During this stage, segmentation and positional identity are determined
both in the rostral-caudal axis and also in the dorsal-ventral axis.
At the rostral end of the neural tube, 3 brain areas are formed:
the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain).
The prosencephalon further develops into the telencephalon (cortex,
basal ganglia, olfactory system) and diencephalon (thalamus and
hypothalamus) and the rhombencephalon becomes subdivided into the
metencephalon (pons and cerebellum) and myelencephalon (medulla).
The telencephalon evaginates to form the 2 cerebral hemispheres. Each
hemisphere is further divided into subdomains, such as the functional
and anatomical subdivisions of the neocortex (motor, somatosensory,
auditory, visual, etc). Programs of regional identity and morphogenesis
in the telencephalon are directed in part by various molecules secreted
by at least 3 patterning centers. Ventral telencephalon and hypothalamus express
Sonic hedgehog (Shh). Along the dorsal midline, secreted molecules
such as bone morphogenetic protein and Wnt families control patterning
of the hippocampus and neocortex. At the anterior margin of the
telencephalon, Fgf8 and Fgf17 promote telencephalic growth and rostral
regionalization. The patterning centers operate in part through
generating graded expression of the transcription factors that control cellular
programs for proliferation, migration, and connectivity. Several
genes encoding transcription factors, including Foxg1 (BF1), COUPTF1,
Emx2, Lef1, Lhx2, Sp8, and Pax6, are expressed in gradients along
the dorsoventral and rostrocaudal axes of the cerebral cortex.2,3