Final answer:
A cystic follicle must be about 30 mm in size to be designated as such. These follicles are part of the folliculogenesis process and can develop to the point of ovulation, releasing a secondary oocyte. The follicle then becomes the corpus luteum in the luteal phase.
Step-by-step explanation:
To be called a cystic follicle, the follicle must be about 30 mm in size. During the menstrual cycle, developing follicles in the ovary undergo a process called folliculogenesis, which includes the transition from primordial follicles to primary, secondary, and ultimately tertiary follicles. Tertiary follicles, which are also referred to as antral follicles, possess a large fluid-filled cavity known as an antrum. It is the growth of such an antrum that eventually leads to the designation of a follicle as cystic once it reaches a significant size. While multiple follicles may reach the tertiary stage, most of these will succumb to a process of degeneration known as atresia, with typically only one developing to the point of ovulation, wherein it will release a secondary oocyte surrounded by granulosa cells.
The follicle that housed the ovulated egg transforms into the corpus luteum during the luteal phase of the menstrual cycle. This structure is significant for hormone production and maintaining the endometrial lining for potential implantation of a fertilized egg. If pregnancy does not occur, the corpus luteum degenerates.