Forebrain Anomalies

Slide 1 · Forebrain Anomalies

Neuronal generation • migration • gyration • midline organization

Pathology of cerebral development

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Trace neuronal production and the radial versus tangential migration routes

Distinguish defects in brain size, gyration, cortical location, and midline formation

Connect distinctive phenotypes with mechanisms and gene examples reported by Robbins

Interpret representative morphologic patterns without overcalling clinical outcomes

Development

Morphology

Mechanism

Interpretation

Schematic: Conceptual sequence

Slide 3 · Four dimensions of abnormal development

Brain volume may be increased or reduced

Cortical surface may have too few or numerous abnormal convolutions

Neurons may form an abnormal cortical architecture or occupy ectopic sites

Midline separation and interhemispheric connections may be defective

Volume

Gyration

Cell location

Midline

Classification schematic

Slide 4 · A lesion can be focal or extensive

Abnormal neuronal production or migration can affect selected regions

Other malformations involve large segments of forebrain

Distribution and anatomy should be described before assigning etiology

Focal involvement

Segmental involvement

Extensive involvement

Phenotype spectrum / alternatives (schematic)

Slide 5 · The germinal matrix is the neuronal source

Proliferating precursor cells reside adjacent to the ventricular system

These precursors contribute neurons to the developing neocortex

Their proliferative behavior influences the final neuronal population

Pial / cortical destination

Ventricular germinal zone

Radial route

Tangential route

Slide 6 · Precursor cell fate determines neuronal number

At each cell cycle, some precursors continue to proliferate

Others leave the proliferative pool and begin migration

The balance between these choices helps determine neuron number

Cycling precursors

Exit cell cycle

Migrating neurons

Cortical population

Schematic: Conceptual sequence

Slide 7 · Radial migration builds cortical populations

One developmental migration route runs radially toward the cerebral cortex

This route with progenitors destined to become excitatory neurons

Successful migration requires coordinated movement from germinal zones

Pial / cortical destination

Ventricular germinal zone

Radial route

Tangential route

Slide 8 · Tangential migration supplies interneurons

A distinct route is tangential rather than primarily radial

Tangential migration with inhibitory interneurons

Separate pathways explain why migration defects may yield different phenotypes

Pial / cortical destination

Ventricular germinal zone

Radial route

Tangential route

Slide 9 · Neuronal generation and migration are distinct

Production governs how many neurons are available

Migration governs where those neurons ultimately reside

Morphologic assessment should ask whether number, location, or both are abnormal

Process

Developmental consequence

Generation

Number of available neurons

Migration

Placement of neurons

Slide 10 · Gyration reflects organized cortical growth

Normal cortical surfaces develop a structured pattern of gyri and sulci

Reduced gyration and irregular excess small convolutions are different defects

Cortical appearance alone does not identify a single molecular cause

Normal

Surface profile

Reduced gyration

Surface profile

Small irregular gyri

Surface profile

Schematic profiles—not microscopic sections

Slide 11 · Cortical layers are a separate clue

Normal neocortex has six principal layers

Polymicrogyria is as four or fewer layers

The number and organization of layers can distinguish malformations

Six-layer pattern

1

2

3

4

5

6

6 illustrated layers

Four-or-fewer layers

1

2

3

4

4 illustrated layers

Schematic; Layer counts reflect only depicted concept

Slide 12 · Phenotype first; Mechanism second

Document brain size, surface, cortical lamination, and ectopic gray matter

Next evaluate midline division and callosal connections

Use molecular examples to explain phenotype, not as interchangeable labels

Identify morphology

Locate defect

Infer process

Correlate genetics

Schematic: Conceptual sequence

Slide 13 · Megalencephaly and microencephaly

Megalencephaly denotes an abnormally large brain volume

Microencephaly denotes an abnormally small brain volume

Microencephaly as much more common in this comparison

Feature

Megalencephaly

Microencephaly

Brain volume

Increased

Decreased

Relative frequency

Less common

More common

Slide 14 · Microencephaly: Brain size and head size

The primary tissue abnormality is reduced brain volume

It is typically accompanied by a small head circumference

Head circumference is a clinical correlate, not a direct measurement of cortical neuronal number

