How do eyes become "eyes that see color" before birth? Vitamin A was the key

Explanation

The human retina has three types of "cone cells" that sense red, green, and blue, but it turns out that in the early stages of development, only blue-sensing cells are actually formed. A research team at Johns Hopkins University in the United States found that when a substance derived from vitamin A (retinoic acid) decreases in the fetal retina, new blue cones stop being formed, and subsequently, thyroid hormone activity causes the remaining blue cones to transform into red and green cones. This mechanism may also be useful for regenerative medicine to treat eye diseases.