Secondary Brain Vesicles

The secondary brain vesicles are five embryonic subdivisions of the developing brain that arise from the three primary brain vesicles. They are the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

These vesicles become recognizable during approximately the fifth week of embryonic development. They provide the basic developmental framework from which the major anatomical divisions of the adult brain arise. Their walls differentiate into neural tissue, while their internal cavities contribute to the ventricular system.

The telencephalon and diencephalon develop from the prosencephalon. The mesencephalon remains undivided. The metencephalon and myelencephalon develop from the rhombencephalon. Through extensive growth, differentiation, migration, and folding, these relatively simple embryonic regions ultimately produce the complex organization of the mature brain.

Formation of the Secondary Brain Vesicles

Development of the secondary brain vesicles begins after formation of the neural tube and establishment of the three primary brain vesicles.

The cranial neural tube initially becomes organized into the prosencephalon, mesencephalon, and rhombencephalon. Further regional growth and subdivision transform these three regions into five secondary vesicles.

The prosencephalon divides into two regions, the rhombencephalon divides into two regions, and the mesencephalon retains its original name.

Primary to Secondary Brain Vesicles

Primary Brain Vesicle	Secondary Brain Vesicle
Prosencephalon	Telencephalon
	Diencephalon
Mesencephalon	Mesencephalon
Rhombencephalon	Metencephalon
	Myelencephalon

Telencephalon

The telencephalon is the most rostral secondary brain vesicle and develops from the prosencephalon.

It undergoes enormous expansion during fetal development and eventually forms the paired cerebral hemispheres. Its derivatives constitute much of the visible surface and internal structure of the adult forebrain.

The telencephalon develops paired lateral outgrowths known as the telencephalic vesicles, which expand around the more centrally positioned diencephalon.

Major Telencephalic Derivatives

The telencephalon gives rise to numerous structures of the cerebral hemispheres.

• Cerebral cortex
• Cerebral white matter
• Basal nuclei
• Hippocampal formation
• Amygdaloid complex
• Olfactory structures
• Commissural pathways connecting the cerebral hemispheres

The telencephalon therefore produces structures involved in sensory perception, voluntary movement, cognition, memory, language, emotion, and many other higher nervous system functions.

Development of the Cerebral Hemispheres

The paired telencephalic vesicles grow rapidly in dorsal, lateral, anterior, and posterior directions.

As they enlarge, the cerebral hemispheres progressively cover portions of the diencephalon and brainstem when viewed externally.

This extensive growth also produces the curved configuration of several internal structures, including portions of the ventricular system and limbic structures.

Cerebral Cortex

The cerebral cortex develops from the superficial walls of the expanding telencephalon.

Neurons generated in proliferative zones migrate toward the developing cortical surface and become organized into cortical layers.

Continued expansion eventually produces the characteristic gyri and sulci that greatly increase the cortical surface area.

Basal Nuclei

Deep regions of the telencephalon contribute to development of the basal nuclei.

These neuronal masses become embedded within the cerebral hemispheres and develop extensive connections with the cerebral cortex, thalamus, brainstem, and other structures.

Their mature circuits participate particularly in motor control as well as cognitive and behavioral functions.

Lateral Ventricles

The cavities within the paired telencephalic vesicles become the lateral ventricles.

As the cerebral hemispheres enlarge and change shape, the ventricles acquire their characteristic curved configuration.

Each lateral ventricle communicates with the third ventricle through an interventricular foramen.

Diencephalon

The diencephalon develops from the central portion of the prosencephalon.

Unlike the rapidly expanding telencephalon, it remains positioned near the midline and becomes largely surrounded by the cerebral hemispheres.

The diencephalon forms several major structures around the third ventricle.

Major Diencephalic Derivatives

• Thalamus
• Hypothalamus
• Epithalamus
• Subthalamic region
• Retina and associated optic structures

These derivatives contribute to sensory integration, autonomic regulation, endocrine control, circadian functions, motor circuitry, and visual development.

Thalamus

The thalamus develops as paired enlargements within the lateral walls of the diencephalon.

The two thalami become major components of the adult diencephalon and lie on either side of the third ventricle.

They form an important interface between numerous subcortical pathways and the cerebral cortex.

Hypothalamus

The hypothalamus develops from ventral portions of the diencephalon.

It contributes to the walls and floor of the third ventricle and becomes closely associated with the pituitary region.

