neural crest cells
• The peripheral processes of spinal ganglion cells pass
in the spinal cord nerves to sensory endings in
somatic or visceral structures
• The central processes enter the spinal cord dorsal
roots of spinal nerves
20. Positional changes of spinal
cord
• In the embryo: the spinal cord extends the entire
length of the vertebral canal.
• Because the vertebral column and dura mater
grow more rapidly than the spinal cord the
caudal end of the spinal cord gradually comes
to lie at relatively higher levels.
• The spinal cord in a newborn terminates at L2 or L3
• The spinal cord in the adult terminates at the
inferior border of L1.
21. Myelination of nerve fibers
• The myelin sheaths surrounding
nerve fibers within the spinal
cord are formed by
oligodendrocytes.
• The myelin sheaths around the
axons of peripheral nerve fibers
are formed by the plasma
membranes of neurolemmal
(Schwann) cells.
23. Development of brain
• Fusion of the neural folds in the cranial region forms 3
primary brain vesicles
– Forebrain “prosencephalon”
– Midbrain “mesencephalon”
– Hindbrain “rhombencephalon”
• During development,
– the forebrain divides into telencephalon and
diencephalon
– the hindbrain divides into metencephalon and
myelencephalon
– The midbrain does not divide
• Consequently, there are 5 secondary brain vesicles
25. Brain flexures
• During development, the embryonic brain grows
rapidly and bends ventrally with the head fold
this produces the
– Midbrain flexure in the midbrain
– Cervical flexure at the junction of the hindbrain
and spinal cord
• Later, unequal growth of the brain between these
flexures produces the pontine flexure.
– This flexure results in thinning of the roof of the
hindbrain
26. Hindbrain
• The cervical flexure demarcates the hindbrain from
the spinal cord
– Later this junction will be defined as the level of the
superior rootlet of the cervical nerve.
• The pontine flexure divides the hindbrain into
– Myelencephalon “caudal” develops into medulla
oblongata
– Metencephalon “rostral-toward the front” develops into
the pons and cerebellum.
• The cavity of the hindbrain becomes the 4th
ventricle
and the central canal in the caudal part of the
medulla
27. Myelencephalon1
• Neuroblasts from the alar plates in the
mylencephalon migrate into the marginal zone
and form isolated areas of the grey matter
gracile nuclei “medial” and cuneate nuclei “laterally
• The ventral area of the medulla contains the
pyramids ( pair of fiber bundles)
• During development, as the walls of the medulla
move laterally, the alar plates lie lateral to the
basal plates motor nuclei medial to sensory
nuclei
28. • Neuroblasts from the basal plate form nuclei:
– General somatic efferent: neurons of hypoglossal
nerve
– Special visceral efferent: neurons innervating
muscles derived from pharyngeal arches
– General visceral efferent: neurons of the vagus and
glossopharyngeal nerves
• Neuroblasts of the alar plate form nuclei:
– General visceral efferent: receive impulses from
viscera
– Special visceral afferent: receive taste fibers
– General somatic afferent: receive impulses from the
surface of the head
– Special somatic afferent: receiving impulses from
ear
– Olivary nuclei
30. Metencephalon
• The cerebellum develops from thickenings of
dorsal parts of the alar plates
• Neuroblasts in the intermediate zoon of the
alar plates migrate and differentiate into the
neurons of the cerebellar cortex
• Cells from the alar plate give rise to the dentate
nucleus “largest nucleus”, pontine nuclei, the
cochlear & vestibular nuclei and the sensory
nuclei of the trigeminal nerve.
• Bands of nerve fibers cross the median plane
and from a bulky ridge pons
32. Midbrain
• The neural canal that passes through the midbrain
narrows cerebral aqueduct “ a canal that connect
the 3rd
and the 4th
ventricles”
• Neuroblasts migrate from the alar plates of the
midbrain into the tectum “roof” and aggregate to form
superior and inferior collicluli concerned with
the visual and auditory reflexes
• Neuroblasts from the basal plates give rise to
neurons in the tegmentum "red nuclei, 3rd
and 4th
cranial nerve nuclei, and the reticular nuclei” and the
substantia nigra
• Cerebral peduncles “fibers from the cerebrum” pass
through the midbrain brain stem spinal cord
34. Forebrain
• Optic vesicles are 2 lateral outgrowths that
appear on each side of the forebrain
primordia of the retinae and optic nerves
• Cerebral vesicles is the second pair of
outgrowths primordia of cerebral
hemispheres and lateral ventricles
• Telencephalon “anterior part of the forebrain”
and diencephalon “posterior part of the
midbrain” contribute to the formation of 3rd
ventricle.
35. Diencephalon
• Swellings develop in the lateral wall of the 3rd
ventricle epithalamus, thalamus and
hypothalamus.
– The thalamus develops rapidly and bulges into the
cavity of the 3rd
ventricle
– The hypothalamus arises by proliferation of
neuroblasts in the intermediate zone of the
diencephalon
– The epithalamus develops form the roof and dorsal
portion of the lateral walls of the diencephalon
37. Telencephalon
• The telencephalon consist of
– Cerebral vesicles:”2 lateral diverticula” primordia of the cerebral
hemispheres
– The median portion of telencephalon forms the anterior part of 3rd
ventricle
• At first, the cerebral vesicles are in communication with
cavity of 3rd
ventricle through the interventricular foramina
as the cerebral hemispheres expand, they meet in the
midline.
