Developmental Neurobiology, edited by Philippe Evrard and Alexandre Minkowski. Nestle Nutrition Workshop Series, Vol. 12. Nestec Ltd., Vevey/Raven Press. Ltd., New York © 1989. Abnormal Development and Destructive Processes of the Human Brain During the Second Half of Gestation Philippe Evrard, Hazim J. Kadhim, Philippe de Saint-Georges, and Jean-Frangois Gadisseux Pediatric Neurology Service and Laboratory of Developmental Neurology, Hopital Universitaire Saint-Luc, B-I200 Brussels, Belgium Most viable developmental brain disorders occurring during the second half of gestation lead to neurological defects that can range from minimal learning or motor deficits to major motor and cognitive handicaps. These disorders are of great public health significance since the affected individuals require special attention or educa- tion as well as long-term medical care for their motor, epileptic, and intellectual handicaps. ABNORMAL HISTOGENETIC EVENTS DURING THE SECOND HALF OF GESTATION IN THE HUMAN FOREBRAIN Most neural cyto- and histogenetic events in the human forebrain occur during the first half of gestation. Most developmental steps in the second half of gestation launch growth and differentiation that persist long into the postnatal life (1,2) (Table 1). Residual, but important, histogenetic activities continue during the last 20 weeks of pregnancy (Table 2). Their disturbances will be reviewed in this section. Precocious Exhaustion of the Telencephalic Germinative Activity in Microcephalia Vera Systematic measurements of the germinative zone in the human fetal brain sup- port the hypothesis of precocious exhaustion of germinative zone mitotic activity in typical cases of genetic microcephalia vera (3). In the normal forebrain, the absolute volume of the germinative layer reaches its maximum around the 26th week of ges- tation, after the end of neuronal production. In fetal microcephalia vera from our 21 22 ABNORMAL DEVELOPMENT I 2ND HALF OF GESTATION TABLE 1. Prenatal developmental steps with examples of abnormal development Cytogenesis and histogenesis (first half of gestation*) Disturbances in the Main steps developmental program" 1. Separation of the main embry- Errors of the program onic sheets Disturbances in the performance of the program 2. Neurulation Role of aleatory aspects during execution of develop- 3. Neuronal multiplication mental program 4. Neuronal migration 5. Regional development of the cerebral vesicles Growth and differentiation (second half of gestation0) Developmental features Main lesional mechanisms" Growth and arborization 1. Disturbances of "residual" histogenesis Connections and synaptogenesis 2. CSF hypertension: prenatal hydrocephalus Myelination 3. Perfusion failures—hypoxias' Gliogenesis 4. Infections' 5. Trauma' 6. "Minor" disturbances Intrauterine steric hindrance Minor cortical disturbances during growth and differ- entiation period "Cyto- and histogenesis not fully completed at mid-gestation. 6Growth and differentiation program can be disturbed by the same mechanisms. 'Growth and differentiation continue during postnatal life. dRole of metabolic disturbances is not detailed in this table but is discussed elsewhere in the chapter. 'These etiological aspects can also interfere with cytogenesis and histogenesis. Reprinted from Evrard et al. (11), with permission of Paul H. Brookes Publishing Company. TABLE 2. Main histogenetic events during the second half of human gestation Continued neuronal production for the cerebellum and allocortex Residual neuronal production in the telencephalic germinal layer Migration of late-generated neurons Brun's layer histogenesis Glial transformation (transformation of RGCs into astrocytes) Glia! production in the germinative zone ABNORMAL DEVELOPMENT I 2ND HALF OF GESTATION 23 collection,* the ventricular zone was completely lacking at the 26th week (Fig. 1). This early exhaustion of the germinative zone which dates back to the last stages of neuron production precludes the normal production of the last migrating neurons destined for layers 3 and 2. This precocious germinative exhaustion gives, there- fore, a pathogenic explanation for the severe neuronal depletion in those superficial neocortical layers which characterizes the cytoarchitectonic pattern of microcephalia vera described by Williams (4). The quantitative estimate of the neuronal comple- ment in the vertical cortical dimension performed with a method modified from Rockel's (5,6) supports this pathogenic explanation (Fig. 1). The study of younger fetal specimens of microcephalia vera would permit one to determine the fetal age at which the germinative activity fails in this disease. Disturbances of Glial Transformation The glycogen staining method combining reduced osmium postfixation