Reduced brain volume

Usually small head circumference

Phenotype spectrum / alternatives (schematic)

Slide 15 · Chromosomal abnormalities are an association

Chromosome abnormalities among conditions linked to microencephaly

The association is broad; The excerpt does not specify a unique chromosome or mechanism

Avoid equating microencephaly with a single chromosomal diagnosis

Chromosomal disorder

Disordered development

Microencephaly

Schematic: Conceptual sequence

Slide 16 · Fetal alcohol syndrome is another association

Microencephaly can accompany fetal alcohol syndrome

This links an in-utero developmental exposure to abnormal brain growth

The source does not quantify risk or specify a cortical histologic pattern for this association

Prenatal exposure

Abnormal cerebral development

Reduced brain volume

Schematic: Conceptual sequence

Slide 17 · Congenital viral infection can affect brain growth

The excerpt names in-utero HIV-1 and Zika virus infection as examples

Both are associations with microencephaly in the supplied source

No infection frequency or diagnostic testing threshold is specified here

Prenatal association

Source examples

Infectious

HIV-1; Zika virus

Noninfectious

Chromosomal abnormality; fetal alcohol syndrome

Slide 18 · Proposed mechanism: Fewer neurons reach cortex

Reduced delivery of neurons to the neocortex

A smaller cortical neuronal population may simplify gyral folding

Experimental mouse data support this model; It is presented as a postulated mechanism

Fewer neurons reach neocortex

Reduced cortical population

Simplified gyral folding

Schematic: Conceptual sequence

Slide 19 · Microencephaly is not a synonym for agyria

Microencephaly describes small brain volume

Agyria describes an extreme loss of cerebral gyral pattern

Reduced brain size and simplified gyration can coexist but are not identical definitions

Term

What is abnormal?

Microencephaly

Brain volume

Agyria

Surface gyration

Slide 20 · Lissencephaly: Diminished gyration

The defining macroscopic feature is reduction in the number of gyri

At the extreme, no discernible gyral pattern remains: Agyria

This phenotype directs attention to disturbances in cortical development

Normal folds

Surface profile

Reduced gyri

Surface profile

Agyria

Surface profile

Schematic profiles—not microscopic sections

Slide 21 · Agyria represents a morphologic extreme

Agyria is absence of the normal gyral pattern

It falls within the severe end of reduced gyration described in lissencephaly

Use anatomic terminology precisely: Not every lissencephalic brain is entirely agyric

Several gyri

Surface profile

Few gyri

Surface profile

No visible gyri

Surface profile

Schematic profiles—not microscopic sections

Slide 22 · Two architectural patterns of lissencephaly

Type I is the classic smooth-surface pattern

Type II has a rough or cobblestone-like surface

The visible differences align with different migration-related mechanisms

Pattern

Surface

Mechanism emphasized

Type I

Smooth

Defective cell migration

Type II

Cobblestone

Defective migration stop signal

Slide 23 · Type I: Impaired migration machinery

Classic lissencephaly is commonly linked to disruption of neuronal movement

Cytoskeletal motor proteins that drive neuroblast migration

The mechanistic emphasis is failure to accomplish appropriately directed movement

Cytoskeletal motor defect

Abnormal neuroblast movement

Smooth-surface lissencephaly

Schematic: Conceptual sequence

Slide 24 · The cytoskeleton is mechanistically relevant

Moving neuroblasts depend on cytoskeletal organization

Disruption of the proteins controlling movement can affect cortical positioning

This is an explanatory mechanism, not a morphologic diagnostic criterion by itself

Motor proteins

Neuronal movement

Cortical organization

Schematic: Conceptual sequence

Slide 25 · Type II: Failure of migration termination

Type II is often associated with alteration of the migration stop signal

The signal requires proteins bearing particular glycosylation patterns

The result is an abnormal rough or cobblestone cortical surface

Abnormal glycosylation

Impaired stop signaling

Cobblestone pattern

Schematic: Conceptual sequence

Slide 26 · Glycosylation is central to type II

A group of specifically glycosylated proteins supports normal migration termination