– The mesenchyme trapped between them gives rise to the falx
cerebri “fold of dura mater”
• Later, corpus striatum develops as a swelling in the floor
of each cerebral hemisphere
40. Embryonic Development of the Brain
• Brain grows rapidly, and changes occur in the
relative position of its parts
– cerebral hemispheres envelop the diencephalon and
midbrain
– wrinkling of the cerebral hemispheres, more neurons
fit within limited space
42. Basic Parts of the Brain
• Divided into four regions
– Cerebral hemispheres
– Diencephalon
– Brain stem includes the midbrain, pons, and
medulla
– Cerebellum
44. • Neural plate to
neural tube
• Neural crest–PNS
• Anterior forms brain
– Forebrain
– Midbrain
– Hindbrain
• Hollow ventricles
• Spinal cord
Embryonic Development of Nervous
System
Figure 9-2: The embryonic nervous system develops into a hollow tube
45
45. Differentiation
• Specialization of structures
• 3 primary vesicles
– rostral end of tube
– develops into brain
• Prosencephalon forebrain
• Mesencephalon midbrain
• Rhombencephalon hindbrain ~
46
60. A lot can go wrong.
• Rate of neurogenesis incredibly rapid.
• Failure to form appropriate connections may
be basis of many neurological and
psychiatric disorders.
61
61. Neurulation
• Neurulation is the formation of the vertebrate
nervous system in embryos.
• The notochord induces the formation of the
CNS by signaling the ectoderm above it to form
the thick and flat neural plate.
• The neural plate then folds in on itself to form
the neural tube, which will then later
differentiate into the spinal cord and brain.
62. • Different portions of the neural tube then
form by 2 different processes in different
species:
1.Primary Neurulation – the neural plate
creases inward until the edges come into
contact and then fuse.
2. Secondary Neurulation – the tube forms by
hollowing out of the interior of a solid precursor
63.
64.
65.
66. Formation of the Neural Tube
• Secondary Neurulation
1. Occurs beyond the caudal neuropore
2. lumbar and tail region
3. Starts with formation of medullary cord
4. Cavitation of cord to form hollow tube
67. Differentiation of Neural Tube
• Major morphological changes:
differentiation of brain vesicles and spinal
cord
• Differentiation of neural tube cells
• Development of peripheral nervous
system
68.
69. Neural Crest Cells
• Induced by organizing cells of notochord
• Main functional groups:
– Cranial neural crest:
• Bones and connective tissue of face
• Tooth primordia
• Thymus, parathyroid, thyroid glands
• Sensory cranial neurons
• Parasympathetic ganglia and nerves
• Parts of the heart (cardiac neural crest)
70. Neural Crest Cells
A group of cells, which breaks away from the
closing neural tube and populate the periphery
(as opposed to the CNS, which will develop
from the tube).
• Neural crest will supply all neurons of the PNS and a
variety of other peripheral structures, ranging from
melanocytes to craniofacial bones to cells of the
adrenals.
• This population of cells separates from the neural
plate shortly after the fusion of the neural folds, and
streams of dividing cells begin their journey through
the embryo.
• The expression of which genes are turned off during
this migratory stage?
• And, this occurs for individual cells as well as for
groups of cells
71. Neural Crest Cells
• For neural crest cells, migratory pathway is
particularly important in cellular determination,
as location (or path) controls the availability of
inducing factors for particular cell fates.
Making Cells
The use of chimeras has been invaluable in the
study of individual cell fates.
What is a chimera?
Cells with a different genome; e.g., chick/quail mix
– heterochromatin marker not found in chick.
[3
H] thymidine labeling has helped in the
delineation of migratory pathways and
development potential of neural crest cells.
72. Neural Crest Cells
Interaction:
Neural crest migration/movement is rigid and
occurs in a ventral (1st
cells give rise to ventral
structures) - to- dorsal order in the head.
Migratory pathways are linked to neuronal fate.
What is the mostly likely result of transplant
experiments when early cells will switch their
fate?
Extrinsic cues ?
Whether these come from the pathway itself or the
final destination (target-derived cues) is not
clear.
As in the CNS, the earlier the cell is, the more
73. Neural Crest Cells
• Main functional groups:
• The stream of neural crest cells migrates via
a ventral route to form:
– Trunk neural crest:
• Melanocytes (via the dorsal route)
• Sensory neurons (DRG)
• Sympathetic ganglia and nerves (ANS)
• Medulla of adrenal glands (chromaffin cells)
Note that they migrate segmentally (sclerotome) –
only in the rostral compartment.
74.
75. Neural Crest Cells
• Migration:
– Epithelial to mesenchyme transition
– Migrational pathways are established by juxtacrine
signals:
• Fibronection, laminin in ECM + integrins
• Ephrin proteins: Restrict movement
• Contact inhibition
• Use of existing structures
– Migration ceases when these signals are reversed
76. Migration of Neural Crest Cells
Unlike cells in the CNS, which migrate radially
along glial fibers, neural crest cells “crawl along”
independently (like fibroblasts).
Motility is promoted by integrins – bind cell surface
to ECM (how does this contrast with cadherins?)
Prominent ECM components along neural crest
cell pathway: fibronectin, laminin, collagen.
The ECM provides attractive (permissive) cues for
movements, as well as a substrate on which to
bind.
A set of repulsive cues in neighboring structures
keeps cells in their precise migratory pathway
77. Migration of Neural Crest Cells
As the cells reach their destination, the expression
of cadherins is once again activated (had been
repressed during free movement) cells
aggregate into ganglia when they undergo
terminal neuronal and glial differentiation.
Side bar: retroviral labeling:
A cell can be labelled permanently and heritably by
injection with a retrovirus carrying a gene (e.g.,
β-galactosidase) incorporated into cells’ DNA
and then expressed.
A substance, which will turn blue from the action of
the enzyme, can then be introduced in a
histochemical test.
78. 128
CNS dev’t begins in the 3rd
wk as a flat plate of
thickened ectoderm called the neural plate.
Its lat edges soon elevate to form the neural
folds. These folds later approach @ other in the
midline & fuse forming the neural tube.
Fusion of the neural tube begins in the cervical
region & proceeds in the cephalic & caudal
directions.
