Behind the Brain

All of our nerve cells stem from the fetus. In fact, every cell stems from a special type of cell in the fetus called a stem cell. So how does a astrocyte become and astrocyte, while a neuron becomes a neuron? If all nerve cells originate from one stem cell, how do they perform different functions and become different types of cells? The answer is Sonic Hedgehog

not this Sonic

Scientifically speaking, Sonic Hedgehog is a signalling molecule/protein released by the mesoderm, the middle membrane of the fetus. Specifically, Sonic hedgehog is released by mesodermal tissue below the developing spinal cord. The amount of Sonic Hedgehog a neuron receives determines what type of nerve cell it becomes. Typically, higher amounts of Sonic Hedgehog result in glial cells, lower amounts result in motor/sensory neurons, and an even lower amount of Sonic Hedgehog results in interneurons. This sort of organization allows glial cells to form before neurons, thus letting glial cells provide a scaffolding that the neurons can grow off of.

Naming

The Sonic Hedgehog signaling molecule’s name has a interesting origin. When researchers were initially investigating fruit flies, they discovered a gene and called it hh. When this gene’s function was lost, the embryos of the fruit flies would be covered in pointy projections that looked very similar to the spikes of a hedgehog.

When trying to find a similar hh gene in vertebrates, scientists found 3 homologous genes. The first two were called desert hedgehog, and Indian hedgehog, named after two different species of hedgehogs. The last one was named by Robert Riddle, who came up with the name sonic hedgehog after reading a magazine with a advertisement for the game “Sonic the Hedgehog”. Therefore, Sonic Hedgehog became the name for the gene, and the protein (the signalling molecule) it produced.

Other Functions of Sonic Hedgehog

1. Lung Development: Sonic Hedgehog is very active throughout the endoderm, the inner layer of the fetus. This is where embryonic lungs are developing. This, coupled with the fact that mice without a deficiency of Sonic Hedgehog have been proven to have poor lung development, suggests that Sonic Hedgehog plays a role in lung branching.
2. Tooth Development: In early tooth development, Sonic Hedgehog is released from the signaling center of the tooth, AKA enamel knot. It then gives information about the position of the tooth an regulates tooth cusp growth. Furthermore, when the secondary enamel knots form, they release Sonic Hedgehog along with other signaling molecules to pattern the crown of the tooth during mineralization. On the flip side Sonic Hedgehog also activates Gli2 and Gli3 molecules, which do result in abnormal incisor growth, and smaller molars. So Sonic Hedgehog does help tooth development, but does have some negative side effects as well.
3. Hair Cell Regeneration: Sonic Hedgehog also plays a role in hair cell regeneration. Sonic Hedgehog inhibits the activity of Retinoblastoma, a protein which suppresses mitosis. Thus Sonic Hedgehog helps protect hair cells from Retinoblastoma, thus letting our hair regrow.