Mutations affecting enzymes that add sugars to these proteins are common causes in this pattern

Defective protein modification can therefore alter brain architecture

Glycosylating enzymes

Modified proteins

Migration stop signal

Schematic: Conceptual sequence

Slide 27 · Compare moving versus stopping defects

Type I emphasizes the machinery that moves migrating neuroblasts

Type II emphasizes the mechanism that stops migration

Both can impair normal cortical structure but should not be conflated

Question

Type I

Type II

Primary issue

Migration machinery

Migration termination

Robbins example

Motor proteins

Glycosylation enzymes

Surface

Smooth

Cobblestone

Slide 28 · Pattern recognition: Too few gyri

First identify whether the number of gyri is reduced

Then determine whether the surface appears smooth or cobblestone-like

Only after the phenotype is defined should the relevant mechanistic class be considered

Fewer gyri

Inspect surface

Type I or type II pattern

Schematic: Conceptual sequence

Slide 29 · Polymicrogyria: Numerous small gyri

The surface contains excessive small, irregularly formed convolutions

Sulci are characteristically shallow

This is morphologically distinct from the diminished gyration of lissencephaly

Normal

Surface profile

Polymicrogyria

Surface profile

Schematic profiles—not microscopic sections

Slide 30 · The cerebral surface is irregular

Many small folds replace the expected gyral pattern

The convolutions are malformed rather than simply more numerous normal gyri

Visual interpretation requires assessment of fold size, irregularity, and sulcal depth

Normal folding

Surface profile

Small irregular folds

Surface profile

Schematic profiles—not microscopic sections

Slide 31 · Polymicrogyria disrupts cortical lamination

Normal cerebral cortex has six principal layers

Polymicrogyric cortex has four or fewer layers in the Robbins description

A surface malformation therefore also reflects abnormal cortical microarchitecture

Normal six layers

1

2

3

4

5

6

6 illustrated layers

Polymicrogyria ≤4 layers

1

2

3

4

4 illustrated layers

Schematic; Layer counts reflect only depicted concept

Slide 32 · Molecular layers may fuse across gyri

Fusion of molecular layers between adjacent small gyri

The altered interface helps explain loss of ordinary sulcal and gyral architecture

This is an architectural description, not an externally visible surface measurement

Adjacent small gyri

1

2

3

4

4 illustrated layers

Fused molecular layers

1

2

3

4

4 illustrated layers

Schematic; Layer counts reflect only depicted concept

Slide 33 · Late migration injury can produce polymicrogyria

Localized tissue injury toward the end of neuronal migration is one possible cause