81. 131
• Closure of the cranial neuropore occurs about
day 25, while the caudal neuropore close at about
2 days later.
• The cephalic end of the neural tube shows 3
dilations called the primary brain vesicles.
Prosencephalon (forebrain)
Mesencephalon (midbrain)
Rhombencephalon (hindbrain)
82. The Brain: embryonic development
• Develops from neural tube
• Brain subdivides into
– Forebrain
– Midbrain
– Hindbrain
• These further divide, each with a fluid filled
region: ventricle, aqueduct or canal
– Spinal cord also has a canal
• Two major bends, or flexures, occur (midbrain
and cervical)
84. 134
• At about week 5, the prosencephalon
divides into:
a) the telencephalon, which consists of a
midportion & 2 lateral out pocketings called
the primitive cerebral hemispheres.
b) the diencephalon.
• The rhombencephalic isthmus is a
furrow which separates the mesencephalon
from the rhombencephalon.
86. • Space restrictions force cerebral
hemispheres to grow posteriorly
over rest of brain, enveloping it
• Cerebral hemispheres grow into
horseshoe shape (b and c)
• Continued growth causes creases,
folds and wrinkles
87. • Gyri (plural of gyrus)
– Elevated ridges
– Entire surface
• Grooves separate gyri
– A sulcus is a shallow groove (plural, sulci)
– Deeper grooves are fissures
88. 138
• The rhombencephalon also consists
of 2 parts:
a) the metencephalon, which later
forms the pons & cerebellum.
b) the myelencephalon.
• The boundary between these 2
portions is marked by the pontine
flexure.
89. 139
• The lumen of the spinal cord called the
central canal is continuous with that of the
brain vesicles.
• The cavity of the cerebral hemispheres are
the lat ventricles, that of the diencephalon is
the 3rd
ventricle & that of the rhombencephalon
is the 4th
ventricle.
• The lumen of the mesencephalon becomes
narrow & connects the 3rd
& 4th
ventricles. Its
called the aqueduct of Sylvius.
90. 140
• The lateral ventricles
communicate with the 3rd
ventricle through the
interventricular foramina of
Monro.
93. • Prenatal life: genes are responsible for creating
the architecture of the brain
– Cortex is the last to develop and very
immature at birth
• Birth: excess of neurons but not inter-connected
– 1st
month of life: a million synapses/sec are
made; this is genetic
• First 3 years of life: synaptic overgrowth
(connections)
– After this the density remains constant though
some grow, some die
• Preadolescence: another increase in synaptic
formation
adapted from Dr. Daniel Siegel, UCLA
94. • Adolescence until 25: brain becomes a
reconstruction site
– Connections important for self-regulation (in
prefrontal cortex) are being remodeled: important
for a sense of wholeness
– Causes personal turbulence
– Susceptible to stress and toxins (like alcohol and
drugs) during these years; affects the rest of one’s
life
• The mind changes the brain (throughout life)
– Where brain activation occurs, synapses happen
– When pay attention & focus mind, neural firing
occurs and brain structure changes (synapses are
formed)
– Human connections impact neural connections
(ongoing experiences and learning include the
interpersonal ones) 144
95. FUNCTIONS OF THE BRAIN:
simplified…
• Back of brain: perception
• Top of brain: movement
• Front of brain: thinking
97. 147
Spinal cord:
The inside of a neural tube consists of a thick
pseudostratified epi of neuroepithelial cells which
divide more rapidly immediately after the closure of
the tube during the neural groove stage.
These cells divide repeatedly to form a
neuroepithelial layer or neuroepithelium.
With further dev’t, the neuroepithelial cells give rise to
primitive nerve cells or neuroblasts xterised by large
round nucleus with pale nucleoplasm & dark staining
nucleolus.
98. 148
Neuroblasts segregate into a mantle layer, around the
neuroepi layer. This layer later forms the grey matter
of the spinal cord.
The outermost layer of the spinal cord, the marginal
layer, contains nerve fibres emerging from the
neuroblasts in the mantle layer. Bse of myelination,
these fibres appear white hence called the white
matter of the spinal cord.
100. 150
With further dev’t, @ side of the neural tube wth
continuos addition of neuroblasts,shows ventral &
dorsal thickenings of the mantle layer.
The ventral thickenings called basal plates form the
motor areas of the spinal cord, while the dorsal
thickenings called alar plates form the sensory areas.
The longitudinal tube,i.e,the sulcus limitans marks the
boundary btn the two plates.
101. 151
Roof & floor plates are dorsal & ventral midline
portions of the neural tube which don’t contain
neuroblasts. They serve primarily as pathways for
nerve fibres crossing from one side to the other.
Positional changes of the cord:
In the 3rd
mth, the cord extends the entire length of the
embryo, with spinal nerves passing thru the
intervertebral foramina at their level of origin.
102. 152
With increasing age, the vtbral column & dura
lengthen rapidly, hence the terminal end of the cord
gradually shifts to a higher level.
At birth, this end is at the level of L3.
The spinal nerves run obliquely from their segment of
origin in the spinal cord to the corresponding level of
the vtbral column.
The Dura remains attached to the vertebral column at
the coccygeal level.
103. 153
In the adult, the cord terminates at the level of
L2 to L3, while the dural sac & subarachnoid
space extend to S2.
Below L2 / L3, a thread-like extension of the pia
mater forms the filum terminale, which marks the
tract of regression of the spinal cord. Its
attached to the 1st
coccygeal vtbra.
Nerve fibres below the terminal end of the cord
collectively constitute the cauda equina.
106. 156
Neural tube defects:
Most of these result from abnormal closure of
the neural folds. The resulting abnormalities
(Neural tube defects, NTDs), may involve the
meninges, vtbrae, mm & skin.
Spina bifida are defects affecting the spinal
region.
