U01.12.002 Regional specification of developing brain

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The embryonic neural tube forms the central nervous system (CNS).
Through regionalization, it differentiates into five main brain vesicles that give rise to the structures of the adult brain and spinal cord.

Primary Brain Vesicles

Primary Vesicle	Region Derived	Future Structure
Prosencephalon (Forebrain)	Rostral end	Telencephalon and Diencephalon
Mesencephalon (Midbrain)	Middle portion	Mesencephalon (remains undivided)
Rhombencephalon (Hindbrain)	Caudal to midbrain	Metencephalon and Myelencephalon

Secondary Brain Vesicles and Their Adult Derivatives

Secondary Vesicle	Adult Brain Structure(s)	Ventricular Derivative
Telencephalon	Cerebral hemispheres	Lateral ventricles
Diencephalon	Thalamus, Hypothalamus	Third ventricle
Mesencephalon	Midbrain	Cerebral aqueduct
Metencephalon	Pons and Cerebellum	Upper part of the fourth ventricle
Myelencephalon	Medulla oblongata	Lower part of the fourth ventricle
Spinal Cord	Spinal cord proper	Central canal

Key Points to Remember

The neural tube forms the entire CNS; the neural crest forms the PNS.
The telencephalon develops first, followed by the diencephalon.
The rest — mesencephalon, metencephalon, and myelencephalon — follow in alphabetical order.
The cavities of each vesicle become the ventricular system of the brain.
Defects in regionalization can lead to congenital anomalies such as holoprosencephaly and anencephaly.

Learning Objective

By the end of this topic, students should be able to:

Identify and describe the five secondary brain vesicles, their adult derivatives, and corresponding ventricular structures, and apply this knowledge clinically to understand neural developmental defects.