The brain develops from the neural tube. Before even fully developing at the posterior, the neural tube (initially straight) begins to form bulges at the anterior that will give rise to the brain. First, three primary vesicles are formed; starting at the most anterior, these are the forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon). Many transcription factors confer identity upon the mammalian brain, including Hox genes.
The anterior of the rhombencephalon is the metencephalon, which gives rise to the cerebellum and pons; the posterior is the myelencephalon, which gives rise to the medulla oblongata. Unlike the rest of the brain, the rhombencephalon segments into totally separate and morphologically distinct rhombomeres. There are seven rhombomeres, and also seven Hox genes whose regions of expression end at the anterior end of each rhombomere.
The rhombencephalon of a knockout for any of these seven Hox genes develops normally anterior to the range of expression of that Hox gene; however, rhombomeres within the range of expression of that Hox gene develop abnormally. This suggests that Hox genes play a role in rhombomere identity, just as they control vertebral identity in the trunk.
|Mesencephalon||The mesencephalon does not subdivide.|
The prosencephalon subdivides to form the telencephalon (at the anterior) and the diencephalon (at the posterior). The telencephacon gives rise to the cerebral hemispheres; the diencephalon evaginates to form much of the eye in addition to the thalamic and hypothalamic brain regions. Subdivision of the prosencephalon is managed by spatially localized expression of transcription factors.
However, the mechanism of subdivision is still vague due to difficulty knocking out relevant genes, and because the segments are not clearly distinct. Outgrowing neurons mark off groups of cells that do not intermix, suggesting there are three or four subdivision (neuromeres or prosomeres) in the telencephalon and diencephalon. There are at least 25 homeobox genes expressed in restricted regions of the forebrain, also implying that the forebrain is divided into functional units. These genes (ie, Emx, Otx and Dlx) often have overlapping expression regions and are homologous to genes (ie, ems, otd, Dll) expressed in the embryonic fly head semental patterns.