Human Embryonic Development

Introduction: The production of a new individual follows a sequence of events coordinated primarily by hormones of the mother and the placenta. Nevertheless, there are other factors which influence the developing individual, and those influences are mediated by the body of the mother. The course of events in the human is characteristic of placental mammals with differences among other species in details and timing. Fertilization: Both sperm and egg are living cells supplied with nutrients to survive for several days, and, while most of those cells never survive those few days, each has the potential to form a new individual. If they meet while still viable, fertilization unites their two sets of chromosomes, one from the mother and one from the father, which will eventually be used as the blueprint for the new individual. Additionally, the process of fertilization initiates development of the embryo (zygote). Earliest development is directed by maternal substances deposited in the egg when it was formed. Enlarge Image Early Development: The first three days of embryonic development occur in the fallopian tube as the embryo moves from the ovary to the uterus. Cell division continues and forms a hollow ball of cells (the blastocyst). Six days after fertilization, the embryo begins implantation into the lining of the uterus. During the next few weeks the placenta begins producing the hormones that will cause the mother to supply the embryo with nutrients and oxygen. In addition, the placenta protects the fetus from immune attack by the mother, removes waste products from the fetus, induces the mother to bring more blood to the placenta, and near the time of delivery, produces hormones that mature the fetal organs in preparation for life outside of the uterus. Complications: Failure of the trophoblasts to establish adequate blood flow between the mother and the fetus can lead to maternal hypertension and possibly to early termination of pregnancy. Infections of the extra-embryonic membranes enclosing the fetus and of the amniotic fluid can lead to difficulties in pregnancy, preterm birth, and permanent effects on the child. The most common source of infection is through the cervix and may be caused by either bacteria or viruses. Although most pregnancies do not experience 'immunologic rejection,' both the fetus and the placenta are foreign tissues within the mother and sometimes they are detected by the immune system of the mother and attacked. Finally, there may be genetic deficiencies in the fetus or the placenta that result in a miscarriage. Such genetic problems are the cause of most cases of first trimester pregnancy loss. Odds: The odds are close to four out of five that a zygote will abort spontaneously due to lethal genetic abnormalities that are manifested in early development. Early pregnancy is precarious and highly tentative. As knowledge of the physiology of human reproduction has improved (thanks in no small measure to the dramatic advances made in in vitro fertilization and assisted reproductive technologies during the 20th century), it has become clear that the vast majority of human fertilizations in vivo die before ever reaching fetal viability. A conservative estimate suggests that at least 75% of all human fertilizations fail to advance to the point of viability, with the vast majority of such failures occurring prior to the development of a clinically recognizable pregnancy (about 4 weeks after conception). Approximately 30% of fertilized oocytes are lost before they even reach the uterus, and another 30% either fail to implant or shrivel up and die shortly after implantation. Most women in whom these events occur are unaware that they are even taking place. Finally, an additional 15% abort spontanously during the early first trimester, shortly after they have become clinically recognizable. After the first trimester of pregnancy the majority of abnormal pregnancies have been eliminated, and loss after 14 weeks is negligible. Risk Factors: Some factors that can reduce the chance of success are: genetic abnormalities in the zygote, smoking or alcohol consumption by the mother, exposure of the father to environmental toxins, extended continence by the father, a mother with genital herpes. A factor that may increase the chance of conception is the father drinking coffee prior to intercourse because that seems to increase the swimming speed of the sperm.

Adapted and excerpted from: From Trophoblast to Human Placenta (No longer on internet; access from link in BIO 301 URLs) Wall, L. Lewis, and Douglas Brown. 2006. Regarding Zygotes as Persons. Perspectives in Biology and Medicine 49(4):602-610 (e-reading in Vista) Tagatz, George. 1990. Medical Techniques for Assisted Reproduction. pp 89-110 In: Beyond Baby M. Ethical Issues in New Reproductive Techniques. Edited by Dianne M. Bartels, Reinhard Priester, Dorothy E. Vawter, and Arthur L Caplan.

Click above for a tutorial assignment question.