Brain Vesicles

Brain Vesicle Formation

Key Points

• The neural tube differentiates into brain vesicles ("-cephalon")
• The earliest subdivisions of the brain have developed by the end of 4 weeks.

3 Primary Brain Vesicles

There are 3 primary brain vesicles:

• Prosencephalon
• Mesencephalon
• Rhombencephalon

5 Secondary Brain Vesicles at 5 Weeks

At 5 weeks, they further develop into the secondary brain vesicles, and there are 5 of them:

• The prosencephalon partitions into the telencephalon and diencephalon.
• The mesencephalon remains as the mesencephalon.
• The rhombencephalon partitions into the metencephalon and myelencephalon.

3 Flexures

There are 3 flexures:

• Cephalic flexure (aka midbrain flexure)
• Pontine flexure
• Cervical flexure

Primary brain vesicles at 4 weeks

Primary Brain Vesicles

From cranial to caudal:

• Prosencephalon
• Mesencephalon
• Rhombencephalon

Points of interest

• The caudal extension of the neural tube later becomes the spinal cord.
• The neural tube runs length of embryo until month 3 when spine growth starts to outpace spinal cord growth.

Secondary brain vesicles at 5 weeks

Secondary Brain Vesicles

From cranial to caudal:

• Telencephalon
• Diencephalon
• Mesencephalon
• Metencephalon
• Myelencephalon

Specifics of development

• The prosencephalon partitions into the telencephalon and diencephalon.
• The mesencephalon remains as the mesencephalon.
• The rhombencephalon partitions into the metencephalon and myelencephalon.

Points of interest

• The pontine flexure within the metencephalon lays the groundwork for the floor of the fourth ventricle (ie, the rhomboid fossa, given its rhomboid shape). The opposite surface of the metencephalon connects as the 4th ventricle roof.
• The optic vesicles lie within the diencephalon.
• Each optic vesicle partitions into the optic cup and the optic stalk.
  • We can remember that the optic vesicle develops from the diencephalon because the optic pathway synapses in the lateral geniculate nuclei along the thalamus.
• The optic cup then becomes the retina.
• And the optic stalk becomes the optic nerve.

The Brain at 6 months

Derivations

• The cerebrum is derived from the telencephalon.
• The thalamic areas are derived from the diencephalon.
• The midbrain is derived from the mesencephalon.
• The pons and cerebellum are derived from the metencephalon.
• The medulla oblongata is derived from the myelencephalon.
• The spinal cord is derived from the neural tube.

Brain Development

• The telencephalon blossoms into the large cerebral hemispheres.
• The diencephalon is buried deep within the brain; it comprises the thalamic areas.
• The brainstem comprises, from superior to inferior: the midbrain, pons, and medulla.
• The cerebellum blossoms from the posterior pons: both are derived from the metencephalon.
• The spinal cord extends from the bottom of the medulla oblongata.