The Ovarian Cycle and Oogenesis

The ovary has an outer layer of cuboidal epithelium that forms the ovarian surface. Deep to this

is the tunica albuginea, which is the dense connective tissue structure that is similar to that in

the male testicle to make the cortex of the ovary. The cortex forms a framework of sorts called

the ovarian stroma, which forms the bulk of the ovary. Oocytes (egg cells) will develop on the

outer layer of the stroma, surrounded by supportive cells. The oocyte plus the supporting cells

is called a follicle. Beneath the cortex is what’s called the inner ovarian medulla, which is where

the blood vessels, nerves, and lymph vessels exist in the ovary.

THE OVARIAN CYCLE AND OOGENESIS

The ovarian cycle involves the predictable development of the female oocyte and the ovarian

follicles during a woman’s reproductive years. This cycle is related to but not the same as the

menstrual cycle. It involves oogenesis (which is the creation of the gamete) and folliculogenesis

(which is the development of the follicle).

The process of creating the female gamete is referred to as oogenesis. The oogonia are the

female stem cells that form in utero during fetal development, dividing by mitosis similar to

how the male spermatogonia divides. Unlike the male, however, the female process of

oogenesis enters the primary oocyte stage before birth with arrest of this process during this

stage of development until the woman becomes fertile. The arrest of development ends with

the primary oogonium undergoing meiosis I but doesn’t go further until menarche, or the onset

of menses in females. The process continues until menopause, when the oocytes no longer

divide. There are millions of oocytes prior to birth but only 1-2 million at the time of the

woman’s birth and this number falls further to 400,000 at the time of puberty. The number

reaches zero at menopause.

Ovulation or the release of an oocyte from an ovary doesn’t happen until sexual maturity is

reached at menarche. Ovulation occurs roughly once every 28 days on average. The luteinizing

hormone in the female reaches a peak prior to ovulation and starts the resumption of the

meiosis I process in the primary oocyte so that it can become a secondary oocyte. While in

males, this results in two cells, this doesn’t happen in females. One secondary oocyte gets most

of the cytoplasm while the other receives very little and becomes a nonfunctional polar body.

The polar body might or might not complete its own meiosis but it will disintegrate. Rather than

the four cells that come from the male spermatogenesis process, only one cell survives to

become ovulated.

There is one oogonium, two diploid primary oocytes (arrested in meiosis I). One goes on to

complete meiosis to become a secondary oocyte, while one becomes a polar body (the

secondary oocyte is haploid, with only 23 chromosomes). The secondary oocyte doesn’t

actually complete its meiotic process until after it gets fertilized.

Meiosis II in the secondary oocyte happens after fertilization leading to a haploid ovum that

becomes the diploid zygote instantly. This is the basis of human life and is the first cell of the

embryo, containing both chromosomes from the male and chromosomes from the female. The

female ovum is considerably larger than the male sperm and contains all of the mitochondria

and organelles of the zygote. The sperm cell does not contribute any cytoplasm to the process. All the mitochondrial DNA is inherited by the maternal side/the ovum.