What are ovarian follicles?

Ovarian follicles are the basic units of female reproductive biology. Each of them contains a single oocyte (egg). We can visualise the follicles through ultrasound imaging. But eggs are very minute structures not visible in naked eye. That is to say, we can monitor the growth of follicles but not eggs. These follicles support the human eggs by feeding them and also consequently release them after maturation.