with a modified Thiery staining technique allows the unequivocal identification of radial glial cells (RGCs) throughout gestation and of radial glial fascicles (7,8). This method permits at the electron microscope level the identification and quantitative study of radial glial fibers (RGFs) and their relationships with migrating neurons and other neuronal elements. Moreover, it permits the study of the transformation of the RGCs into astrocytes during the second half of gestation through a process of nuclear migration (Fig. 2) and autophagy of glial processes (Fig. 3) (7,8). The ap- plication of this method in studying human abnormal neuronal patterns enabled us to describe a severe depletion of RGFs or an early glial transformation in holo- prosencephaly (9) and to suggest the existence of abnormal radial glial fascicles in the Potter syndrome (10) with total renal dysplasia (11). In fetal Hunter disease, the glycogen staining method showed the distribution of storage material between glial and neuronal phases at early prenatal stages of the process (12). Disturbances of Late Neuronal Migration Subpial ectopias are cellular masses containing neurons that either did not nor- mally discontinue or abnormally resumed their migration at the junction between the cortical plate and the plexiform zone and "overmigrated" into the extraneural com- partment. Scars between adjacent plexiform zones and discontinuities of the subpial basal membrane are often associated with such subpial ectopias which suggests that normal arrest of migration is dependent on the integrity of the pial-glial interface. Massive neuronal ectopias are fairly common in fetal alcohol syndrome (FAS) *The human brains used for this study were collected in accordance with the ethical rules of our labo- ratory. Human fetal brains are obtained for neuropathological and neuroanatomical studies from cases involving miscarriage, abortion, and deceased premature infants. In cases of abortion, we accept speci- mens only if the obstetrician submitting the specimen formally certifies that the ethical rules applicable in his department were respected. 24 ABNORMAL DEVELOPMENT I 2ND HALF OF GESTATION FIG. 1. Premature exhaustion of the germinative zone in microcephalia vera. a: Microcephalia vera, human fetal forebrain, 26th week of gestation, b: Normal human fetal forebrain, 26th week, same cortical region, for comparison. In microcephalia vera the germinative zone is exhausted at this age, and the intermediate zone is almost devoid of late migrating glial and neuronal cells. In the neocortex the number of neurons calculated in the vertical columns by a modified Rockel's method is 20% less than in the control; layers 6 to 4 are normal, whereas the two superficial lay- ers are almost missing. LV, lateral ventricle; GL, germinative zone. (From ref. 11, with permis- sion of Paul H. Brookes Publishing Company.) (Fig. 4) without being pathognomonic for this syndrome. Subpial ectopias are also encountered in a heterogeneous group of malformations and can occasionally be present in the normal brain (13-18). In our fetal material, subpial neuronal ectopias prevail between 20 and 25 weeks of gestation, suggesting that they are a late migra- tory event. Abnormalities of the Subpial Granular Layer Brun's layer, a transitory subpial granular layer characteristic of the human fetal brain, is generated from the postolfactory germinative zone. Its volume increases until the 20th week of gestation and finally disappears around the 34th week (19,20). The fetal case of microcephalia vera shown in Fig. 1 was devoid of the sub- pial granular layer; this feature is in line with a precocious generalized exhaustion of the germinative zones, including that of the ganglionic eminence and postolfactory ABNORMAL DEVELOPMENT I 2ND HALF OF GESTATION 25 FIG. 2. Appearance of glial cell bodies within the cortical plate at the end of the period of neu- ronal migration, a: Glycogen labeling according to Gadisseux and Evrard (7). These glial nuclei appear within the cortical plate from 21 st week in the human fetal neocortex and from embryonic day 18 in the mouse, b: Insert from a. G, Glial cell body, with cytoplasm containing glycogen. RGF, isolated radial glial fiber. FIG. 3. Lysosomal proliferation in glial processes of radial glial cells. Human fetal molecular layer, 24th week of gestation. Glycogen staining according to ref. 8. G1 and G2, glial profiles filled with glycogen. Lysosomal proliferation in glial profile G1. Proliferation of the lysosomal ap- paratus and autophagy become intense, after the period of neuronal migration, in segments of RGFs during the RGC transformation into astrocytes. r^-rVV.'-* V*V ?J FIG. 4. Subpiai neuronal ectopia. Human fetal neocortex, 24th week of gestation.