Human reproduction requires healthy gametes (sperm and eggs) as well as a healthy uterus to nurture the fetus until it is born. 6-7% of heterosexual couples will experience infertility, defined as the inability to achieve pregnancy after 12 months of trying. Because our reproductive systems age as we age, women over the age of 35 are considered infertile after 6 months of trying and not getting pregnant.
In the natural system, ejaculated semen is deposited in the vagina during intercourse. Motile sperm swim out of the semen and travel through the female reproductive tract from the vagina, through the cervix, through the uterus, and finally into the fallopian tube(s). If this happens during or just after ovulation (the release of an egg from the ovary), the egg and sperm meet inside the fallopian tube. The sperm then interact with the eggshell (zona pellucida) and one sperm is allowed inside to fertilize the egg. The newly fertilized embryo (zygote) then travels towards the uterus as it develops. First the one cell divides into two cells, then four, then 8, then 16. Around the 16 cell stage (Day 3 of development) the embryo begins to morulate and the cells talk to each other and decide which part of the embryo they will become. After morulation the embryo must blastulate (form a fluid-filled cavity and two distinct tissue masses, the inner cell mass and the trophectoderm). The embryo must reach the blastocyst stage before it is ready to implant into the uterine lining and form a placenta.
The human female reproductive system includes a vagina, cervix (opening between the vagina and uterus), a uterus with two fallopian tubes on either side, two ovaries as well as breast tissue and external genatalia. The reproductive organs are controlled by the reproductive hormones which are secreted by the pituitary gland in response to hormones released by the hypothalamus in the brain.
Hormones are inter-organ communication messages. They allow the organs to “talk” to each other. In the human female, the hypothalamus releases gonadotropin-releasing hormone (GnRH) to “tell” the pituitary gland to release the gonadotropins. The gonadotropins include follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH “tells” the ovaries to recruit and nurture follicles (small sacs filled with nurturing cells that help an egg develop). In response to this message, follicular cells nurture the egg and make estrogen. LH “tells” the follicle it is time to rupture and release the egg (ovulation). It also “tells” the follicular cells to luteinize (stop making estrogen and start making progesterone) as well as tell the egg that it is time to complete development. This progesterone is very important if an egg fertilizes as it keeps the uterine lining (endometrium) thick and vascularized and ready to form a placenta for the growing embryo.
The male reproductive system includes the testes (where sperm are produced), vas deferens (the tube that sperm travel to exit the body), Cowper's gland, the seminal vesicles, prostate, and external genitalia (penis and testicles which house the testes). Sperm precursor cells inside the testes are constantly cycling through sperm production in the adult male. It takes about 3 months from the time a precursor cell begins its development to the time that it is ready to be ejaculated as a mature sperm.
The testes are controlled by the same hormones as the female reproductive system. Gonadotropin-releasing hormone is released by the hypothalamus which then released the gonadotropins (FSH and LH) in response to this signal. These hormones stimulate sperm production in the testes as well as the production of the sex hormone testosterone.