Fertilization of the ovulated by a oocyte by a spermatazoan normally occurs in the fallopian tubes of the female reproductive tract within 18 to 24 hours of oculation. Transport of gametes to the fallopian tubes, fertilization, embryo development, formation of the placenta and pregnancy depend on a coordinated sequence of events.

Disruption of these events can result in structural and or functional anomalies in the fetus and or termination of the pregnancy. After ovulation, the cumulus-oocyte complex is transported along the surface of the ovary and through the ostium facilitated by the beating of the cilia lining the fimbria. The spermatozoon is ejaculated into the vagina near the external orifice of the cervix during coitus which usually occurs within 10 minutes of intermission.

The alkaline semen buffers the acidic with vaginal fluid which is about ph 5. After ejaculation, the process of capacitation is completed and spermatozoa undergo hyperactivation process by which they become hypermotile. The seminal plasma coagulates upon ejaculation which helps trap sperm in the cervical mucus until they become hypermotile. Fibrinolysin a proteolytic enzyme secreted into seminal plasma by the prostate gland causes liquefaction within 20 to 30 minutes maximal sperm motility is achieved within about one hour.

Sperm migrate through the cervical mucus and travel about 2 to 3 millimeters per minute aided by contractions of the uterine and cervical musculature the 1st sperm reach the fallopian tubes about 5 minutes after ejaculation usually less than 200 spermatozoa are present in the fallopian tubes at any one time but they can remain intact for 24 to 72 hours.

Contractions of the oviductal muscles direct the oocyte into the ampula of the fallopian tube where it remains for about 3 days while the ampullary isthmic sphincter remains constricted. Note that oocytes remain fertile for only 15 to 18 hours after ovulation and sperm remain fertile for perhaps 24 hours after ejaculation.

Hyaluronidase on the outer surface of the acrosome of the spermatazoan facilitates migration through the cumulus oocyte complex. By degrading hyaluronic acid containing mucal polysaccharides in the cumulus. Fertilization is the fusion of a spermatozoan and an oocyte takes place in one of the oviducts or fallopian tubes. It is necessary for the gametes to be delivered to the fallopian tubes in a fertilizable state.

When a spermatozoan encounters the zona pellucida, it undergoes the acrosome reaction. This reaction breaks down the acrosomal membrane and requires high calcium acrosin, a proteolytic enzyme is released and it aids in penetration of the zona pellucida by the spermatozoan. The sperm head membrane binds to the sperm receptor which is followed by fusion with the oolemma.

Microvilli on the oocyte surface surround the sperm head and the oocyte undergoes the cortical reaction which is the release of cortical granules. The zona pellucida hardens and no other spermatozoa can penetrate the oolemma. The nucleus completes maturation to yield the female pronucleus and the 2nd polar body. The sperm nucleus forms the male pronucleus.

The zygote is kept in the fallopian tube for about 3 days by the spastic contractions of the estrogen dominated isthmus. In the fallopian tube, the zygote undergoes cleavage division from one cell to eight-cell. Approximately 7 days after fertilization the blastocyst bursts from the zona pellucida which is called hatching and implants in the wall of the uterus which is called nidation.

Implantation requires prior conditioning of the endometrium by progesterone which causes the stromal cells to swell and accumulate glycogen, lipids, and protein. The presence of HCG from the blastocyst stimulates the corpus luteum of the maternal ovary to secrete progesterone. Anti progestins such as RU486 block implantation. The blastocyst attaches to the wall of the uterus fundus at the embryonic pole.

The inner cell mass becomes the fetus and the outer cells become the placenta and fetal membranes. Embryonic truffle blast cells invade through the endometrial epithelium into the endometrial stroma aided by proteases. Stromal cells decidualize, a process by which they enlarge and become transcriptionally active and surround the blastocyst.