The timing of injury can influence the resulting cortical pattern

Not every case is attributed to injury by the supplied source

Late migration period

Localized tissue injury

Polymicrogyria

Schematic: Conceptual sequence

Slide 34 · Lissencephaly versus polymicrogyria

Lissencephaly: Diminished or absent gyri

Polymicrogyria: Many small, abnormal gyri with shallow sulci

Polymicrogyria additionally has a characteristic lamination abnormality

Feature

Lissencephaly

Polymicrogyria

Gyration

Reduced

Excess small, irregular

Surface

Smooth or cobblestone

Many shallow folds

Layers

Mechanism-dependent

Four or fewer

Slide 35 · Neuronal heterotopia: Misplaced cells

Heterotopia denotes neurons collected at abnormal subcortical sites

The ectopic location lies along the developmental route of migration

The defining defect is neuronal position, not necessarily total brain size

Cortical destination

Ventricle / germinal surface

Ectopic neurons

Migration / heterotopia schematic

Slide 36 · Migration can fail before neurons leave origin

A specific location for heterotopic neurons is the ventricular surface

These cells as if they had never left their birthplace

This explains the name periventricular nodular heterotopia

Cortical destination

Ventricle / germinal surface

Ectopic neurons

Migration / heterotopia schematic

Slide 37 · Periventricular nodules are anatomically specific

Nodular aggregates reside along the lateral ventricular surface

These represent misplaced neurons, not additional ventricular chambers

Location provides a clue to defective departure or movement of neurons

Cortical destination

Ventricle / germinal surface

Ectopic neurons

Migration / heterotopia schematic

Slide 38 · Filamin A links motility to periventricular heterotopia

The FLNA gene encodes the actin-binding protein filamin A

Filamin A contributes to cytoskeletal organization and cell motility

Mutations can cause periventricular nodular heterotopia

FLNA alteration

Altered actin organization

Abnormal cell motility

Periventricular heterotopia

Schematic: Conceptual sequence

Slide 39 · FLNA is located on the X chromosome

The gene encoding filamin A is X-linked

That the mutant allele can be lethal in individuals with a single X chromosome

This source-specific pattern must not be generalized to all X-linked neural malformations

Genetic feature

Source description

Gene

FLNA

Chromosomal location

X chromosome

Single-X mutant allele

May be lethal in described pattern

Slide 40 · X inactivation can create neuronal mosaicism

In individuals with two X chromosomes, X inactivation creates distinct cell populations

Cells expressing the normal allele may reach the correct location

Cells expressing the mutant allele may remain in heterotopic locations

Mosaic populations after X inactivation

Normal allele expressed

Mutant allele expressed

Cells shown for inheritance mechanism, not tissue evidence

Slide 41 · Doublecortin participates in migration

DCX encodes doublecortin, a microtubule-associated protein

The gene is X-linked in the Robbins account

Its disruption gives sex-chromosome–dependent neuronal migration phenotypes

DCX mutation

Microtubule-linked migration defect

Abnormal cortical neuronal position

Schematic: Conceptual sequence

Slide 42 · DCX: Phenotype with a single X chromosome

DCX mutation with lissencephaly in individuals having a single X chromosome

This phenotype belongs to the migration-defect spectrum

It differs from the bandlike heterotopia seen in many individuals with two X chromosomes

DCX variant

Single X chromosome

Lissencephaly

Schematic: Conceptual sequence

Slide 43 · DCX: Phenotype with two X chromosomes

DCX mutation with subcortical bandlike heterotopia in individuals with two X chromosomes

An extra band of gray matter appears beneath the ordinary cortex

The pattern is described as a double cortex

Cortical destination

Ventricle / germinal surface

Ectopic neurons

Migration / heterotopia schematic

Slide 44 · Why a double cortex is not a second hemisphere

The phrase describes two vertically separated bands of cortical gray matter

The deeper band is formed by heterotopic neurons

It does not imply duplicated cerebral hemispheres or normal cortical anatomy

Depth

Anatomic element

Superficial

Cortical gray matter

Intermediate

White matter

Deeper

Heterotopic gray band

Slide 45 · FLNA and DCX produce distinct migration phenotypes

FLNA is linked to periventricular nodular heterotopia

DCX is linked to lissencephaly or subcortical band heterotopia depending on X-chromosome context

The protein roles differ: Actin binding versus microtubule association

Feature

FLNA

DCX

Protein

Filamin A

Doublecortin

Cytoskeletal link

Actin

Microtubules

Phenotype example

Periventricular nodules

Lissencephaly / band

Slide 46 · Holoprosencephaly: Incomplete separation

The defining abnormality is incomplete midline separation of the cerebral hemispheres

It represents a spectrum rather than a single uniform gross pattern

Severity may be reflected in associated craniofacial abnormalities

Normal separation

Separated

Incomplete separation

Midline continuity

Conceptual dorsal shapes; Not clinical imaging

Slide 47 · The midline is a developmental boundary

Normal hemispheres develop as distinct left and right compartments

Holoprosencephaly disrupts the formation of this separation

The principal diagnostic question is extent of hemispheric nonseparation

Separated

Separated

Partly united

Midline continuity

Severely united

Midline continuity

Conceptual dorsal shapes; Not clinical imaging

Slide 48 · Severe forms may have major facial anomalies

Midline facial abnormalities with severe holoprosencephaly

Cyclopia is an extreme example mentioned in the source

Facial findings are associated clues, not a substitute for defining forebrain anatomy

