Brain Regionalization and Vesicle Formation

Brain regionalization is the patterning of the cranial neural tube into distinct territories that become the major divisions of the brain. Soon after closure, the rostral tube dilates into three primary vesicles, the prosencephalon, mesencephalon, and rhombencephalon, which subdivide into five secondary vesicles whose walls give rise to the cerebral hemispheres, diencephalon, midbrain, pons and cerebellum, and medulla.

Definition

Brain regionalization and vesicle formation is the process by which the cranial neural tube acquires distinct anteroposterior and dorsoventral identities and dilates into the three primary brain vesicles (prosencephalon, mesencephalon, rhombencephalon) and subsequently the five secondary vesicles that prefigure the major brain divisions.

Scope

The entry covers the emergence of the primary and secondary brain vesicles, the anteroposterior and dorsoventral patterning that assigns regional identity, the role of local signalling centres such as the isthmic organizer, and the proliferation of neural progenitors that builds the brain wall. It is an educational reference to developmental anatomy, not clinical guidance.

Key concepts

• Primary brain vesicles
• Secondary brain vesicles
• Anteroposterior patterning
• Isthmic (midbrain-hindbrain) organizer
• Rhombomeres and Hox gene expression
• Dorsoventral patterning
• Neural progenitors and the ventricular zone
• Cephalic flexures

Mechanisms

Regional identity along the cranial neural tube is set by graded and localized signals. Signalling centres, including the anterior neural ridge, the zona limitans intrathalamica, and the isthmic organizer at the midbrain-hindbrain boundary, secrete factors that pattern adjacent neuroepithelium and establish boundaries between territories. In the hindbrain, transient segments called rhombomeres form with nested Hox gene expression that confers positional value. Along the dorsoventral axis, opposing signals from the roof and floor regions specify dorsal and ventral cell types. As patterning proceeds the rostral tube balloons into the primary and then secondary vesicles, and proliferating progenitors in the ventricular zone generate the neurons and glia that thicken the vesicle walls, with flexures bending the elongating brain within the head.

Clinical relevance

Disturbances of forebrain patterning are associated with malformations of the brain's midline and divisions, such as holoprosencephaly, in which the prosencephalon fails to separate into paired hemispheres. This entry describes the normal developmental anatomy as background and is not a basis for individual diagnosis or treatment.

History

The three- and five-vesicle description of the developing brain is a long-standing framework of descriptive embryology. The molecular era added the concept of local organizers, most influentially the isthmic organizer at the midbrain-hindbrain boundary, and clarified how segmental gene expression patterns the hindbrain, as synthesised in modern reviews.

Key figures

• Wolfgang Wurst
• Laure Bally-Cuif
• Wieland B. Huttner

Seminal works

• wurst-2001
• florio-2014

Frequently asked questions

What are the three primary brain vesicles?

The prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain); these later subdivide into five secondary vesicles that prefigure the major brain divisions.

What is the isthmic organizer?

It is a localized signalling centre at the midbrain-hindbrain boundary that secretes patterning factors to organize the identity of the adjacent midbrain and anterior hindbrain.