THE INFLUENCE OF EXTRINSIC FACTORS UPON INTRAUTERINE LIFE AND DEVELOPMENTAL DISTURBANCES OF THE BRAIN

Abstract

Feeblemindedness is caused by diversified origins. There are feeblemindednesses resulting from extrinsic factors which affect intrauterine life, perinatal life and further post natal life. Here, causes that act on intrauterine life and developmental disturbances of the brain resulting from such extrinsic factors will be discussed. Special reference to extrinsic factors which have the possibility to cause feeblemindedness and developmental disturbances will be stated in a little detail. Further, on experimental approaches to study developmental disturbances of the brain due to extrinsic factors will be referred to. I. Intrauterine life is divided into preimplantation period, period of organogenesis and fetal period. Each period is influenced on either common or specific extrinsic factors. A. It is said that during the preimplantation period severe physical and chemical effects kill cells while effects below a certain level do not involve cells in damage. Therefore, extrinsic factors do not, in general, cause malformations. B. Period of organogenesis : In this period, embryos are the most susceptible and various kinds of developmental disturbances with special reference to those of the brain are caused. Following factors are known teratogenic : 1. Ionizing radiations with special reference to medical irradiation and exposure to the atomic bomb. 2. Infections such as rubella, cytomegalic inclusion disease, and toxoplasmosis. 3. Chemical substances such as thalidomide, aminopterine, etc. 4. Hormonal aberrations with special reference to diabetes of the mother, hypo- or hyperthy-roidism, etc. are also conceivable. 5. Other factors such as maternal age, birth ranks, etc. are also suspected. Among them, ionizing radiations and the above infections only involve the brain. C. Fetal period : In human beings, following factors cause birth defects. 1. Ionizing radiations. 2. The same infectious agents indicated in item B. 3. Poisoning of organic mercuric compounds (congenital MINAMATA disease). 4. Masculinization by synthetic gestagens. 5. Possible oxygen deficiency such as caused by unsuccessful attempts at suiside by hanging, fuel gas poisoning, etc. 6. Abnormal immnologic condition in the mother (suspected) . Among the above factors, except for synthetic gestagens, are causes of possible microcephalias or hydrocephalias which are related to feeblemindedness. II. Experimental approach : On external agents which cause developmental disturbances of the brain related to feeblemindedness, such as hydrocephalia, microcephalia are stated. Further, other malformations of the central nervous system which will occure besides those related to feeblemindedness will also be stated. However, microcephalias that manifest themselves in laboratory animals are usually accompanied by enlargement of the brain cavity, i.e., hydromicro-cephalia in microscopic examinations. Involvements are so severe that it is difficult to find similarities in morphologic changes in experimental specimens compared with those seen in man. However, it is verified that fetal stages will be vulnerable to ionizing radiation according to the dosage factor.