Brain Growth & Induction

The mouse is a strange organism and is not what most puppeteers would consider to be normal.  This organism has only one head which holds its brain and both of its eyes.  How does this strange and simple animal’s brain develop?  After months of research we have pinpointed the major differences and now understand why the mouse’s brain is located in its head instead of its trunk.

As far as the development of the brain, the mouse and puppeteer are almost identical.  The neural plate forms and begins to fold upwards.  When the two sides of the folding epidermis meet, the neural tube is formed.  After the neural tube closes there is a build up of fluid in the anterior most section of the neural tube which causes a large lumen to form.  This fluid pressure causes increased mitosis in the brain region.  The blockage of fluid is key in both mouse and puppeteer brain development.

The central nervous system (CNS) develops differentially along both the anterior/posterior axis, and the dorsal/ventral axis.  The dorsal/ventral axis is induced by a TGF-beta gradient supplied by the ectoderm dorsal to the neural tube and Sonic Hedge Hog (SHH) is supplied by the notochord ventral to the neural tube.  Meanwhile, differential development along the anterior/posterior axis is controlled by Hox gene expression. 

The combination of Hox gene expression and ectodermal/notochord interactions cause the neural tube to development into several different layers and regions.  The neural tube differentiates into the forebrain, mid-brain, and hind brain.  These sections further differentiate into the telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon, and the spinal cord, respectively.

                              Mouse Brain Development                                                       Puppeteer Brain Development

The main difference between the mouse and puppeteer brain formation is the placement of the brain.  The mouse’s brain forms at the anterior most portion of the neural tube where as the puppeteer’s brain is located at the base of the neck.  In the mouse one fluid block is necessary at the base of the brain.  In the puppeteer three fluid blocks are necessary.  One blockage is located at the base of the brain and the other two blockages are located where the two anterior neural tubes meet the diencephalon.  After Henson's node migrates, and the neural tube is formed, the diencephalon then stimulates optic vesicle formation, and thus mitosis of the underlying cells.  This induction by the diencephalon is not present in the mouse and is the cause for only one head encasing the brain.  

Copp AJ, Greene NDE and Murdoch JN (2003). The genetic basis of mammalian neurulation. Nature, 4, pp. 784-793.

Sources:

Gilbert, Scott F.  Developmental Biology.  7^th ed.  Sunderland: Sinauer Associates, Inc., 2003. p. 364-368