Science and Profession (Magill’s Medical Guide, Sixth Edition)
The study of human embryology is the study of human prenatal development. The three stages of development are cleavage (the first week), embryonic development (the second through eighth weeks), and fetal development (the ninth through thirty-eighth weeks).
After an egg is fertilized by sperm in the uterine, or Fallopian, tube, the resulting zygote begins to divide rapidly. This period of rapid cell division is known as cleavage. By the third day, the zygote has divided into a solid ball containing twelve to sixteen cells. The small ball of cells resembles a mulberry and is called the morula, which is Latin for “mulberry.” The morula moves from the uterine tube into the uterus.
The morula develops a central cavity as spaces begin to form between the inner cells. At this stage, the developing human is called a blastocyst. The ring of cells on the outer edge of the hollow ball is called the trophoblast and will form a placenta, while the cluster of cells within becomes the inner cell mass and will form the embryo. By the end of the first week, the surface of the inner cell mass has flattened to form an embryonic disc, and the blastocyst has attached to the lining of the uterus and begun to embed itself.
During the second week of development, the trophoblast makes connections with the uterus into which it has burrowed to form the placenta. Blood vessels from the embryo link it to the placenta through the...
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Diagnostic and Treatment Techniques (Magill’s Medical Guide, Sixth Edition)
Knowledge of normal embryonic development is very important both in helping women provide optimal prenatal care for their children and in promoting scientific research for improved prenatal treatment, better understanding of malignant growths, and insight into the aging process.
Environmental stress to the embryo during the fourth through eighth weeks can cause abnormal development and result in congenital malformation, which may be defined as any anatomical defect present at birth. Environmental agents that cause malformations are known as teratogens. Malformations may develop from genetic or environmental factors, but most often they are caused by a combination of the two. Some of the common teratogens are viral infections, drug use, a poor diet, smoking, alcohol consumption, and irradiation.
The genetic makeup of some individuals makes them particularly sensitive to certain agents, while others are resistant. The abnormalities may be immediately apparent at birth or hidden within the body and discovered later. Embryos with severe structural abnormalities often do not survive, and such abnormalities represent an important cause of miscarriages. In fact, up to half of all conceptions spontaneously abort, with little or no notice by the prospective mother.
Genetic birth defects are passed on from one generation to another and result from a gene mutation at some time in the past. Mutations are...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
The first recorded observations of a developing embryo were performed on a chick by Hippocrates in the fifth century b.c.e. In the fourth century b.c.e., Aristotle wondered whether a preformed human unfolded in the embryo and enlarged with time, or whether a very simple embryonic structure gradually became more and more complex. This question was debated for nearly two thousand years until the early nineteenth century, when microscopic studies of chick embryos were carefully conducted and described.
Understanding human embryology is foundational for recognizing the relationships that exist between the body systems and congenital malformations in newborns. This field of study takes on new importance in the light of advances of modern technology, which have made prenatal diagnosis and treatment a reality.
The study of embryology is also making contributions toward finding the causes of malignant growth. Malignancy is a breakdown in the mechanisms for normal growth and differentiation first seen in the early embryo. Questions about uninhibited malignant growth may be answered by studying embryonic tissues and organs.
The study of old age is another area in which embryological research is valuable. Understanding the clock mechanisms of embryonic cells has led to greater understanding of the “winding down” of cells in old age. It is also important that researchers discover how environmental conditions...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Mader, Sylvia S. Inquiry into Life. Dubuque, Iowa: William C. Brown, 2010. An introductory-level college text designed to cover the entire range of biological topics. Gives a clear description of typical early developmental stages of all vertebrates and offers a section on human embryology and fetal development, adulthood, and aging.
Marieb, Elaine N. Essentials of Human Anatomy and Physiology. 9th ed. San Francisco: Pearson/Benjamin Cummings, 2009. This introductory anatomy and physiology textbook, easily accessible to those with little science background, is richly illustrated with diagrams and photographs, which help to illuminate body systems and processes.
Moore, Keith L., and T. V. N. Persaud. The Developing Human. 8th ed. Philadelphia: Saunders/Elsevier, 2008. An outstanding textbook on human embryonic development, with specific information about the causes of congenital malformations and common defects occurring in each of the body’s systems. This widely used textbook gives a clear and careful description of normal human development during the entire prenatal period.
Riley, Edward P., and Charles V. Vorhees, eds. Handbook of Behavioral Teratology. New York: Plenum Press, 1986. An informative compilation of learning in the field of behavioral teratology. Covers historical context, general principles, and specific drugs and environmental agents that act as...
(The entire section is 302 words.)