The female gonads are the ovaries. They are responsible for the production of the sex cell, oocyte. During fetal development, the egg cells start to populate the organ. However, many of them undergo atresia and die before a woman reaches the puberty stage. Aside from the egg cell, the ovaries are also responsible for the production of the sex hormones, which are the progesterone and estrogen. They account for the secondary female characteristics, like the enlargement of the breasts, widening of the hips and the monthly menstruation. Inside the organ, you will see numerous follicles. Here are the ovarian follicles at different stages of development. 


Primordial Follicle

The primordial follicles are the most primitive type of ovarian follicle. They house the primary oocyte. A single layer of cells called the granulose cells lines the ovarian follicle. From the time of birth until you reach the adolescent stage, the number declines to become only 400 and continue the oogenesis stage. During puberty, the follicle-stimulating hormone released from the pituitary gland stimulates the maturation of the primordial to become the primary follicle. 

Primary Follicle

During histological examination, the primary follicles are discernible under the microscope. Trained microscopists can easily locate them. The enlarged oocyte and the developed layer of granulosa becoming cuboidal cells characterize the primary follicle. With further development, a clear fluid surrounds the primary oocyte. This fluid is the zona pellucida. 

Secondary Follicle

Some of the primary follicles continue to develop and become the secondary follicle. At this stage, the granulose layer increases and surrounds the oocyte. Vesicles are found scattered in the granulose layer filled with clear fluid. As development proceeds, the follicle creates two discernible capsules called the theca interna and theca externa. 

Mature Graafian Follicle

Once the vesicles in the secondary follicle fuse to become one fluid-rich vesicle, the anthrum develops. The mature follicle houses the secondary oocyte. As fluid accumulates around the anthrum, a crown-like substance surrounds the secondary oocyte, thus given the name corona radiate. Generally, only a single graafian follicle progress to maturation; the rest degenerate once one follicle becomes dominant. During this time, more fluid accumulates greater than what the capsule can take. As a result, the mature follicle ruptures and releases the secondary oocyte. The release of the cell is called the ovulation. The ovulation often occurs on the mid-cycle of menstruation.  

Corpus Luteum

The corpus luteum is the remnant of the mature follicle after ovulation. The cells inside the structure secrete progesterone and a small amount of estrogen. The hormones prepare the uterus in preparation for pregnancy. It is only functional for the next 10-14 days. 

Corpus Albicans

Once the corpus luteum starts to degenerate, it becomes the corpus albicans. This tissue is no longer capable of producing the hormone and, eventually, it will disappear. Due to the loss of hormones maintaining the lining of the uterus, the layer starts to shed off in the form of menstruation. However, if fertilization occurs, the placenta saves the function of the albicans and keeps the integrity of the uterus. 

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