The mature hypothalamus participates in autonomic, endocrine, behavioral, and homeostatic regulation.

Epithalamus

The epithalamus develops from dorsal and posterior portions of the diencephalon.

Its derivatives include the pineal gland and habenular structures.

These structures remain anatomically associated with the posterior region of the third ventricle.

Optic Vesicles and Retina

Paired optic vesicles arise as lateral outgrowths of the developing forebrain, particularly the diencephalic region.

They contribute to development of the optic cups and neural components of the eyes.

The neural retina is therefore derived from neuroectoderm associated with the developing brain rather than from surface ectoderm.

Third Ventricle

The cavity of the diencephalon becomes the third ventricle.

This narrow midline cavity communicates with the lateral ventricles through the interventricular foramina.

Caudally, it communicates with the fourth ventricle through the cerebral aqueduct.

Mesencephalon

The mesencephalon is unique among the primary brain vesicles because it does not subdivide into a differently named secondary vesicle.

It remains the mesencephalon and develops into the midbrain.

Although relatively small compared with the cerebral hemispheres, the mature midbrain contains important sensory, motor, autonomic, and integrative structures.

Dorsal Mesencephalon

The dorsal region of the mesencephalon develops into the tectum.

The tectum becomes organized into paired superior and inferior colliculi.

These structures participate in visual and auditory reflexes and orientation toward sensory stimuli.

Ventral Mesencephalon

Ventral and central portions of the mesencephalon contribute to the tegmentum and cerebral peduncular regions.

The developing midbrain contains numerous neuronal populations and fiber pathways.

Adult structures include the red nucleus, substantia nigra, periaqueductal gray, cranial nerve nuclei, and major ascending and descending tracts.

Cerebral Aqueduct

The cavity of the mesencephalon becomes markedly narrowed during development.

It persists as the cerebral aqueduct, which connects the third and fourth ventricles.

The narrow diameter of the aqueduct makes it an important potential site of cerebrospinal fluid obstruction.

Metencephalon

The metencephalon develops from the rostral portion of the rhombencephalon.

Its two principal adult derivatives are the pons and cerebellum.

These structures develop through extensive differentiation of ventral and dorsal hindbrain regions.

Development of the Pons

The pons develops within the ventral metencephalon.

Its mature structure contains pontine nuclei, cranial nerve nuclei, ascending and descending pathways, and extensive connections with the cerebellum.

Transverse pontocerebellar fibers become a prominent feature of its ventral anatomy.

Development of the Cerebellum

The cerebellum develops predominantly from specialized dorsal regions of the metencephalon associated with the rhombic lips.

These regions enlarge and extend toward the midline, eventually contributing to formation of the cerebellar hemispheres and vermis.

Subsequent proliferation, migration, and differentiation produce the cerebellar cortex and deep cerebellar nuclei.

Metencephalic Cavity

The cavity associated with the metencephalon contributes to the rostral portion of the fourth ventricle.

Expansion of this cavity profoundly alters the arrangement of the dorsal hindbrain.

This process helps establish the characteristic organization of sensory and motor territories within the brainstem.

Myelencephalon

The myelencephalon is the most caudal secondary brain vesicle.

It develops from the caudal portion of the rhombencephalon and gives rise primarily to the medulla oblongata.

The medulla forms the transition between the pons and spinal cord.

Development of the Medulla Oblongata

The walls of the myelencephalon differentiate into the complex neuronal and fiber architecture of the medulla.

The medulla contains cranial nerve nuclei, reticular formation, ascending sensory pathways, descending motor pathways, and nuclei involved in essential autonomic functions.

Its organization reflects modifications of the basic neural tube pattern caused by expansion of the fourth ventricle.

Myelencephalic Cavity

The cavity of the rostral myelencephalon contributes to the caudal portion of the fourth ventricle.

More caudally, the cavity narrows and becomes continuous with the central canal of the spinal cord.

This continuity reflects the common origin of the brain and spinal cord from the neural tube.

Five Secondary Brain Vesicles and Their Adult Derivatives

Secondary Vesicle	Major Adult Derivatives
Telencephalon	Cerebral hemispheres, cerebral cortex, cerebral white matter, basal nuclei and associated structures
Diencephalon	Thalamus, hypothalamus, epithalamus, subthalamic region and associated optic structures
Mesencephalon	Midbrain
Metencephalon	Pons and cerebellum
Myelencephalon	Medulla oblongata

Ventricular Derivatives

The cavities within the secondary brain vesicles remain continuous and form the ventricular system of the mature brain.

