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Some Aspects of Developmental Neurology: a Review SAMUELP

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Some Aspects of Developmental Neurology: a Review SAMUELP Some Aspects of Developmental Neurology: A Review SAMUELP. HICKS (New England Deaconess Hospital and Harrard Medical School, Boston, Mass.) The increasing interest in the ontogenetic and with in basic physics and chemistry. ... As we genetic aspects of abnormal development and neo now view it, development is an assembly-line plasia in the nervous system makes a presentation process in which countless component events are of some aspects of neuroembryology timely. Seem brought together in orderly patterns in regular ingly unrelated medical problems of growth are succession and are interwoven with one another by coming more and more to have a common meeting innumerable specific interactions." ground in basic fields of developmental biology. What are some of these engineering perform In medical practice, for example, a newborn anen- ances, and what lies behind them? In the present cephalic monster, an idiot child who excretes phen- state of our knowledge, with its numerous gaps, we ylpyruvic acid in its urine, a young adult with a are still at the stage of studying the component retinoblastoma, and a victim of an hereditary events and describing the parts that go into what ataxia might seem to have nothing in common but we see as morphogenesis. We are a long way from simply to be the result of "environmental acci Weiss' ideal of understanding the assembly process dents." Actually, these are no accidents but repre as a whole, but a number of areas have been of sent specific problems in development, with a especial interest to experimental embryologists, promise of real answers. These particular abnor biochemists, anatomists, and geneticists. Some of malities have a certain inevitableness now, because these may be enumerated. One concerns the prob their origin can be traced back to the primary lem of how the nervous system makes its first genetic material in the germ cells; but progress is appearance—-the so-called induction of the nerv being made toward a better understanding of the ous system. The means by which one group of steps that lie between a peculiar chromosome con cells "induces" another group to differentiate is a figuration and such deviations from normal ontog major problem, not only in the inception of the eny. When those steps are understood, prevention neural plate but as a basic phenomenon through may be possible. out embryogenesis. Another area is the study of The study of how the nervous system may de the mechanics by which cells that make up the velop abnormally must be based on a knowledge nervous system originate, differentiate, migrate, of its normal embryology, for what we call abnor aggregate, and interconnect to form the complex malities are really variations or deviations from functioning organization that we know as the normal or average pathways of development. A nervous system. During ontogeny each neuron primary goal in any such study is to learn how the acquires a biochemical submicroscopic specificity nervous system functions and what things disturb that makes almost every nerve cell different from it. The foundation for this is how it was put to every other, and the relation of this to integrated gether in the first place, since morphogenesis de behavior has been much studied. How the genie termines the structural basis of function. The material at the beginning of development trans morphogenesis of the nervous system is compli lates itself into the specific form and function of a cated, and it cannot, unfortunately, be reduced to mature organism is receiving increased attention only a few simple laws based on physical chemis from multiple disciplines. The impact of embry try or enzyme action. Actually, there is a great ology has been more superficial in oncology, but deal more to the problem. Weiss (1950) has empha some interrelations have attracted interest. sized this by saying that "The complex engineer This review will tell about some of the work go ing performances of technology are a much more ing on in the areas just enumerated, and emphasis pertinent model of the nature of morphogenesis will be on early morphogenesis. This can perhaps than are the more elementary phenomena dealt be done best by first refreshing the reader's mind * Some of the work referred to is supported by AEC Con about how animals with backbones develop, then tracts AT (30-1) 901 and 1452, USPHS Grant B382, and UCP going on into the subjects of neural induction, how Research and Educational Foundation. the components of the nervous system are put to Received for publication December 22, 1956. gether, the early development of functional specific- 251 Downloaded from cancerres.aacrjournals.org on September 24, 2021. © 1957 American Association for Cancer Research. 