Spina bifida occulta, is a defect in the vertebral
arches that is covered by skin & does not involve
the underlying neural tissue.
107. 157
Spina bifida cystica, is when neural tissue &
meninges protrude thru defects in the vertebral
arches to form a cyst-like sac.
Fluid may fill the protruding meninges hence
called spina bifida with meningocele, or neural
tissue may protrude in the sac hence called spina
bifida with meningomyelocele.
Causes of NTDs is multifactorial eg, hyperthermia,
hyper-vitaminosis A, folate deficiency and other
teratogens.
Somatic (voluntary) nervous system (SNS)
neurons from cutaneous and special sensory receptors to the CNS
motor neurons to skeletal muscle tissue
Autonomic (involuntary) nervous systems (ANS)
sensory neurons from visceral organs to CNS
motor neurons to smooth & cardiac muscle and glands
sympathetic division (speeds up heart rate)
parasympathetic division (slow down heart rate)
Enteric nervous system (ENS)
involuntary sensory & motor neurons control GI tract
neurons function independently of ANS & CNS
3 weeks of gestation in humans
Step 1: Neural groove forms.
Step 2: Neural tube closes at midbrain level, then "zips up" in both rostral and caudal directions.
birth defects related to folic acid deficiency
“Holoprosencephaly” is an anomaly resulting from disturbances of early forebrain development. Because of the influence of the brain on surrounding structures facial defects are common. Fully developed cases frequently include facial defects, microcephaly, and closely set eyes.
Cyclopia: During early normal development the eye fields form on either side of the diencephalon. Failure of development of the ventral midline portion of the prosencephalon allows the eye fields to form in close proximity to one another resulting in cyclopia. In the example shown, the near absence of upper and mid-facial tissue allowed the fusion of the optic primordia below a tubular proboscis (nose).
As the spinal cord develops neuroblasts of the neural tube’s mantle layer proliferate in 2 zones. In cross section the mantle layer develops a characteristic “butterfly”-shape of gray matter. The lateral walls of the tube thicken but leave a shallow, longitudinal groove called the sulcus limitans which separates the developing gray matter into a dorsal alar plate and a ventral basal plate. These plates signal the future locations of sensory and motor functions, respectively. The sulcus limitans extends the length of the spinal cord and through the mesencephalon.
Alar plate - Neuronal cell bodies here form nuclei which constitute the uninterrupted dorsal gray matter (or columns) that receive and relay input from afferent (sensory, somatic and visceral) neurons. It extends the length of the cord. These neurons receive sensory information from axons in the dorsal roots of spinal nerves.
Basal plate - Cell bodies here form the uninterrupted ventral gray matter (or columns) of efferent neurons that extend the length of the cord. Axons of these neurons project motor fibers to skeletal muscle and form the ventral roots of the spinal nerves.
Proliferation of alar and basal plates causes accompanying changes. Bulges of the plates result in the formation of the longitudinally running dorsal and ventral median septi. The lumen of the neural tube becomes reduced to a small central canal.
The marginal layer increases in mass due to the addition of longitudinally running intersegmental axons, long ascending axons from the gray matter, long descending axons from supraspinal levels and incoming dorsal root sensory fibers. The mass of fibers in the marginal layer subsequently becomes myelinated (beginning in the 4th month) and is called white matter. The dorsal gray and ventral gray columns partially segregate the white matter into dorsal, ventral and lateral white funiculi.
During week 4, the neural groove closes to form a neural tube beginning in the region of the 4th - 6th somites; fusion of neural folds proceeds cranially and caudally forming the brain and spinal cord respectively. Ectodermal cells of the early tube develop 3 concentric zones, 1) germinal (or matrix), 2) mantle, and 3) marginal. Cells of the original single-layered tube divide to form a pseudostratified neuroepithelium whose cells extend from the neural canal to the tube’s external surface. Cells near the central canal are called the germinal layer. They divide rapidly, thickening the walls of the tube and eventually producing neuroblasts and glioblasts.
Newly formed undifferentiated cells of the pseudostratified neuroepithelium migrate outward forming a 3-layered tube consisting of 1) an internal, columnar ependymal layer - becomes the ependymal lining and epithelium of choroid plexus, 2) a middle, densely packed layer of mantle cells - becomes the gray matter of the CNS, and 3) an external marginal layer composed mainly of the processes of cells of the mantle layers - becomes the white matter of the CNS.
Neurons develop from neuroblasts of the neuroepithelium and migrate into the mantle layer. The neuroblast develops into a bipolar cell having a primitive axon and dendrite. The single dendrite degenerates and is replaced by multiple dendrites forming a multipolar neuroblast. Axons have few branches and their axonal growth cones are directed to their targets by tropic factors. Primitive supporting cells are glioblasts and migrate into the mantle and marginal layers to become astrocytes and oligodendrocytes. Microglia are derived from invading blood monocyte
Neuroblasts of the brainstem develop in a manner similar to the spinal cord. From the medulla through the midbrain, alar and basal plates form motor and sensory columns of cells that supply cranial nerves. However, the organization of alar and basal plates differ from of the spinal cord in that, 1) in the medulla and pons the alar plate lies lateral to the basal plate, not dorsal to it, since the 4th ventricle is “open”, 2) there are migrations of neuroblasts of both plates from the ventricular floor to other locations, and 3) “special” sensory and motor structures of the head require new/different cell groups for innervation. Rostral to the midbrain i.e., diencephalon and cerebral hemispheres develop from the alar plate. The cerebellum also develops from alar plate.
Brainstem development from the medulla through the midbrain resembles that of the spinal cord i.e., gray matter is derived from alar and basal plates. Neurons of these plates form cell columns and, as occurred in the cord, they form sensory nuclei of termination and motor nuclei of origin. Unlike the spinal cord, the brainstem receives special sensory information which is different from that of the spinal cord. Whereas the spinal cord supplies skeletal muscle which originated from somites, in the head, there is skeletal muscle which in addition to (head) somite origin originates from branchial arches.
