#cyclopia

The top bulb is the prosencephalon, which will later differentiate into the telencephalon and diencephalon. These will become the cerebral hemispheres, basal ganglia, thalamus, hypothalamus, and amygdala. The middle bulb is the mesencephalon, which will become the midbrain. The bottom bulb is the rhombencephalon, which will differentiate into the metencephalon and myelencephalon. These will become the pons, cerebellum, and medulla.

So, this entire thing has to split in half anteriorly. The anterior portion of this structure deals with motor function, and the posterior half deals with sensory function. This holds true as an adult. The front half of your spinal cord carries motor nerves, and the back half carries sensory info. If you want to move your eyes, you use the front part of your brain. If you want to know what you are seeing, you use the back half.

If you look at your spinal cord, the front half has this fissure down the middle (anterior median fissure is such a fitting name, then right??). Your brain is also split down the middle. Since the eyes are part of your brain, you get one for each half of the brain (two). Now, let's say you never have this anterior fissure form... you won't get two eyes, either. The condition where the fissure doesn't form is called holoprosencephaly (holo means whole).

There are five types of this (in order of decreasing severity): alobar, semilobar, lobar, syntelencephaly, and microform.

*note that the more severe the condition is, the more likely the fetus will die and not form all structures mentioned*

Alobar: there will be one eye, a proboscis (fleshy tube on the face), and severe mental impairment.

Semilobar: eyes will be close together, the nasal bridge will be flat, the palate and lip will be cleft, and there will be mental and ocular impairment.

Lobar: the eyes will be close together, the nasal bridge will be flat, the nostrils will be close together, and there will be some mental impairments.

Syntelencephaly: flat nasal bridge, shallow philtrum, and possible mental delays.

Microform: eyes may be close, nasal bridge will be sharp, and there will be one central upper incisor.

Okay, so what does this have to do with Sonic the Hedgehog? Sonic hedgehog (SHH) genes, of course! This gene codes for the sonic hedgehog protein, which controls what happens during organogenesis (how everything differentiates and forms). A mutation in this gene can cause issues with how embryos form, and is indicated specifically in holoprosencephaly. SHH genes are still important in adults, and mutations can be linked to certain cancers.