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An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries.It secretes hormones that influence stages of the menstrual cycle.At the time of puberty, women have approximately 200,000 to 300,000 follicles, [1] [2] each with the potential to release an egg cell (ovum) at ovulation for fertilization. [3]
During ovarian follicle activation, the granulosa cells of the primordial follicles change from a flat to a cuboidal structure, marking the beginning of the primary follicle. The oocyte genome is activated and genes become transcribed .
The outer layer is the ovarian cortex, consisting of ovarian follicles and stroma in between them. Included in the follicles are the cumulus oophorus, membrana granulosa (and the granulosa cells inside it), corona radiata, zona pellucida, and primary oocyte. Theca of follicle, antrum and liquor folliculi are also contained in the follicle.
The rise in FSH levels recruits five to seven tertiary-stage ovarian follicles (this stage follicle is also known as a Graafian follicle or antral follicle) for entry into the menstrual cycle. These follicles, that have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. [5]
A depiction of the ovarian cycle in animals. In human adult females, the primordial follicle is composed of a single oocyte surrounded by a layer of closely associated granulosa cells. In early stages of the ovarian cycle, the developing follicle acquires a layer of connective tissue and associated blood vessels. This covering is called the theca.
Ovarian follicle activation can be defined as primordial follicles in the ovary moving from a quiescent (inactive) to a growing phase. The primordial follicle in the ovary is what makes up the “pool” of follicles that will be induced to enter growth and developmental changes that change them into pre-ovulatory follicles, ready to be released during ovulation.
Ovarian stroma differs from typical connective tissue in that it contains a high number of cells. The stroma cells are distributed in such a way that the tissue appears to be whorled. Stromal cells associated with maturing follicles may acquire endocrine function and secrete estrogens. The entire ovarian stroma is highly vascular. [1]
Ovarian follicle dominance is the process where one or more follicles are selected per cycle to ovulate. In female mammals , each ovulatory cycle , or menstrual cycle in humans , a set number of ovarian follicles ovulate, each follicle releasing an egg that can be fertilised .