Hence, in addition to the 4 components of spinal nerves i.e., 1) general somatic afferents (GSA), 2) general visceral afferents (GVA), 3) general somatic efferents (GSE), and 4) general visceral efferents (GVE) cranial nerves contain 3 additional components 5) special somatic afferents - auditory & vestibular (SSA), 6) special visceral afferents - taste & smell (SVA) and 7) special visceral efferents - muscles of the jaw, face & larynx (SVE). (Vision and olfaction are cranial nerves associated with the diencephalon and cerebral hemispheres.)
In the 6 mm embryo the thin ependymal roof of the neural tube, viz.-a-viz. the spinal cord, becomes even thinner as the ventricle of the neural tube begins to widen in the early stages of the development of the 4th ventricle. With continued development, alar and basal plates shift laterally and become located in the floor of the ventricle. The sulcus limitans continues to be identifiable helping to mark the boundary between sensory and motor areas. The basal plate forms the motor nuclei of the cranial nerves, medial to the sulcus limitans in the ventricular floor. Lateral to the sulcus, the alar plate forms sensory relay nuclei; portions of the alar plate migrate ventrally and form the inferior olivary nucleus, a cerebellar relay nucleus. Medullary pyramids consist of fibers from the cerebral cortex and develop on the ventral surface near the midline.
The single cell ependymal layer plus adjacent pia (tela choroidea) forms the roof of the medulla. When vascularized, the tela choroidea forms the paired choroid plexuses in the roof. Continuity of the ventricular system and the subarachnoid spaces occurs through the 4th ventricle when the two lateral foramina of Luschka and the midline foramen of Magendi form.
Spinal nerves contain 4 functional fiber types: general somatic afferents (GSA), general visceral afferents (GVA), general somatic efferents (GSE) and general visceral efferents (GVE). In cranial nerves there are 3 additional functional components; special somatic afferents (SSA), special visceral (branchial) efferents (SVE) and special visceral (branchial) afferents (SVA). Cranial nerves may contain anywhere from 1 to 5 fiber types. Hence, representatives of their cell columns will only be present at those levels of the brain where there is a cranial nerve that requires them i.e., many cell columns will be discontinuous.
In the spinal cord, the components are represented by columns of cells, extending longitudinally, that are more or less continuous. The basal plate gives rise to the GSEs and GVEs in the ventral gray horn. The GSE cell column is a continuous cell column (supplying skeletal muscle of somite origin) throughout the cord. The GVE cell column is an interrupted one - it is represented by the intermediolateral cell column from a) C8 - L2 and b) S2 - S4. The alar plate gives rise to the a) continuous GSA cell column represented in the dorsal gray by nucleus posteromarginalis, substantia gelatinosa, and nucleus proprius, b) nucleus dorsalis (of Clarke) found between C8 - L3 at the base of the dorsal gray horn, and c) nucleus gracilis and cuneatus which is present in the low medulla but which receives discriminative touch and proprioception from spinal cord levels. Sensory fibers from the viscera enter the cord and synapse on the GVA cell column at the base of the dorsal gray horn between C8 - L2 and S2 - S4.
The brainstem develops similarly to the cord i.e., basal and alar plates, but as the roof of the 4th ventricle thins and the medulla and pons “open up”, alar plate derivatives come to lie laterally to those of the basal plate. Also, the addition of special sensory and special motor components introduce certain changes from the plan of the cord. None-the-less, the concept of cell columns of specific neuronal components, continuous from the cord and extending into the brain stem, is a useful one. The concept allows for the anticipation of the location of even discontinuous cell columns; functional cell columns will appear in approximately the same dorsal-ventral or medial-lateral positions in whatever level of the brainstem they appear. This concept provides a logical scheme to the CNS in spite of what at first appears to be random.
1 - The continuous GSA cell column of the spinal cord dorsal gray horn which handles pain, temperature and touch sensations is continued without interruption into the medulla, pons and midbrain. In these locations it is variably known as the spinal nucleus of V, principal sensory nucleus of (of CN V) and the mesencephalic nucleus (of CN V). Any CN carrying GSA information will terminate in these trigeminal nuclei, derived from the alar plate.
Proprioceptive GSA information destined for the cerebellum from the lower extremity terminated in the nucleus dorsalis of the cord. Similar proprioceptive information from the upper extremity passes to the analagous rostral representation called the accessory cuneate nucleus in the low medulla.
2 - The discontinuous GVA cell column of the spinal cord dorsal gray horn is recognized throughout the medulla when it resumes as the nucleus of the tractus solitarius. Any CN carrying GVA information will terminate in this cell column.
3 - The continuous GSE cell column of the spinal cord ventral gray horn is represented throughout the brainstem by an interrupted cell column of neurons supplying skeletal muscle of head somite origin. In the medulla there is the motor nucleus of CN XII, in the pons there is the motor nucleus of CN VI, in the midbrain it is represented by the the nearly continuous cell column of CNs III and IV.
4 - The discontinuous GVE cell column of the spinal cord ventral gray horn resumes in the medulla as the dorsal motor nucleus of the vagus and the slightly more rostral inferior salivatory nucleus (of CN IX). In the caudal pons this discontinuous cell column is represented by the superior salivatory nucleus (of CN VII). In the midbrain, this interrupted cell column appears again as the nucleus of Edinger-Westphal (of CN III) at the level of the superior colliculus.
5 - Special visceral (branchial) afferents (SVA) are fibers from taste buds and are carried by CNs VII, IX and X. Upon entering the brainstem, they travel with the GVAs, in the tractus solitarius to the most rostral end of its nucleus (in the pons) where it is called the gustatory portion of the solitary nucleus.
