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The initial divisions of the fertilized ovum (zygote) create a mass of cells (blastocyst) consisting of an inner cell mass (the embryonic disk) surrounded by an outer layer (the trophoblast). The cells of the trophoblast actively penetrate the endometrium and form the placenta.

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The cells of the embryonic disk are totipotential (ie, they have the capacity to produce all cells of the body). Initial differentiation into the three primary germ layers—ectoderm, mesoderm, and endoderm—ultimately gives rise to the organs of the body through division, organization, and differentiation. The mass of primitive cells from which an organ develops is known as the anlage. A fully developed organ is composed of highly differentiated cells committed to the performance of particular functions and having limited residual capacity for division. Most human organs are fully formed and functional at birth. Organs such as the heart complete development earlier in fetal life than others such as the lung, which reaches maturity after the thirty-fourth week. The brain shows considerable development after birth, attaining maturity at about age 7 years. Sexual development culminates at puberty.

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Definitions

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Agenesis

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Failure of development of a primitive organ anlage in the embryo results in agenesis—complete absence of the organ. Agenesis of a vital organ such as the heart or brain leads to death of the fetus in utero. If the tissue is not vital or is one of a pair of organs, such as a kidney, the remainder of the embryo may develop normally.

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Dysgenesis

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Abnormal differentiation of the anlage leads to a structurally abnormal organ. For example, in renal dysgenesis, a mass of tissue composed of abnormal epithelium-lined cysts and mesenchymal tissues such as cartilage is found instead of a normal kidney (Figure 15-2). Dysgenesis sometimes affects only part of an organ.

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Figure 15–2.
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Renal dysgenesis. The kidney is grossly abnormal and shows multiple cysts. (The nodular structures seen on this surface view were filled with fluid when cut open.)

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Hypoplasia and Aplasia

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When the anlage differentiates normally but growth or development ends prematurely, a structurally normal but small organ results (hypoplasia). In aplasia, the organ is completely absent. Aplasia can be distinguished from agenesis only if an undeveloped anlage or its vascular connections can be identified. In agenesis, there is no anlage or vascular pedicle.

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Causes of Fetal Abnormalities

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In most instances, the exact cause of fetal abnormalities is unknown. Known causes fall into two major groups: those affecting the genome and those acting mainly on the proliferating cells of the embryo or fetus. Almost any ...

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