Secondary Vesicle	Ventricular Derivative
Telencephalon	Lateral ventricles
Diencephalon	Third ventricle
Mesencephalon	Cerebral aqueduct
Metencephalon	Rostral portion of fourth ventricle
Myelencephalon	Caudal portion of fourth ventricle, continuing toward the central canal

Development of the Ventricular System

The ventricular system represents the persistent lumen of the embryonic neural tube.

Expansion of the cerebral hemispheres produces the lateral ventricles, while thickening of the diencephalic walls leaves a relatively narrow third ventricle. The mesencephalic lumen becomes the narrow cerebral aqueduct, and the expanded hindbrain cavity forms the fourth ventricle.

These cavities remain connected and later contain circulating cerebrospinal fluid.

Brain Flexures

The secondary brain vesicles develop within a neural tube that is undergoing substantial bending.

Three important flexures help establish the mature spatial relationships of the developing brain: the cephalic flexure, cervical flexure, and pontine flexure.

These bends are produced by differential growth and are essential for understanding why adult brain regions are not arranged along a simple straight axis.

Cephalic Flexure

The cephalic flexure develops in the region of the mesencephalon.

It bends the cranial neural tube ventrally and contributes to the angle between the forebrain and brainstem.

Unlike some other embryonic flexures, much of this curvature remains evident in the mature brain.

Cervical Flexure

The cervical flexure forms near the junction between the hindbrain and spinal cord.

It creates an early bend between the developing brain and spinal neural tube.

The cervical flexure becomes less prominent later in development.

Pontine Flexure

The pontine flexure forms within the developing rhombencephalon.

It bends in the opposite direction to the cephalic and cervical flexures and contributes to expansion of the fourth ventricular region.

This flexure has major consequences for the arrangement of developing sensory and motor regions in the brainstem.

Alar and Basal Plates

The embryonic brainstem retains the basic organization of the neural tube into alar and basal plates.

Alar plate derivatives are predominantly associated with sensory functions, while basal plate derivatives are predominantly associated with motor functions.

The opening of the roof of the hindbrain around the fourth ventricle changes their relative positions compared with the spinal cord.

Reorganization in the Hindbrain

As the fourth ventricle expands, the dorsal walls of the metencephalon and myelencephalon spread laterally.

This moves alar plate derivatives laterally while basal plate derivatives remain relatively medial.

This developmental rearrangement helps explain the general organization of cranial nerve nuclei in the adult brainstem, with many motor nuclei positioned medially and sensory nuclei positioned more laterally.

Rhombic Lips

The rhombic lips are specialized dorsal regions associated with the alar plates of the developing hindbrain.

They are particularly important in formation of the cerebellum and several neuronal populations that migrate within the developing brainstem.

Their extensive proliferation illustrates the complexity of regional differentiation within the metencephalon.

Rhombomeres

The embryonic hindbrain is transiently divided into segmental units known as rhombomeres.

These segments possess distinct patterns of gene expression and developmental identity.

Rhombomeric organization contributes to patterning of cranial nerve nuclei, neural crest populations, and other structures derived from the hindbrain.

Regional Patterning

The identities of the secondary brain vesicles are established through coordinated molecular patterning along the cranial-caudal axis of the neural tube.

Different signaling environments and patterns of gene expression establish boundaries between developing brain territories.

These regional identities guide later neuronal differentiation, migration, axonal growth, and anatomical organization.

Forebrain Patterning

Patterning of the prosencephalic derivatives establishes distinct telencephalic and diencephalic territories.

Further regionalization within these areas produces the developing cerebral cortex, basal nuclei, thalamic regions, hypothalamus, and other forebrain structures.

Proper early patterning is essential because disturbances can affect multiple anatomically related structures.

Midbrain-Hindbrain Boundary

The junction between the mesencephalon and metencephalon contains an important developmental organizing region commonly called the midbrain-hindbrain boundary.

Signals generated near this boundary influence the development and identity of adjacent midbrain and anterior hindbrain tissues.

It is particularly important for normal development of the midbrain and cerebellar region.