252 Cancer Research ity, genetic and chromosomal aspects, and finally blastic and a hypoblastic layer. Above this is the a brief reference to neoplasia. The available data amniotic sac, below the yolk sac. Mesoderm forms are by no means sufficient to form a smooth ac at the disc rim and gradually forms a middle count of the origin of the nervous system at pres "germ layer" by the time that a primitive streak ent, and they can only suggest some of the mech forms. A mesodermal node forms in the streak and anisms by which its development goes awry. apparently by gastrulation forms the prechordal and chordal mesoderm whose importance will be VERTEBRATEDEVELOPMENT discussed. The lateral mesoderm is not believed to The earliest vertebrates perhaps had simply an flow into the streak to become redistributed by elementary receptor-effector neural axis. During gastrulation. evolution there was added a variety of increasingly These vertebrates have arrived by different complex associative and information-storing de paths at a stage where mesoderm underlies ecto vices that integrated and modified neural function derm. The mesoderm is essential to the transfor in adaptation to surroundings. Despite the great mation, or induction, of the ectoderm into neurec- array and different complexities of present-day toderm, which forms the nervous system. With vertebrate nervous systems, there are common mammals used now as examples of further devel features in their development (9, 12, 18, 48, 49, opment, the transformed central ectoderm, now a 54, 55, 65, 66, 72). neural plate, broadens anteriorly to be shaped like Following fertilization (or parthenogenesis) and a long blunt triangle. Caudally the plate lengthens various patterns of cleavage, the vertebrate cell but is narrower. The anterior brain-to-be grows mass comes to a stage called a blástula in which laterally, the edges curl up and inward, and the the beginnings of what we arbitrarily call an em caudal spinal cord-to-be does the same on a bryo first appear (63, 67, 84, 95, 104, 115). A proc smaller scale. Both close medially to form the ess called gastrulation ("forming a little stomach") neural tube, enlarged anteriorly. By the time the occurs in which relatively •¿undifferentiatedcells lateral folds have become well developed the an actively migrate and stream into certain relative terior end is bent ventrally over the early heart positions that give rise to a "layered" early em and gut structures, and the optic pits are recog bryo. The amphibian blástula is a hollow sphere nizable ventrolaterally. At about the 20-somite of cells with a thin roof and a thick yolk-cell floor. stage in the rat and at about the same in man, An indentation on the upper surface appears, in- cerebral vesicles are becoming evident as bulges vaginates, and outer cells stream toward and into of the anterior neural tube, and the neuraxis may this opening (blastopore) to underlie cells on the be arbitrarily divided into forebrain, in-between- outside and form by their relative positions the brain (diencephalon), midbrain, hindbrain, and so-called germ layers. The relations are so rapidly spinal cord. Subsequent growth involves a com changing that in point of time ectoderm, ento- plex sequence of gross flexures and disproportion derm, and mesoderm are very arbitrary terms ate growths. The cerebral vesicles with their ven- applied to the layers. In the bird the blástulaforms trolateral striatal masses bulging into the ven a disc, instead of a vesicle, on a sea of yolk, and tricles increase greatly in size. The thalamic struc during gastrulation cells stream toward and into tures develop medially in the diencephalic region, a groove in the primitive streak, a central concen and the cerebellum arises from the anterior part of tration of cells in the disc. The rat and man, be the rhomboid flexure of the medulla still later on cause of placentation, are somewhat more com (see Chart 1). plex. The hollow blástulaof the rat develops a cell At the cellular level, the early nervous system mass at one pole, the outer part of which becomes derives from the primitive neural plate, a layer of ectoplacenta. The inner part of the cell mass forms proliferating cells. Similar cells later line the neural a hollow cylinder with an inner ectoderm and an tube and are called neurectoderm, germinal layer, outer entoderm. The embryo proper then develops or ependymal layer. Their derivatives are the pre as a focal thickening of the ectoderm called a cursors of neurons and glia, and these accumulate primitive streak. Between the entoderm and the at first just outside the lining to form a "mantle." thin blástula vesicle wall is a space, the yolk sac, By varying processes of proliferation, migration, sometimes containing maternal erythrocytes. It and grouping, these germinal and differentiating is assumed that the layered embryo that follows cells form the future nuclei, cell layers, and zones results from gastrulation analogous to that in the of the gray matter.
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