6 - Special visceral (branchial) efferents (SVE) are represented in the brainstem by an interrupted cell column that has no equivalent in the cord. These neurons innervate skeletal muscle that developed from the mesoderm of the branchial arches e.g., larynx, pharynx, facial muscles, muscles of mastication. In the medulla it is represented by nucleus ambiguus (CN IX and X) while in the pons it is represented by the motor nuclei of CN VII and V.
7 - Special somatic afferents (SSA) are found only in supraspinal locations and are represented by the laterally-lying vestibular and auditory nuclei in the medulla and pons. They develop from branchial arch structures.
Cranial nerves I and II serve vision (SSA) and olfaction (SVA). However, these are not true nerves. Rather they develop as evaginations of the telencephalon (olfactory nerve) and the diencephalon (optic nerve).
Neuroblasts of the brainstem develop in a manner similar to the spinal cord. From the medulla through the midbrain, alar and basal plates form motor and sensory columns of cells that supply cranial nerves. However, the organization of alar and basal plates differ from of the spinal cord in that, 1) in the medulla and pons the alar plate lies lateral to the basal plate, not dorsal to it, since the 4th ventricle is “open”, 2) there are migrations of neuroblasts of both plates from the ventricular floor to other locations, and 3) “special” sensory and motor structures of the head require new/different cell groups for innervation. Rostral to the midbrain i.e., diencephalon and cerebral hemispheres develop from the alar plate. The cerebellum also develops from alar plate.
Brainstem development from the medulla through the midbrain resembles that of the spinal cord i.e., gray matter is derived from alar and basal plates. Neurons of these plates form cell columns and, as occurred in the cord, they form sensory nuclei of termination and motor nuclei of origin. Unlike the spinal cord, the brainstem receives special sensory information which is different from that of the spinal cord. Whereas the spinal cord supplies skeletal muscle which originated from somites, in the head, there is skeletal muscle which in addition to (head) somite origin originates from branchial arches.
Hence, in addition to the 4 components of spinal nerves i.e., 1) general somatic afferents (GSA), 2) general visceral afferents (GVA), 3) general somatic efferents (GSE), and 4) general visceral efferents (GVE) cranial nerves contain 3 additional components 5) special somatic afferents - auditory & vestibular (SSA), 6) special visceral afferents - taste & smell (SVA) and 7) special visceral efferents - muscles of the jaw, face & larynx (SVE). (Vision and olfaction are cranial nerves associated with the diencephalon and cerebral hemispheres.)
In the 6 mm embryo the thin ependymal roof of the neural tube, viz.-a-viz. the spinal cord, becomes even thinner as the ventricle of the neural tube begins to widen in the early stages of the development of the 4th ventricle. With continued development, alar and basal plates shift laterally and become located in the floor of the ventricle. The sulcus limitans continues to be identifiable helping to mark the boundary between sensory and motor areas. The basal plate forms the motor nuclei of the cranial nerves, medial to the sulcus limitans in the ventricular floor. Lateral to the sulcus, the alar plate forms sensory relay nuclei; portions of the alar plate migrate ventrally and form the inferior olivary nucleus, a cerebellar relay nucleus. Medullary pyramids consist of fibers from the cerebral cortex and develop on the ventral surface near the midline.
The single cell ependymal layer plus adjacent pia (tela choroidea) forms the roof of the medulla. When vascularized, the tela choroidea forms the paired choroid plexuses in the roof. Continuity of the ventricular system and the subarachnoid spaces occurs through the 4th ventricle when the two lateral foramina of Luschka and the midline foramen of Magendi form.
The cerebellum is first noted at 5-6 weeks as the rhombic lips of the cranial edge of the thinned roof of the 4th ventricle of the hindbrain. The lips develop from the dorsolateral portions of the alar plates forming the cerebellar primordia. Growth causes the 2 rhombic lips to fuse in the midline to form the cerebellar plate which covers the 4th ventricle caudal to the mesencephalon.
The fused rhombic lips assume a dumbbell shape - the central unpaired part is the vermis while the lateral knobs are the hemispheres. By the end of the 4th month growth causes fissures on the surface. The first fissure to develop is the posterolateral fissure which separates the flocculonodular lobe from the corpus cerebelli which rostral to it. The corpus cerebelli grows more rapidly than the flocculonodular lobes and becomes divided by the primary fissure into the anterior and posterior lobes. The surface of the lobes are further marked by the development of many closely packed transverse gyri called folia.
Histogenesis of the cerebellar cortex - In the 2nd month of development the cerebellum consists of a ventricular (inner germinal layer), mantle, and marginal zones. By 19 weeks neuroblasts dividing in inner germinal layer migrate to the marginal layer, proliferate and form the external granular (germinal) layer. The outer germinal layer produces basket, granule and stellate cells. The inner germinal layer produces the Purkinje and Golgi cells and the cells of the deep cerebellar nuclei (dentate, globose, emboliform and fastigii). Radial glial cells extend from the ventricular layer to the surface of the marginal layer and guide the migration of the developing neurons. Neuroblasts of both dividing cell layers produce glia.
The adult mesencephalon (midbrain) is the least modified of the brainstem structures with regard to basal and alar plates. Neuroblasts of alar plates migrate to form inferior and superior colliculi (tectum). Inferior and superior colliculi are related to the auditory and visual systems respectively. The general somatic afferent portion of CN V, the mesencephalic nucleus, is also found here. Oculomotor neurons (GSE) arise from mesencephalic neuroblasts while trochlear motor (GSE) neurons migrate to this location from the metencephalon.
The basal plates and the floor plate expand. They form the tegmentum of the cerebral peduncles in which the motor nuclei of CN III and IV are found in the central gray along with the general visceral efferents to the eye i.e., the Edinger-Westphal nucleus. Corticofugal fiber tracts help form ventrolateral bulges of the cerebral peduncles. The embryologic origin of the red nucleus and substantia nigra (from alar or basal plates) in the cerebral peduncles are uncertain.