Developmental Timeline

Approximate Period	Developmental Event
Third week	Neural plate forms and neurulation begins
Late third to fourth week	Neural tube forms and closes
Fourth week	Prosencephalon, mesencephalon and rhombencephalon become recognizable
Fifth week	Five secondary brain vesicles become established
Subsequent fetal development	Secondary vesicles undergo extensive growth, differentiation and regional specialization

Developmental Sequence

1. The neural tube forms from neuroectoderm.
2. The cranial neural tube expands.
3. Three primary brain vesicles become recognizable.
4. The prosencephalon divides into telencephalon and diencephalon.
5. The mesencephalon remains undivided.
6. The rhombencephalon divides into metencephalon and myelencephalon.
7. The five secondary brain vesicles undergo regional differentiation.
8. The telencephalon expands into the cerebral hemispheres.
9. The diencephalon forms central forebrain structures.
10. The mesencephalon forms the midbrain.
11. The metencephalon forms the pons and cerebellum.
12. The myelencephalon forms the medulla oblongata.
13. The vesicular lumina develop into the ventricular system.

Clinical Significance

Developmental Brain Malformations

Abnormal subdivision or patterning of the secondary brain vesicles can produce congenital abnormalities affecting one or several related brain structures.

The resulting anatomy depends on the developmental stage, embryonic territory, and molecular processes involved.

Knowledge of vesicular derivatives can therefore help explain why particular congenital abnormalities involve characteristic combinations of brain regions.

Holoprosencephaly

Holoprosencephaly is characterized by incomplete separation of the developing forebrain.

It particularly affects structures derived from the prosencephalon and can interfere with normal formation and separation of the cerebral hemispheres.

The anatomical severity varies widely and may be accompanied by craniofacial abnormalities.

Aqueductal Stenosis

The cerebral aqueduct is derived from the lumen of the mesencephalon and is much narrower than the major ventricles.

Congenital narrowing or obstruction can impair cerebrospinal fluid passage from the third ventricle to the fourth ventricle.

This can result in enlargement of the lateral and third ventricles and produce obstructive hydrocephalus.

Hindbrain Malformations

Developmental abnormalities affecting the metencephalon or myelencephalon can involve the cerebellum, pons, medulla, fourth ventricle, or combinations of these structures.

Because posterior fossa structures share closely related developmental origins, abnormalities may involve multiple anatomical components.

Embryological relationships are therefore important when interpreting congenital posterior fossa abnormalities.

Secondary Brain Vesicles at a Glance

Secondary Vesicle	Origin	Major Adult Region	Cavity
Telencephalon	Prosencephalon	Cerebral hemispheres	Lateral ventricles
Diencephalon	Prosencephalon	Thalamus, hypothalamus and related structures	Third ventricle
Mesencephalon	Mesencephalon	Midbrain	Cerebral aqueduct
Metencephalon	Rhombencephalon	Pons and cerebellum	Rostral fourth ventricle
Myelencephalon	Rhombencephalon	Medulla oblongata	Caudal fourth ventricle

Key Anatomical Features

Feature	Key Point
Number	Five secondary brain vesicles
Telencephalon origin	Prosencephalon
Diencephalon origin	Prosencephalon
Mesencephalon origin	Mesencephalon
Metencephalon origin	Rhombencephalon
Myelencephalon origin	Rhombencephalon
Major telencephalic derivative	Cerebral hemispheres
Major diencephalic derivatives	Thalamus and hypothalamus
Mesencephalic derivative	Midbrain
Metencephalic derivatives	Pons and cerebellum
Myelencephalic derivative	Medulla oblongata
Approximate appearance	Fifth week of development

Anatomical Importance

The secondary brain vesicles provide one of the most useful developmental frameworks for understanding adult neuroanatomy. Nearly every major brain region can be related to one of these five embryonic subdivisions.

The telencephalon produces the cerebral hemispheres, the diencephalon forms central forebrain structures, the mesencephalon persists as the midbrain, the metencephalon produces the pons and cerebellum, and the myelencephalon develops into the medulla oblongata.

Their internal cavities simultaneously become the lateral ventricles, third ventricle, cerebral aqueduct, and fourth ventricle. Understanding these relationships connects the apparently complex anatomy of the mature brain to the simpler organization of the embryonic neural tube and provides an anatomical basis for interpreting many congenital abnormalities of the central nervous system.