Brain cells have been shown to rejuvinate. What kills off a lot of them is chronic stress. What is interesting is that SSRIs, like Prozac, can cause the nucleus accumbens to grow, and along with that, your memory improves. So, as you think better, your depression might ease.
Funny, it was the stress that casued the brain shrinkage in the first place, like in PTSD, and that the treatment makes a part of the brain grow back.
But as far as your question: stress takes its toll in terms of brain shrinkage all the way back in the womb. Women under extreme stress have children with smaller, dysfunctional brains. The stress hormones in the mother's blood restructure the growing brain to handle stress when the child is born.