An atretic follicle is an ovarian follicle that degenerates before coming to maturity. This is a normal process and several of these occur each month in a woman's body.
If atretic follicles are retrieved in preparation for in vitro fertilization (IVF), they cannot be used for fertilization and transfer.
FertilitySmarts explains Atretic Follicle
Female bodies are born with millions of ovarian follicles, sacs that contain eggs, but only around 400 of these will eventually mature and release an egg during ovulation. Most of the follicles will degenerate, or become atretic.
This is a normal process and occurs throughout a woman's life. In fact, each month about 20 follicles mature, but only one becomes dominant and is ovulated. The exact reason for this is unknown.
Atretic Eggs & IVF
When eggs are retrieved for IVF, they are inspected for health and maturity. Sometimes, atretic eggs are collected during the retrieval process. These are essentially dead eggs and cannot be used for fertilization and transfer.