Which part of the hair is responsible for its growth?

Premature babies have lanugo hair on their bodies. It disappears as they begin to mature towards their correct due date.

The last sentence contains nonsense. One cannot have "hot and cold temperatures". A temperature is a number on a scale. It cannot be "hot" or "cold". It can be "high" or "low".

Anorexia suffers in the late stages of the condition sometimes grow lanugo hair.