The Experimental Embryology of the Foetal Nervous System

Abstract

SUMMARY It is feasible to submit the mammalian foetus to surgery and the types of operation thus far performed are reviewed. Other techniques for affecting mammalian organs during development, such as irradiation, organ explantation, and antibody‐antigen reactions, are mentioned. The reactions to injury of mammalian foetal nerve tissue are reviewed. The severed spinal cord does not regenerate under ordinary circumstances in postnatal or foetal animals. Spinal ganglion cells regenerate. The reactions of the nervous system to injury are more extensive in younger foetuses than in older ones, and more marked in foetuses than in postnatal animals. This is seen in the results of eye extirpation in guinea‐pig foetuses compared with those in newborn rats. Gliosis is limited in foetal and young animals, and is not so marked in young foetuses as in old ones, owing probably to the immaturity of the neuroglial cells and to a relatively reduced glial cell number. The life history of nerve cells can be divided into three stages: the phase of proliferation, the phase of differentiation, and the phase of maturation, including growth and the completion and maintenance of this process. The methods of determining which stage of development of neurones is affected after operation on a mammalian foetus are explained. There are many cases in chick and amphibian embryos where cellular hypoplasia after operation is caused by retrograde or transneuronal regression or degeneration of cells, and is a result of effects on the maintenance of neurones, rather than on their proliferation or differentiation. Cases of foetal malformations, such as lack of development of a limb or eye, are reviewed. The hypoplastic effects in these cases are reinterpreted and are seen to be due also to effects on the maintenance of neurones, not on their proliferation or differentiation. Experiments on limb or eye removal in foetuses are reviewed. The resulting hypoplasia or surmised hypoplasia in the spinal ganglia, spinal cord, superior col‐liculus and lateral geniculate body are caused partly by retrograde or transneuronal degeneration or regression and are interpreted as effects, at least in part, on the maintenance of neurones. More work must be done on mammals before effects on proliferation and differentiation of neurones can be eliminated as contributing to hypoplasia after operation. The mammalian foetal experiments which produce hypoplasia after some or all of the nerve fibres have already reached their destination (limb, spinal cord, superior colliculus, lateral geniculate body) are not unlike many experiments producing hypoplastic effects in amphibian and chick embryos, where the nerve fibres have not yet innervated their respective parts at the time of operation. In both, the resulting hypoplasia is due, at least in part, to retrograde or transneuronal degeneration or regression of already differentiated nerve cells; it is frequently a direct effect on the maintenance of cells, rather than an indirect effect on proliferation or differentiation, and...

Abnormal spontaneous potentials in distal muscles in animal models of spinal cord injury

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