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Teratology is the study of physiological defects in organisms. The defects could be due to several factors like the environmental factors. These factors work together to impair the normal development of an embryo. The level of vulnerability to these defects depends on several factors like the stage of development of an embryo at the time the organism is exposed to the heterogenic agents.
These agents act on the cells of the embryo or its tissues hence causing abnormalities in the development. The level of defects mostly depends on the nature of the teratogenic agents, the way in which the cells are exposed to these agents and the level of placental transfer between the mother and the fetus as well as the level of the dosage (Wilson 200).
When an embryo is exposed to the teratogenic agents, the end result might be, retardation, malfunctioning, malformation or in the extreme cases death. Some of the congenital defects are however genetic and they are passed from one generation to the other. Some are usually internal while others are external.
The manifestation might be large or small depending on a number of factors like the level of exposure to the teratogenic agents. Some of the parts affected during the malfunctioning, deformation or retardation include the nervous system, facial features, the gut, the skeletal as well as the genital systems (Nau 111).
Some of the malformations in vertebrates are usually minor and to some extend negligible. Some involve the malformation of the external features like the facial features, the ear, and hands among others. Some of these malformations are due to the viral effects, radiation as well as the chemical substances that are ingested or find their way into the body. The malformations can be categorized as follows;
- Malformation: This is a structural defect that is caused by a localized morphogenetic error.
- Deformation: It is the change in the structure or shape that occurs on an organ that was previously normal.
- Anomalad: This is a malformation that takes place with subsequent structural changes.
- Syndrome: It is a known malformation pattern that might not necessarily be due to morphogenetic errors.
- Association: It is a malformation pattern that is considered to be neither a syndrome nor an anomalad.
A single etiological factor is able to lead to a number of these malfunctions. Mutation of an organism’s genes plays a very significant role in the development of these malformations. In the cases where such malformations are hereditary, careful insight into the organism’s history and family tree is able to determine the genesis of such malformations.
It is usually not very easy to determine whether the defects that occur after birth are malformations or deformations. Some of them might not necessarily be due to the primary developmental error. When different agents cause the similar defects in the same organisms, then it is quite obvious that such agents would be acting on one particular field of development (Carlson 134).
Gene mutations usually lead to misinformation as the information contained in such genes is usually altered which in turn leads to malformation for instance in the cases of deafness, cataract as well as heart complications. The altered information that is coded in the genes is usually passed to the subsequent generations and the probability of similar malformations occurring in the subsequent generations is very high. Some of the defects might not be recognized until the organism is a few months or years old.
A good example is the deafness that is caused by rubella which might not be evident until the child is one or two years old. The rubella virus rarely crosses the placenta from the mother to the fetus although whenever this happens the results are tragic. It might for instance result in the deafening of the fetus, retardation, abortions and in some cases death. It has been found out that some of the prenatal defects that are eminent in mammals are due to irradiation (Needham 109-111).
Vitamins A are very vital in the fetal development in the vertebrates. When the mother does not get these vitamins, the fetus will most probably be prone to malformation. Whenever such vitamins are deficient in the diet of the mother, there are high chances of the malformations in the kidneys, ovaries, ears among other parts.
It has also been discovered that the malformations might be due to the chemical substances that are ingested or those that in one way or the other find their way into the body. Pregnant mothers who take the thalidomide medication are prone to having babies with malformations.
The magnitude of the malformation might depend on the amount of these substances that has been ingested. Some of the other effects include deaths as well as abortions. Some of the defects include the malformation of the limbs, shoulders, and the skeleton as well as other non-skeletal tissues. The placenta is usually porous to some of the substances that are likely to cause defects when they come into contact with the fetus.
It is quite common for the blood cells of the fetus to find their way into the mother’s blood system and this might be due to placental defects or it could happen at the time of delivery. It is possible for the fetus’s erythrocytes to have the Rh antigens while the mother is negative. Whenever they find their way into the mother’s system, her immune system produces the anti-Rh antibodies which are aimed at countering the Rh antigens. The effect is not much in the firstborns as the antibodies are not in large quantities.
The subsequent pregnancies are however affected significantly as the mother’s immune system is well prepared to counter any Rh antigens. The antibodies are able to penetrate the placental walls attacking the Rh positive erythrocytes in the unborn hence causing hemolysis. This results into the hemolytic disease. In severe cases, the young one would be prone to brain damage and anemia. In humans, the conditions might be averted through blood transfusion in the baby.
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Placental Pathological Conditions
There are several pathological conditions of the placenta that might affect the normal development of the fetus. Abnormalities in the implantation of the placenta might have adverse effects on the unborn. In some cases for instance, the placenta might be implanted in such a way that part of it covers the uterine cavity on the cervical outlet.
This is a common cause of hemorrhage which usually leads to the early separation of parts of the placenta from the uterine wall of the mother hence leading to fetal malformation. The uterus is also prone to malignant tumors which are likely to cause impairment in the fetal development (Streeter 62).
Principles of Teratology
The vulnerability to these defects and malformations is very much dependent on the organism’s genotype and its interaction with the environment. The stage of development at the time of exposure is very significant in determining the vulnerability of the organism.
The teratogenic agents act in certain ways to the organism’s cells as well as tissues hence causing abnormality in the development of the organism. This usually results in malformations, retardation, and disorder and to some extend death might occur. Environmental factors might also determine the development of a tissues or cells of that organism but this largely depends on the nature of such influence. The degree of the influence is usually proportional to the level of dosage of the teratogenic agents (Fabiola Muller 213).
The causes of the malformations can therefore be summarized into three major categories; they include genetic disorders of which 15% are attributed to the internal factors. The single gene defects and the chromosomal aberrations fall under this category.
The other causes are environmental factors of which 10% is attributed to the external factors. They include chemical substances, drugs, certain hormones as well as Vitamins. Others include physical agents as well as infectious agents. The other environmental factors that cause the malformations include the maternal conditions.
Idiopathic disorders and other multifactorial disorders also lead to malformations. Some of the chemical substances that might alter the normal development of the embryo include environmental pollutants like lead and mercury, ethanol and retinoid (O’Rahilly 333-340). The malformations usually vary from one organism to another depending on the exposure to the teratogenic agents and this usually results into miscarriages and still birth, retardation, congenital anomalies as well as functional defects.
Carlson, B. M. Patten’s Foundations of Embryology. 6. New York: McGraw-Hill, Inc., 1996.
Fabiola Muller, Ronan O’Rahilly. Human Embryology & Teratology. New York: Wiley-Liss, 2001.
Nau, Hopwood. A history of normal plates, tables and stages in vertebrate embryology. New York: PubMed PMID, 2007.
Needham, Joseph. History of Embryology. Cambridge: Cambridge: University Press, 1959.
O’Rahilly, Ronan. Developmental Stages in Human Embryos. Washington: Carnegie Institution of Washingto, 1973.
Streeter, George. Developmental horizons in human embryos. Contributions to Embryology. Oxford: Oxford University Press, 1996.
Wilson, James G. und F. Clarke Fraser. Handbook of Teratology. London: Plenum Press, 1977.