The cavity of the original neural tube is little modified in the adult midbrain except to be narrowed by growth of the surrounding midbrain structures; it remains as the narrow cerebral aqueduct.
The structure of the early prosencephalon closely resembles the early neural tube i.e., thick lateral walls are connected by thinner floor and roof plates. The early prosencephalon develops an ocular cup from its ventrolateral walls in the position of the future diencephalon.
In the 5th week, when the embryo reaches the 7 mm stage, the simple plan of the prosencephalon changes; paired telencephalic vesicles and the diencephalon begin to form; the optic cup continues its development. Rapid bilateral expansion of the telencephalon results in rearward overgrowth of the diencephalon. An early separation of the telencephalon and diencephalon occurs through a shallow tele-diencephalic sulcus which deepens and persists in the adult as the horizontal transverse cerebral fissure (filled with duplicated pia and blood vessels, called velum interpositum). As development progresses the growing hemispheres continue their caudal expansion so as to cover the mesencephalon and more.
In the embryo, as in the adult, the roof of the diencephalon is very thin comprised only of ependyma plus adjacent pia called the tela choroidea. When blood vessels invade the tela choroidea the choroid plexus of the 3rd (and lateral) ventricles develop and invade the ventricles. The thin medial wall of the ventricle is the choroid fissure; its most rostral point is at the interventricular foramen. In the adult the choroid fissure is “C”-shaped and continuous in the medial walls of the parietal and temporal lobes.
Early in development, the diencephalon develops 2 pairs of prominent swellings in the walls of the 3rd ventricle. The swellings represent only the alar plate; there is no basal plate. The largest mass is the thalamus dorsally, separated by the hypothalamic sulcus from the ventral hypothalamus.
Where the roof plate thickens along the medial wall of the thalamus are the smaller swellings of the epithalamus comprised of the a) midline pineal gland, b) paired habenular nuclei, and c) paired stria medullaries.
The anterior wall of the diencephalon’s slit-like 3rd ventricle is the lamina terminalis. At this anterior location in the 3rd ventricle, the interventricular foramina (of Monro) connects the 3rd ventricle with the lateral ventricles of the developing hemispheres.
The lamina terminalis (represents the membrane formed at the point of closure of the anterior neuropore) is the most rostral structure of the early telencephalon. By 10 weeks it contains the rudiments of the commissural bundles i.e., corpus callosum, optic chiasm and anterior commissure. The lamina terminalis provides the only location where nerves interconnect the cerebral hemispheres. Crossing fibers of the optic chiasm develop in the lamina ventrally, the anterior commissure connects olfactory bulbs and temporal lobes, the commissure of the fornix joins the hippocampal formations, and most dorsally is the corpus callosum which connects non-olfactory cortical areas. Other crossing fibers do not connect the hemispheres e.g., the posterior commissure connects preoptic areas, the habenular commissure connects habenular nuclei.
The interventricular foramina, which connects the 3rd ventricle with the lateral ventricles, lies just caudal to the lamina terminalis.). The stria medullaris and tela choroidea form the roof of the 3rd ventricle.
he largest commissural bundle is the corpus callosum. It begins as a small bundle but it grows as the cerebral hemispheres expand, encircling the diencephalon and becoming “C”-shaped. First, the hemispheres form parietal and frontal lobes; posterior expansion next forms occipital lobes followed by growth in an inferior direction producing temporal lobes. By its caudalward growth the hemispheres arch over the tela choroidea and choroid plexus of the roof of the diencephalon. This “C”-shaped expansion causes many underlying structures to also become “C”-shaped e.g., corpus callosum, lateral ventricle, choroid plexus, caudate nucleus, fornix. The insula is a lobe of the brain that remains relatively undeveloped as it retains a fixed position while the rest of the hemisphere grows around and over it.
Early in development hippocampal ridges form on the medial aspect of the telencephalic vesicles. These ridges are joined by the commissure of the fornix. When the temporal lobes grow downward and forward in the older embryo, the hippocampal formation forms a ring of gray matter (just above the choroid fissure) on the medial aspect of the hemispheres which is continuous with the “C”-shaped fornix. As both the fornix and corpus callosum grow rearward they become separated and the tissue remaining between them is the septum pellucidum. The anterior commissure connect olfactory bulbs and anterior temporal lobes.
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Expansion of the hemispheres backward and inferiorly over the roof of the diencephalon induces many structures of the CNS to assume a “C”-shape. The result is that when sections of the brain are studied many structures are seen twice. In the sample section shown below the following structures are shown in two locations: 1) lateral ventricles, body and inferior horn, 2) caudate nucleus, body and tail, 3) hippocampal formation, hippocampus and fornix, 4) choroid plexus.
The corpus striatum is comprised of two structures a) caudate-putamen and b) globus pallidus. In the adult the caudate and putamen are partially separated by fibers of the anterior limb of the internal capsule. The posterior limb of the internal capsule separates the globus pallidus from the diencephalon (thalamus).
Early in the development of the cerebral vesicles. The caudate-putamen develops from neuroblasts of the floor of the developing telencephalic vesicle called the striatal ridge while the globus pallidus originates from neuroblasts in the wall of the 3rd ventricle of the diencephalon. Pallidal neuroblasts eventually migrate laterally to join the caudate-putamen. Both parts of the corpus striatum originate near the foramen of Monro.