Severe midline patterning defect

Forebrain nonseparation

Midline facial anomaly

Schematic: Conceptual sequence

Slide 49 · Arrhinencephaly illustrates milder variants

Arrhinencephaly for less severe variants of this spectrum

The olfactory cranial nerves and associated structures may be absent

The key morphological clue here is loss of olfactory-related anatomy

Severe spectrum

Less severe example

Marked nonseparation

Arrhinencephaly

May include cyclopia

Olfactory system absent

Slide 50 · Trisomy 13 is a documented association

Holoprosencephaly can accompany trisomy 13

The excerpt also mentions other genetic syndromes

A syndromic association does not mean all cases share the same karyotype

Chromosomal or syndrome context

Altered forebrain development

Holoprosencephaly

Schematic: Conceptual sequence

Slide 51 · Sonic hedgehog signaling is implicated

Mutations in genes encoding sonic hedgehog pathway components may cause holoprosencephaly

This links molecular patterning to the anatomy of hemispheric division

The supplied source does not provide a complete gene-level testing algorithm

SHH pathway alteration

Midline patterning disruption

Hemispheric nonseparation

Schematic: Conceptual sequence

Slide 52 · Severe cases may be recognized prenatally

Ultrasonography can identify severe holoprosencephaly in utero

Fetal MRI is another imaging modality

The excerpt gives no sensitivity estimates or gestational-age thresholds

Prenatal ultrasound

Fetal MRI if used

Anatomic assessment

Schematic: Conceptual sequence

Slide 53 · Holoprosencephaly is not callosal agenesis

Holoprosencephaly concerns incomplete separation of two hemispheres

Callosal agenesis concerns absence of interhemispheric commissural white matter

These entities have fundamentally different anatomic definitions

Malformation

Primary structural defect

Holoprosencephaly

Failure of hemispheric separation

Callosal agenesis

Absence of callosal white matter

Slide 54 · The corpus callosum links both hemispheres

The corpus callosum consists of white matter commissural bundles

Its fibers carry cortical projections from one hemisphere to the other

The normal structure presupposes distinct cerebral hemispheres

Left hemisphere

Callosal fibers

Callosal bridge

Callosal fibers

Schematic commissural connection

Slide 55 · Agenesis means absence of those bundles

Agenesis of the corpus callosum is absence of normal interhemispheric callosal fibers

It is described as a relatively common forebrain malformation

The anatomic definition is not dependent on a particular cognitive phenotype

Normal commissure

Callosal fibers

Agenesis

No callosal bridge

Schematic commissural connection

Slide 56 · Lateral ventricular morphology can provide a clue

Misshapen lateral ventricles on radiologic studies

The stated imaging appearance is a batwing deformity

The ventricular pattern supports recognition but should be correlated with callosal anatomy

Callosal agenesis

Altered lateral ventricles

Batwing deformity on imaging

Schematic: Conceptual sequence

Slide 57 · Clinical expression may vary widely

Callosal agenesis can occur in healthy individuals

In other cases, it may accompany intellectual disability

Imaging morphology alone does not establish a uniform developmental prognosis

Apparently healthy

Associated disability

Phenotype spectrum / alternatives (schematic)

Slide 58 · Callosal agenesis can be sporadic or familial

The supplied source describes both sporadic and familial occurrence

Genetic context varies across affected individuals

No particular Mendelian inheritance frequency is specified

Occurrence

Source-defined possibilities

Family history

Sporadic or familial

Additional anomalies

Isolated or associated

Slide 59 · Isolated and syndromic patterns differ

Some patients have callosal agenesis without another recognized anomaly

Others have additional malformations

Associated anatomy is essential context when interpreting clinical significance

Callosal agenesis

Isolated form / Associated anomalies

Context-dependent interpretation

Schematic: Conceptual sequence

Slide 60 · Map morphology to developmental process

Small volume points toward a disturbance in neuronal population size

Reduced gyration and abnormal folding point toward cortical construction defects

Ectopic neuronal bands or nodules point toward migration abnormalities

Unseparated hemispheres and absent commissure are distinct midline-related defects

Observed feature

First developmental question

Small brain

Were adequate neurons produced/recruited?

