Instructions: In this task, we ask you to elaborate the sentence without changing its general meaning. You can do so by explaining further the input sentence, using more precise wording, adding qualifiers and auxiliary information etc.
Input: In humans , the pigment of the iris varies from light brown to black .
Output:
In humans , the pigmentation of the iris varies from light brown to black , depending on the concentration of melanin in the iris pigment epithelium ( located on the back of the iris ) , the melanin content within the iris stroma ( located at the front of the iris ) , and the cellular density of the stroma .