At first, during the caudal expansion of the telencephalic vesicles, the hemispheres are separate from the diencephalon (except at the foramen of Monro). Later, when the hemispheres begin their complex bending, the medial aspects of the growing hemispheres approach the diencephalon and their apposing surfaces fuse. After the fusion, corticofugal fibers heading to and from the cerebral cortex incompletely divides the caudate nucleus from the putamen. Later, then putamen merges with the globus pallidus forming the lenticular nucleus. As the hemisphere grows backward, the body of the caudate nucleus portion of the striatal ridge in the floor of the lateral ventricle follows the wall of the developing lateral ventricle downward into the roof of the inferior horn of the lateral ventricle where it forms the tail of the caudate nucleus.
The pattern of the ventricular system is established early. The prosencephalon separates into 2 expanding telencephalic vesicles (lateral ventricles) and a slit-like diencephalic vesicle (3rd ventricle). Thickening of its walls narrows the lumen of this segment to a vertical slit, the 3rd ventricle. The lumina of the lateral and 3 ventricles connect just behind the the lamina terminalis via the paired foramina of Monro. Expansion of the mesencephalon narrows its vesicle forming the cerebral aqueduct, which connects the 3rd and 4th ventricle of the rhombencephalon. The rhombencephalic roof is thin and covers the 4th ventricle which forms a shallow, diamond-shaped depression. The expansion of the cerebral hemispheres affects the shape of the lateral ventricles which become “C”-shaped.
A rich capillary plexus develops in the connective tissue around the neural tube of the 6-8 mm embryo. The roofs of the prosencephalon and rhombencephalon are thin, composed of ependyma that becomes invaded by the capillaries. Ependyma plus pia mater is called tela choroidea; vascularized tela choroidea is choroid plexus.
In the adult, cerebrospinal fluid (CSF) is formed by choroid plexus in the 1) body and inferior horns of the lateral ventricles, 2) roof of the 3rd ventricle of the diencephalon, and 3) roof of the 4th ventricle in the medulla. Apertures in the 4th ventricle (foramina of Magendi and Luschka), which develop during the 3rd month, permit the flow of CSF from within the ventricles to subarachnoid spaces surrounding the brain and spinal cord.
The ventricle follows the cerebral hemisphere as it makes its “C”-shaped curve. The choroid fissure and its associated choroid plexus is at first on the thinned medial wall on the dorsal aspect of the developing hemisphere. Later, it continues into the roof of the medial aspect of the inferior horn of the lateral ventricle.
The eyes develop from: 1 neuroectoderm of the diencephalon, 2) surface ectoderm, and 3) intervening neural crest/mesoderm. Neurectoderm gives rise to the retina, epithelium of the ciliary body/iris, and optic nerves. Surface ectoderm gives rise to the lens and anterior surface of the cornea. The surrounding mesenchyme is of neural crest origin and contributes to the sclera, part of the cornea, choroid, ciliary body/iris, and blood vessels of the eye. Early in the 4th week optic sulci appear and are transformed into hollow outgrowths of the diencephalon called optic vesicles which approach the surface ectoderm. Continued growth of the vesicle produces an invaginated optic cup which is connected to the diencephalon by the constricted, hollow optic stalk. When optic vesicles come in contact with the overlying ectoderm it forms the lens. At first the ectoderm thickens to form the lens placode, which forms a lens vesicle, and it finally becomes the solid lens.
Early in the 4th week optic vesicles extend from the 3rd ventricle and wall of the forebrain (diencephalon). As the vesicle reaches the surface ectoderm it flattens (a) and progressively invaginates (b) to form the optic cup, which remains attached to the forebrain by the optic stalk (precursor of the optic nerve). The asymmetric invagination leaves a groove, the choroid fissure, in the stalk. The adjacent ectoderm thickens to form the lens placode (a & b) which invaginates and (eventually) separates (c) from the ectoderm to form the lens vesicle. The primary optic vesicle (a) becomes a double-walled optic cup (c). With continued invagination the original lumen of the optic vesicle is reduced to a slit between the 1) inner neural layer and 2) outer pigment layer of the optic cup (c). Mesenchyme around the optic vesicle will contribute to the fibrous coats of the eye (sclera/cornea) externally and the choroid layer adjacent to the pigment layer. It also forms the hyaloid vessels (c) which pass in the choroid fissure, across the vitreous chamber, to supply the lens.
Lens: As the lens sits in the optic cup the vitreous body develops in the space between the lens and the inner wall of the optic cup. The early lens vesicle is hollow. Later, cells of its posterior pole begin to elongate to form primary lens fibers that run antero-posteriorly obliterating the cavity. The cells of the anterior pole remain cuboidal and become the anterior epithelium. The hyaloid artery passes through the vitreous space to vascularize the posterior aspect of the developing lens. Later, the part supplying the lens disappears; its proximal part remains as the central artery of the retina. Cornea - The lens transforms the overlying surface ectoderm into the anterior epithelium of the cornea and its primary stroma (acellular). Neural crest/mesenchymal cells, surrounding the optic cup, migrate to form the posterior corneal endothelium and invade the primary stroma forming the secondary stroma (cellular).
Optic Cup - Retina, Ciliary body, Iris - Retina: The cells of the inner wall of the optic cup divide to form the multilayered, sensory neural retina; the outer wall of the cup forms the pigment layer of the retina. The neural retina comes to consist of a 3-neuron chain:1) rods and cones, 2) bipolar cells, and 3) ganglion cells. Growth of the axons of the ganglion cells into the optic stalk forms the optic nerve. The edge of the optic cup transforms into the non-light sensitive portions of the retina, on the posterior aspect of the iris and ciliary bodies. Ciliary body - Its outer epithelial layer is pigmented, a forward extension of the pigment layer of the optic cup; its inner epithelial layer is a non-pigmented extension of the neural retina. The stroma of the ciliary body, including its muscle cells, arises from mesenchyme. Iris - The inner and outer epithelial layers of the iris are pigmented. The stroma of the iris arises from mesenchyme; in the stroma are the sphincter and dilator muscles (which arise from its pigmented epithelial cell layer).
