Embryonic development of the brain
The earliest phase of brain development begins at three weeks in the embryo . The ectoderm, which is the cell layer at the dorsal surface, thickens along the midline axis of the embryo to form the neural plate.
The neural plate then folds inward to form a groove with neural folds on either side. As this
neural groove deepens, the
top-most edge of the fold fuses, forming the neural tube.
The neural tube soon detaches from the surface ectoderm and sinks inward. This neural tube has formed by week four and develops into the central nervous system (CNS).The brain forms from the top portion and the spinal cord forms from the bottom portion of the neural tube which will be discussed in the section "spinal cord". The neural folds move between the surface ectoderm and neural tube forming neural crests which will form most of the peripheral nervous system. Shortly after the formation of the neural tube, the top portion begins to form the three primary brain vesicles: the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain).
During the fifth week, these primary vesicles develop into the secondary brain vesicles. The forebrain divides into two parts: thetelencephalon (giving rise to the cerebrum and the diencephalon. The hindbrain constricts to form the metencephalon (giving rise to the brain stem: pons and the cerebellum) and the myelencephalon (giving rise to the brain stem: medulla oblongata). And, finally, the midbrain remains undivided, giving rise to the brain stem: midbrain. Each of these five secondary vesicles develops very quickly into the major structures of the adult brain. The telencephalon sprouts two lateral swellings that become the two cerebral hemispheres that make up the cerebrum. The diencephalon section specializes to form the hypothalamus, thalamus, epithalamus, and retina of the eye. the mesencephalon forms the midbrain, the metencephalon forms the pons and cerebellum, and the myelencephalon forms the medulla oblongata. All of these midbrain and hindbrain structures, except for the cerebellum, make up the brain stem. The center cavity of the neural tube remains and enlarges in four areas to form ventricles filled with fluid. This will be discussed further in the subsection "ventricles".
The brain grows far more rapidly than the membranous skull enclosing it, therefore, two major flexures develop: the midbrain and cervical flexures. These flexures move the forebrain toward the brain stem. Another result of the limited space is that the cerebral hemispheres form in a horseshoe-shape, growing out to the sides and turning backward. Because of this growth pattern, they almost completely envelop the diencephalon and midbrain. At week twenty-six, they continued growth of the hemispheres has caused them to crease and fold, forming convolutions and increasing their surface area, which allows for more neurons to occupy the very limited space.