Chapter 25 - Physiology of Conception

Implantation (continued)

After the blastocyst is free in the uterine cavity for 1-2 days, the exposed cell walls of the blastocyst called the trophoblast secrete enzymes that are able to break down protein and penetrate cell membranes. These enzymes allow the blastocyst to enter the endometrium and implant. The action of the enzymes normally stops short of the myometrium but may cause slight bleeding in some individuals (implantation bleeding). The bleeding may confuse some women, who think they have a very light and short menstrual cycle when really they are pregnant.

Implantation (continued)

Chorionic villi secrete human chorionic gonadotropin (HCG), a hormone that stimulates the continued production of progesterone and estrogen by the corpus luteum. This is why ovulation and menstruation cease during pregnancy. This villi become the fetal portion of the placenta. The primary villi also synthesize protein and glucose for approximately 12 weeks, until the fetus is adequately developed to meet its own needs. The chorionic villi become the fetal portion of the placenta.

Fertilization

During sexual intercourse, the sperm carried in ejaculatory semen enters the vagina. The sperm travels through the mucus of the cervical canal (if the mucus is receptive), enter the uterine chamber, and move into the ampulla-the outer third of the Fallopian tube. If the timing is right, an ovum has been produced and is also within the ampulla of the tube; in such cases fertilization may occur. The fusion of the sperm into the ovum requires approximately 24 hours. Once fertilization has occurred, the new cell is referred to as a zygote (fertilized ovum).

Implantation (continued)

Ectopic pregnancy also poses serious problems. During the first few weeks after implantation, primary villi appear. These villi use maternal blood vessels as a source of nourishment and oxygen for the developing embryo. The villi nourish the embryo from the time of implantation (2 weeks after conception) until the seventh or eighth week. Also during these first few weeks the first stages of the chorionic villi occur.

Implantation (continued)

The condition of the uterine lining is critical for implantation of the zygote. During the secretory phase of the menstrual cycle the endometrium has an enriched vascular bed with enlarged blood vessels and an increased store of glycogen. Implantation usually occurs in the fundus of the uterus on either the anterior or posterior surfaces. If uterine conditions are not suitable, implantation is unlikely to occur. If intrauterine vascular or hormonal conditions cannot sustain the implanted embryo, a spontaneous abortion occurs, usually during the first 8 weeks of pregnancy.

Implantation

The zygote moves through the uterine tube through ciliary action and some irregular peristaltic activity. It takes 3-4 days to enter the uterine cavity. During this time the zygote is in a phase of rapid cell division (mitosis); further changes result in the formation of a structure called the morula. The morula develops into the blastocyst. STEM CELLS are derived from the inner cell mass of the blastocyst.