Abnormal folds

Was cortical construction organized?

Ectopic neurons

Did neurons migrate correctly?

Midline defect

Did division or commissure form?

Slide 61 · Do not interchange the surface phenotypes

Lissencephaly features abnormally few gyri

Polymicrogyria features many small and malformed gyri

Heterotopia describes ectopic neurons and need not be named by surface folding

Lissencephaly

Surface profile

Polymicrogyria

Surface profile

Heterotopia ≠ surface term

Surface profile

Schematic profiles—not microscopic sections

Slide 62 · A mechanistic classification sharpens the differential

Precursor proliferation and cell-cycle exit influence neuronal number

Motility and stopping cues influence neuronal position and cortical architecture

Midline patterning and commissural development govern different gross structures

Neuronal number

Migration + stop cues

Midline / connections

Schematic: Conceptual sequence

Slide 63 · Genes are examples, not one-to-one labels

FLNA and DCX illustrate distinct cytoskeletal routes to migration anomalies

SHH pathway components illustrate a midline patterning mechanism

Phenotype is necessary context when evaluating a proposed molecular cause

FLNA → nodules

DCX → band / lissencephaly

SHH → midline

Classification schematic

Slide 64 · Morphology exercise: A smooth cortex

Observed: Markedly reduced cortical convolutions

Classify the primary morphologic dimension before suggesting a gene

Ask whether the surface is smoothly reduced or cobblestone-like

Smooth cortex

Surface profile

Few folds

Surface profile

Schematic profiles—not microscopic sections

Slide 65 · Morphology exercise: Many shallow sulci

Observed: Numerous small irregular cerebral convolutions

The corresponding Robbins entity is polymicrogyria

Histologic context: Simplified cortical lamination and fused molecular layers

Small folds

Surface profile

Shallow sulci

Surface profile

Schematic profiles—not microscopic sections

Slide 66 · Migration exercise: Nodules beside ventricles

Observed: Subcortical neuronal collections along the ventricular surface

This corresponds to periventricular nodular heterotopia

FLNA illustrates a relevant actin-associated migration pathway

Cortical destination

Ventricle / germinal surface

Ectopic neurons

Migration / heterotopia schematic

Slide 67 · Midline exercise: Unseparated hemispheres

Observed: The cerebral hemispheres remain joined across the midline

This is the defining feature of holoprosencephaly

Trisomy 13 and sonic hedgehog pathway changes are documented associations

Separated (normal)

Separated

Unseparated

Midline continuity

Conceptual dorsal shapes; Not clinical imaging

Slide 68 · Imaging exercise: Absent commissural bridge

Observed: Absence of callosal white matter bundles

Lateral ventricles may show the characteristic batwing deformity

Interpretation must account for possible isolated or associated malformations

Normal callosal bridge

Callosal fibers

Agenesis

No callosal bridge

Schematic commissural connection

Slide 69 · Distinct entities; Distinct anatomic questions

Microencephaly: Is the brain abnormally small?

Lissencephaly / polymicrogyria: How is the cerebral surface organized?

Heterotopia: Are neurons located where they belong?

Holoprosencephaly / callosal agenesis: Are midline structures properly formed?

Volume

Folding

Location

Midline

Classification schematic

Slide 70 · References and source boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th ed. (2026), Chapter 28, pp. 1134–1135

Section: Forebrain Anomalies; nearby material used only to establish topic boundaries

Mechanism diagrams are original editable teaching schematics, not clinical images

No source figures or authentic MRI scans have been reproduced

Scope: source-based teaching

No independent diagnostic thresholds or outcome rates are asserted