Below is a great picture that shows the amazing nature of the amniotic sac. Take a look and I'll explain afterwards.
The amniotic sac is made of two individual membranes connected by friction; the amnion and the chorion. In this picture, you can't tell which is which, but you can see that one of the two has slid down, some air even getting between the two, making that bubble.
Each membrane has a function. The amnion is closest to the baby and is responsible for making the amniotic fluid (easy to remember that the amniotic fluid is around the baby, so the amnion is next to the baby). The chorion, among other things, helps change the mom's nutrition into a form the baby can absorb via his blood. When there is chorionic villi sampling early in pregnancy, it is the chorion they are taking the sample from.
While I knew hospitals "donated" (sold) placentas to a variety of places from cosmetic companies to (rumored) dog food makers, I hadn't ever considered the medical use of this amazing part of the birthing process. Until Sarah got ocular lymphoma - eye cancer.
Quick aside is that when they removed the cancer, they had to use something to cover the open wound in her eye. The membrane was also going to act as scaffolding so the eye cells could grow across and meet each other, the 40 very tiny sutures falling out, thereby losing the membrane. The whole piece removed was smaller than the eraser on a pencil. When we were sitting with the surgeons, they were talking around us, thinking we didn't know what they were saying - even though I'd told them more than once I was a midwife. "Blah blah blah... a graft will go over the wound... blah blah blah." Our ears perked up and asked what the graft was going to be made of. Monotone, they said it would be one of two things, either the foreskin of a newborn's penis.... WHAT?!? We were horrified. Although... Sarah had to make a joke about it saying she couldn't possibly use the foreskin, then she'd be cock-eyed. We laughed our heads off. They were stone-faced. She then said she wouldn't allow the foreskin to be put in her eye... what was the other choice.
Amnion. I perked up and said, "Amnion? Like amnion and chorion?" And they said yes, that they only use the amnion (not sure why they only use that side). Joking, I told them I could bring in many samples, we could try and match the best one to the color of her eyes. They didn't think it was funny and droned on about pathology and sterility, blah blah. We thought it was funny.
The day of surgery, we gave thanks to the mother and baby for their gift of the amnion that would protect Sarah's eye from the surgery insult.
I am always amazed by the amniotic membranes.